-Sections
+Frequently Asked Questions Cryptsetup/LUKS
+Sections
1. General Questions
2. Setup
3. Common Problems
6. Backup and Data Recovery
7. Interoperability with other Disk Encryption Tools
8. Issues with Specific Versions of cryptsetup
-9. References and Further Reading
+9. The Initrd question
+10. LUKS2 Questions
+11. References and Further Reading
A. Contributors
+1. General Questions
+
+
+ * 1.1 What is this?
+
+ This is the FAQ (Frequently Asked Questions) for cryptsetup. It covers
+ Linux disk encryption with plain dm-crypt (one passphrase, no
+ management, no metadata on disk) and LUKS (multiple user keys with one
+ master key, anti-forensic features, metadata block at start of device,
+ ...). The latest version of this FAQ should usually be available at
+ https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions
-1. General Questions
-
-
- * 1.1 What is this?
-
- This is the FAQ (Frequently Asked Questions) for cryptsetup. It
- covers Linux disk encryption with plain dm-crypt (one passphrase,
- no management, no metadata on disk) and LUKS (multiple user keys
- with one master key, anti-forensic features, metadata block at
- start of device, ...). The latest version of this FAQ should
- usually be available at
- http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
-
-
- * 1.2 WARNINGS
-
- ATTENTION: If you are going to read just one thing, make it the
- section on Backup and Data Recovery. By far the most questions on
- the cryptsetup mailing list are from people that managed to damage
- the start of their LUKS partitions, i.e. the LUKS header. In
- most cases, there is nothing that can be done to help these poor
- souls recover their data. Make sure you understand the problem and
- limitations imposed by the LUKS security model BEFORE you face
- such a disaster! In particular, make sure you have a current header
- backup before doing any potentially dangerous operations.
-
- SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
- unclear how to get LUKS or plain dm-crypt to run on them with the
- full set of security features intact. This may or may not be a
- problem, depending on the attacher model. See Section 5.19.
-
- BACKUP: Yes, encrypted disks die, just as normal ones do. A full
- backup is mandatory, see Section "6. Backup and Data Recovery" on
- options for doing encrypted backup.
-
- CLONING/IMAGING: If you clone or image a LUKS container, you make a
- copy of the LUKS header and the master key will stay the same!
- That means that if you distribute an image to several machines, the
- same master key will be used on all of them, regardless of whether
- you change the passphrases. Do NOT do this! If you do, a root-user
- on any of the machines with a mapped (decrypted) container or a
- passphrase on that machine can decrypt all other copies, breaking
- security. See also Item 6.15.
-
- DISTRIBUTION INSTALLERS: Some distribution installers offer to
- create LUKS containers in a way that can be mistaken as activation
- of an existing container. Creating a new LUKS container on top of
- an existing one leads to permanent, complete and irreversible data
- loss. It is strongly recommended to only use distribution
- installers after a complete backup of all LUKS containers has been
- made.
-
- UBUNTU INSTALLER: In particular the Ubuntu installer seems to be
- quite willing to kill LUKS containers in several different ways.
- Those responsible at Ubuntu seem not to care very much (it is very
- easy to recognize a LUKS container), so treat the process of
- installing Ubuntu as a severe hazard to any LUKS container you may
- have.
-
- NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from
- STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
- warning that you are about to format (and e.g. will lose any
- pre-existing LUKS container on the target), as it assumes it is
- used from a script. In this scenario, the responsibility for
- warning the user and possibly checking for an existing LUKS header
- is shifted to the script. This is a more general form of the
- previous item.
-
- LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
- used in deriving the master key. It is used in decrypting a master
- key that is randomly selected on header creation. This means that
- if you create a new LUKS header on top of an old one with
- exactly the same parameters and exactly the same passphrase as the
- old one, it will still have a different master key and your data
- will be permanently lost.
-
- PASSPHRASE CHARACTER SET: Some people have had difficulties with
- this when upgrading distributions. It is highly advisable to only
- use the 95 printable characters from the first 128 characters of
- the ASCII table, as they will always have the same binary
- representation. Other characters may have different encoding
- depending on system configuration and your passphrase will not
- work with a different encoding. A table of the standardized first
- 128 ASCII characters can, e.g. be found on
- http://en.wikipedia.org/wiki/ASCII
+ * 1.2 WARNINGS
- * 1.3 System specific warnings
+ LUKS2 COMPATIBILITY: This FAQ was originally written for LUKS1, not
+ LUKS2. Hence regarding LUKS2, some of the answers found here may not
+ apply. Updates for LUKS2 have been done and anything not applying to
+ LUKS2 should clearly say LUKS1. However, this is a Frequently Asked
+ Questions, and questions for LUKS2 are limited at this time or at least
+ those that have reached me are. In the following, "LUKS" refers to both
+ LUKS1 and LUKS2.
+
+ The LUKS1 on-disk format specification is at
+ https://www.kernel.org/pub/linux/utils/cryptsetup/LUKS_docs/on-disk-format.pdf
+ The LUKS2 on-disk format specification is at
+ https://gitlab.com/cryptsetup/LUKS2-docs
+
+ ATTENTION: If you are going to read just one thing, make it the section
+ on Backup and Data Recovery. By far the most questions on the
+ cryptsetup mailing list are from people that managed to damage the start
+ of their LUKS partitions, i.e. the LUKS header. In most cases, there
+ is nothing that can be done to help these poor souls recover their data.
+ Make sure you understand the problem and limitations imposed by the LUKS
+ security model BEFORE you face such a disaster! In particular, make
+ sure you have a current header backup before doing any potentially
+ dangerous operations. The LUKS2 header should be a bit more resilient
+ as critical data starts later and is stored twice, but you can decidely
+ still destroy it or a keyslot permanently by accident.
+
+ DEBUG COMMANDS: While the --debug and --debug-json options should not
+ leak secret data, "strace" and the like can leak your full passphrase.
+ Do not post an strace output with the correct passphrase to a
+ mailing-list or online! See Item 4.5 for more explanation.
+
+ SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
+ unclear how to get LUKS or plain dm-crypt to run on them with the full
+ set of security assurances intact. This may or may not be a problem,
+ depending on the attacker model. See Section 5.19.
+
+ BACKUP: Yes, encrypted disks die, just as normal ones do. A full backup
+ is mandatory, see Section "6. Backup and Data Recovery" on options for
+ doing encrypted backup.
+
+ CLONING/IMAGING: If you clone or image a LUKS container, you make a copy
+ of the LUKS header and the master key will stay the same! That means
+ that if you distribute an image to several machines, the same master key
+ will be used on all of them, regardless of whether you change the
+ passphrases. Do NOT do this! If you do, a root-user on any of the
+ machines with a mapped (decrypted) container or a passphrase on that
+ machine can decrypt all other copies, breaking security. See also Item
+ 6.15.
+
+ DISTRIBUTION INSTALLERS: Some distribution installers offer to create
+ LUKS containers in a way that can be mistaken as activation of an
+ existing container. Creating a new LUKS container on top of an existing
+ one leads to permanent, complete and irreversible data loss. It is
+ strongly recommended to only use distribution installers after a
+ complete backup of all LUKS containers has been made.
+
+ UBUNTU INSTALLER: In particular the Ubuntu installer seems to be quite
+ willing to kill LUKS containers in several different ways. Those
+ responsible at Ubuntu seem not to care very much (it is very easy to
+ recognize a LUKS container), so treat the process of installing Ubuntu
+ as a severe hazard to any LUKS container you may have.
+
+ NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from STDIN
+ (e.g. via GnuPG) on LUKS format, it does not give you the warning that
+ you are about to format (and e.g. will lose any pre-existing LUKS
+ container on the target), as it assumes it is used from a script. In
+ this scenario, the responsibility for warning the user and possibly
+ checking for an existing LUKS header is shifted to the script. This is
+ a more general form of the previous item.
+
+ LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not used
+ in deriving the master key. It is used in decrypting a master key that
+ is randomly selected on header creation. This means that if you create
+ a new LUKS header on top of an old one with exactly the same parameters
+ and exactly the same passphrase as the old one, it will still have a
+ different master key and your data will be permanently lost.
+
+ PASSPHRASE CHARACTER SET: Some people have had difficulties with this
+ when upgrading distributions. It is highly advisable to only use the 95
+ printable characters from the first 128 characters of the ASCII table,
+ as they will always have the same binary representation. Other
+ characters may have different encoding depending on system configuration
+ and your passphrase will not work with a different encoding. A table of
+ the standardized first 128 ASCII characters can, e.g. be found on
+ http://en.wikipedia.org/wiki/ASCII
+
+ KEYBOARD NUM-PAD: Apparently some pre-boot authentication environments
+ (these are done by the distro, not by cryptsetup, so complain there)
+ treat digits entered on the num-pad and ones entered regularly
+ different. This may be because the BIOS USB keyboard driver is used and
+ that one may have bugs on some computers. If you cannot open your
+ device in pre-boot, try entering the digits over the regular digit keys.
- - Ubuntu as of 4/2011: It seems the installer offers to create
- LUKS partitions in a way that several people mistook for an offer
- to activate their existing LUKS partition. The installer gives no
- or an inadequate warning and will destroy your old LUKS header,
- causing permanent data loss. See also the section on Backup and
- Data Recovery.
- This issue has been acknowledged by the Ubuntu dev team, see here:
- http://launchpad.net/bugs/420080
+ * 1.3 System specific warnings
- Update 4/2013: I am still unsure whether this has been fixed by
- now, best be careful. They also seem to have added even more LUKS
- killer functionality to the Ubuntu installer. I can only strongly
- recommended to not install Ubuntu on a system with existing LUKS
- containers without complete backups.
+ - The Ubuntu Natty uinstaller has a "won't fix" defect that may destroy
+ LUKS containers. This is quite old an not relevant for most people.
+ Reference:
+ https://bugs.launchpad.net/ubuntu/+source/partman-crypto/+bug/420080
- * 1.4 My LUKS-device is broken! Help!
+ * 1.4 My LUKS-device is broken! Help!
First: Do not panic! In many cases the data is still recoverable.
Do not do anything hasty! Steps:
- - Take some deep breaths. Maybe add some relaxing music. This may
- sound funny, but I am completely serious. Often, critical damage is
+ - Take some deep breaths. Maybe add some relaxing music. This may
+ sound funny, but I am completely serious. Often, critical damage is
done only after the initial problem.
- - Do not reboot. The keys mays still be in the kernel if the device
- is mapped.
+ - Do not reboot. The keys may still be in the kernel if the device is
+ mapped.
- Make sure others do not reboot the system.
- - Do not write to your disk without a clear understanding why this
- will not make matters worse. Do a sector-level backup before any
- writes. Often you do not need to write at all to get enough access
- to make a backup of the data.
+ - Do not write to your disk without a clear understanding why this will
+ not make matters worse. Do a sector-level backup before any writes.
+ Often you do not need to write at all to get enough access to make a
+ backup of the data.
- Relax some more.
- Ask on the mailing-list if you need more help.
- * 1.5 Who wrote this?
+ * 1.5 Who wrote this?
- Current FAQ maintainer is Arno Wagner <arno@wagner.name>. If you
- want to send me encrypted email, my current PGP key is DSA key
- CB5D9718, fingerprint 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D
- 9718.
+ Current FAQ maintainer is Arno Wagner <arno@wagner.name>. If you want
+ to send me encrypted email, my current PGP key is DSA key CB5D9718,
+ fingerprint 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718.
- Other contributors are listed at the end. If you want to contribute,
- send your article, including a descriptive headline, to the
- maintainer, or the dm-crypt mailing list with something like "FAQ
- ..." in the subject. You can also send more raw information and
- have me write the section. Please note that by contributing to this
- FAQ, you accept the license described below.
+ Other contributors are listed at the end. If you want to contribute,
+ send your article, including a descriptive headline, to the maintainer,
+ or the dm-crypt mailing list with something like "FAQ ..."
+ in the subject. You can also send more raw information and have
+ me write the section. Please note that by contributing to this FAQ,
+ you accept the license described below.
- This work is under the "Attribution-Share Alike 3.0 Unported"
- license, which means distribution is unlimited, you may create
- derived works, but attributions to original authors and this
- license statement must be retained and the derived work must be
- under the same license. See
- http://creativecommons.org/licenses/by-sa/3.0/ for more details of
- the license.
+ This work is under the "Attribution-Share Alike 3.0 Unported" license,
+ which means distribution is unlimited, you may create derived works, but
+ attributions to original authors and this license statement must be
+ retained and the derived work must be under the same license. See
+ http://creativecommons.org/licenses/by-sa/3.0/ for more details of the
+ license.
- Side note: I did text license research some time ago and I think
- this license is best suited for the purpose at hand and creates the
- least problems.
+ Side note: I did text license research some time ago and I think this
+ license is best suited for the purpose at hand and creates the least
+ problems.
- * 1.5 Where is the project website?
+ * 1.6 Where is the project website?
- There is the project website at http://code.google.com/p/cryptsetup/
- Please do not post questions there, nobody will read them. Use
- the mailing-list instead.
+ There is the project website at
+ https://gitlab.com/cryptsetup/cryptsetup/ Please do not post
+ questions there, nobody will read them. Use the mailing-list
+ instead.
- * 1.6 Is there a mailing-list?
+ * 1.7 Is there a mailing-list?
Instructions on how to subscribe to the mailing-list are at on the
- project website. People are generally helpful and friendly on the
+ project website. People are generally helpful and friendly on the
list.
- The question of how to unsubscribe from the list does crop up
- sometimes. For this you need your list management URL, which is
- sent to you initially and once at the start of each month. Go to
- the URL mentioned in the email and select "unsubscribe". This page
- also allows you to request a password reminder.
+ The question of how to unsubscribe from the list does crop up sometimes.
+ For this you need your list management URL, which is sent to you
+ initially and once at the start of each month. Go to the URL mentioned
+ in the email and select "unsubscribe". This page also allows you to
+ request a password reminder.
- Alternatively, you can send an Email to dm-crypt-request@saout.de
- with just the word "help" in the subject or message body. Make sure
- to send it from your list address.
+ Alternatively, you can send an Email to dm-crypt-request@saout.de with
+ just the word "help" in the subject or message body. Make sure to send
+ it from your list address.
The mailing list archive is here:
- http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
+ https://marc.info/?l=dm-crypt
- * 1.7 Unsubscribe from the mailing-list
+ * 1.8 Unsubscribe from the mailing-list
- Send mail to dm-crypt-unsubscribe@saout.de from the subscribed
- account. You will get an email with instructions.
+ Send mail to dm-crypt-unsubscribe@saout.de from the subscribed account.
+ You will get an email with instructions.
Basically, you just have to respond to it unmodified to get
- unsubscribed. The listserver admin functions are not very fast. It
- can take 15 minutes or longer for a reply to arrive (I suspect
- greylisting is in use), so be patient.
+ unsubscribed. The listserver admin functions are not very fast. It can
+ take 15 minutes or longer for a reply to arrive (I suspect greylisting
+ is in use), so be patient.
- Also note that nobody on the list can unsubscribe you, sending
- demands to be unsubscribed to the list just annoys people that are
- entirely blameless for you being subscribed.
+ Also note that nobody on the list can unsubscribe you, sending demands
+ to be unsubscribed to the list just annoys people that are entirely
+ blameless for you being subscribed.
- If you are subscribed, a subscription confirmation email was sent
- to your email account and it had to be answered before the
- subscription went active. The confirmation emails from the
- listserver have subjects like these (with other numbers):
+ If you are subscribed, a subscription confirmation email was sent to
+ your email account and it had to be answered before the subscription
+ went active. The confirmation emails from the listserver have subjects
+ like these (with other numbers):
- Subject: confirm 9964cf10.....
-
- and are sent from dm-crypt-request@saout.de. You should check
- whether you have anything like it in your sent email folder. If
- you find nothing and are sure you did not confirm, then you should
- look into a possible compromise of your email account.
+ Subject: confirm 9964cf10.....
+ and are sent from dm-crypt-request@saout.de. You should check whether
+ you have anything like it in your sent email folder. If you find
+ nothing and are sure you did not confirm, then you should look into a
+ possible compromise of your email account.
-2. Setup
+2. Setup
- * 2.1 LUKS Container Setup mini-HOWTO
+ * 2.1 LUKS Container Setup mini-HOWTO
This item tries to give you a very brief list of all the steps you
- should go though when creating a new LUKS encrypted container, i.e.
+ should go though when creating a new LUKS encrypted container, i.e.
encrypted disk, partition or loop-file.
- 01) All data will be lost, if there is data on the target, make a
+ 01) All data will be lost, if there is data on the target, make a
backup.
- 02) Make very sure you have the right target disk, partition or
+ 02) Make very sure you use the right target disk, partition or
loop-file.
- 03) If the target was in use previously, it is a good idea to
- wipe it before creating the LUKS container in order to remove any
- trace of old file systems and data. For example, some users have
- managed to run e2fsck on a partition containing a LUKS container,
- possibly because of residual ext2 superblocks from an earlier use.
- This can do arbitrary damage up to complete and permanent loss of
- all data in the LUKS container.
+ 03) If the target was in use previously, it is a good idea to wipe it
+ before creating the LUKS container in order to remove any trace of old
+ file systems and data. For example, some users have managed to run
+ e2fsck on a partition containing a LUKS container, possibly because of
+ residual ext2 superblocks from an earlier use. This can do arbitrary
+ damage up to complete and permanent loss of all data in the LUKS
+ container.
To just quickly wipe file systems (old data may remain), use
- wipefs -a <target device>
-
+ wipefs -a <target device>
+
To wipe file system and data, use something like
- cat /dev/zero > <target device>
-
- This can take a while. To get a progress indicator, you can use
- the tool dd_rescue (->google) instead or use my stream meter "wcs"
- (source here: http://www.tansi.org/tools/index.html) in the
- following fashion:
+ cat /dev/zero > <target device>
+
+ This can take a while. To get a progress indicator, you can use the
+ tool dd_rescue (->google) instead or use my stream meter "wcs" (source
+ here: http://www.tansi.org/tools/index.html) in the following fashion:
+
+ cat /dev/zero | wcs > <target device>
+
+ Plain "dd" also gives you the progress on a SIGUSR1, see its man-page.
- cat /dev/zero | wcs > <target device>
-
Be very sure you have the right target, all data will be lost!
- Note that automatic wiping is on the TODO list for cryptsetup, so
- at some time in the future this will become unnecessary.
+ Note that automatic wiping is on the TODO list for cryptsetup, so at
+ some time in the future this will become unnecessary.
- 04) Create the LUKS container:
- cryptsetup luksFormat <target device>
-
- Just follow the on-screen instructions.
+ Alternatively, plain dm-crypt can be used for a very fast wipe with
+ crypto-grade randomness, see Item 2.19
- Note: Passphrase iteration is determined by cryptsetup depending on
- CPU power. On a slow device, this may be lower than you want. I
- recently benchmarked this on a Raspberry Pi and it came out at
- about 1/15 of the iteration count for a typical PC. If security is
- paramount, you may want to increase the time spent in iteration, at
- the cost of a slower unlock later. For the Raspberry Pi, using
+ 04) Create the LUKS container.
- cryptsetup luksFormat -i 15000 <target device>
-
- gives you an iteration count and security level equal to an average
- PC for passphrase iteration and master-key iteration. If in doubt,
- check the iteration counts with
+ LUKS1:
+
+ cryptsetup luksFormat --type luks1 <target device>
+
+ LUKS2:
+
+ cryptsetup luksFormat --type luks2 <target device>
- cryptsetup luksDump <target device>
-
- and adjust the iteration count accordingly by creating the container
- again with a different iteration time (the number after '-i' is the
- iteration time in milicesonds) until your requirements are met.
+
+ Just follow the on-screen instructions.
+
+ Note: Passprase iteration count is based on time and hence security
+ level depends on CPU power of the system the LUKS container is created
+ on. For example on a Raspberry Pi and LUKS1, I found some time ago that
+ the iteration count is 15 times lower than for a regular PC (well, for
+ my old one). Depending on security requirements, this may need
+ adjustment. For LUKS1, you can just look at the iteration count on
+ different systems and select one you like. You can also change the
+ benchmark time with the -i parameter to create a header for a slower
+ system.
+
+ For LUKS2, the parameters are more complex. ARGON2 has iteration,
+ parallelism and memory parameter. cryptsetup actually may adjust the
+ memory parameter for time scaling. Hence to use -i is the easiest way
+ to get slower or faster opening (default: 2000 = 2sec). Just make sure
+ to not drop this too low or you may get a memory parameter that is to
+ small to be secure. The luksDump command lists the memory parameter of
+ a created LUKS2 keyslot in kB. That parameter should probably be not
+ much lower than 100000, i.e. 100MB, but don't take my word for it.
05) Map the container. Here it will be mapped to /dev/mapper/c1:
- cryptsetup luksOpen <target device> c1
-
- 06) (Optionally) wipe the container (make sure you have the right target!):
- cat /dev/zero > /dev/mapper/c1
-
- Note that this creates a small information leak, as an attacker can
- determine whether a 512 byte block is zero if the attacker has
- access to the encrypted container multiple times. Typically a
- competent attacker that has access multiple times can install a
- passphrase sniffer anyways, so this leakage is not very
- significant. For getting a progress indicator, see step 03.
-
- Note that at some time in the future, cryptsetup will do this for
- you, but currently it is a TODO list item.
-
- 07) Create a file system in the mapped container, for example an
- ext3 file system (any other file system is possible):
- mke2fs -j /dev/mapper/c1
-
- 08) Mount your encrypted file system, here on /mnt:
- mount /dev/mapper/c1 /mnt
-
- Done. You can now use the encrypted file system to store data. Be
- sure to read though the rest of the FAQ, these are just the very
- basics. In particular, there are a number of mistakes that are
- easy to make, but will compromise your security.
+ cryptsetup luksOpen <target device> c1
+ 06) (Optionally) wipe the container (make sure you have the right
+ target!):
- * 2.2 How do I set up encrypted swap?
+ cat /dev/zero > /dev/mapper/c1
- As things that are confidential can end up in swap (keys,
- passphrases, etc. are usually protected against being swapped to
- disk, but other things may not be), it may be advisable to do
- something about the issue. One option is to run without swap, which
- generally works well in a desktop-context. It may cause problems
- in a server-setting or under special circumstances. The solution to
- that is to encrypt swap with a random key at boot-time.
+ This will take a while. Note that this creates a small information
+ leak, as an attacker can determine whether a 512 byte block is zero if
+ the attacker has access to the encrypted container multiple times.
+ Typically a competent attacker that has access multiple times can
+ install a passphrase sniffer anyways, so this leakage is not very
+ significant. For getting a progress indicator, see step 03.
- NOTE: This is for Debian, and should work for Debian-derived
- distributions. For others you may have to write your own startup
- script or use other mechanisms.
+ 07) Create a file system in the mapped container, for example an
+ ext3 file system (any other file system is possible):
- 01) Add the swap partition to /etc/crypttab. A line like the following
- should do it:
+ mke2fs -j /dev/mapper/c1
- swap /dev/<partition> /dev/urandom swap,noearly
-
- Warning: While Debian refuses to overwrite partitions with a
- filesystem or RAID signature on it, if your disk IDs may change
- (adding or removing disks, failure of disk during boot, etc.), you
- may want to take additional precautions. This is not a concern if
- you have only one disk. One possibility is to make sure the
- partition number is not present on additional disks or also swap
- there. Another is to encapsulate the swap partition (by making it a
- 1-disk RAID1 or by using LVM), so that it gets a persistent
- identifier. Specifying it directly by UUID does not work,
- unfortunately, as the UUID is part of the swap signature and that
- is not visible from the outside due to the encryption and in
- addition changes on each reboot with this setup.
-
- Note: Use /dev/random if you are paranoid or in a potential
- low-entropy situation (embedded system, etc.). This may cause the
- operation to take a long time during boot. If you are in a "no
- entropy" situation, you cannot encrypt swap securely. In this
- situation you should find some entropy, also because nothing else
- using crypto will be secure, like ssh, ssl or GnuPG.
-
- Note: The "noearly" option makes sure things like LVM, RAID, etc.
- are running. As swap is non-critical for boot, it is fine to start
- it late.
+ 08) Mount your encrypted file system, here on /mnt:
+
+ mount /dev/mapper/c1 /mnt
+
+ 09) Make a LUKS header backup and plan for a container backup.
+ See Section 6 for details.
+
+ Done. You can now use the encrypted file system to store data. Be sure
+ to read though the rest of the FAQ, these are just the very basics. In
+ particular, there are a number of mistakes that are easy to make, but
+ will compromise your security.
+
+
+ * 2.2 LUKS on partitions or raw disks? What about RAID?
+
+ Also see Item 2.8.
+ This is a complicated question, and made more so by the availability of
+ RAID and LVM. I will try to give some scenarios and discuss advantages
+ and disadvantages. Note that I say LUKS for simplicity, but you can do
+ all the things described with plain dm-crypt as well. Also note that
+ your specific scenario may be so special that most or even all things I
+ say below do not apply.
+
+ Be aware that if you add LVM into the mix, things can get very
+ complicated. Same with RAID but less so. In particular, data recovery
+ can get exceedingly difficult. Only add LVM if you have a really good
+ reason and always remember KISS is what separates an engineer from an
+ amateur. Of course, if you really need the added complexity, KISS is
+ satisfied. But be very sure as there is a price to pay for it. In
+ engineering, complexity is always the enemy and needs to be fought
+ without mercy when encountered.
+
+ Also consider using RAID instead of LVM, as at least with the old
+ superblock format 0.90, the RAID superblock is in the place (end of
+ disk) where the risk of it damaging the LUKS header is smallest and you
+ can have your array assembled by the RAID controller (i.e. the kernel),
+ as it should be. Use partition type 0xfd for that. I recommend staying
+ away from superblock formats 1.0, 1.1 and 1.2 unless you really need
+ them.
+
+ Scenarios:
+
+ (1) Encrypted partition: Just make a partition to your liking, and put
+ LUKS on top of it and a filesystem into the LUKS container. This gives
+ you isolation of differently-tasked data areas, just as ordinary
+ partitioning does. You can have confidential data, non-confidential
+ data, data for some specific applications, user-homes, root, etc.
+ Advantages are simplicity as there is a 1:1 mapping between partitions
+ and filesystems, clear security functionality and the ability to
+ separate data into different, independent (!) containers.
+
+ Note that you cannot do this for encrypted root, that requires an
+ initrd. On the other hand, an initrd is about as vulnerable to a
+ competent attacker as a non-encrypted root, so there really is no
+ security advantage to doing it that way. An attacker that wants to
+ compromise your system will just compromise the initrd or the kernel
+ itself. The better way to deal with this is to make sure the root
+ partition does not store any critical data and to move that to
+ additional encrypted partitions. If you really are concerned your root
+ partition may be sabotaged by somebody with physical access (who would
+ however strangely not, say, sabotage your BIOS, keyboard, etc.), protect
+ it in some other way. The PC is just not set-up for a really secure
+ boot-chain (whatever some people may claim).
+
+ (2) Fully encrypted raw block device: For this, put LUKS on the raw
+ device (e.g. /dev/sdb) and put a filesystem into the LUKS container, no
+ partitioning whatsoever involved. This is very suitable for things like
+ external USB disks used for backups or offline data-storage.
+
+ (3) Encrypted RAID: Create your RAID from partitions and/or full
+ devices. Put LUKS on top of the RAID device, just if it were an
+ ordinary block device. Applications are just the same as above, but you
+ get redundancy. (Side note as many people seem to be unaware of it: You
+ can do RAID1 with an arbitrary number of components in Linux.) See also
+ Item 2.8.
+
+ (4) Now, some people advocate doing the encryption below the RAID layer.
+ That has several serious problems. One is that suddenly debugging RAID
+ issues becomes much harder. You cannot do automatic RAID assembly
+ anymore. You need to keep the encryption keys for the different RAID
+ components in sync or manage them somehow. The only possible advantage
+ is that things may run a little faster as more CPUs do the encryption,
+ but if speed is a priority over security and simplicity, you are doing
+ this wrong anyways. A good way to mitigate a speed issue is to get a
+ CPU that does hardware AES as most do today.
+
+
+ * 2.3 How do I set up encrypted swap?
+
+ As things that are confidential can end up in swap (keys, passphrases,
+ etc. are usually protected against being swapped to disk, but other
+ things may not be), it may be advisable to do something about the issue.
+ One option is to run without swap, which generally works well in a
+ desktop-context. It may cause problems in a server-setting or under
+ special circumstances. The solution to that is to encrypt swap with a
+ random key at boot-time.
+
+ NOTE: This is for Debian, and should work for Debian-derived
+ distributions. For others you may have to write your own startup script
+ or use other mechanisms.
+
+ 01) Add the swap partition to /etc/crypttab. A line like the
+ following should do it:
+
+ swap /dev/<partition> /dev/urandom swap,noearly
+
+ Warning: While Debian refuses to overwrite partitions with a filesystem
+ or RAID signature on it, as your disk IDs may change (adding or removing
+ disks, failure of disk during boot, etc.), you may want to take
+ additional precautions. Yes, this means that your kernel device names
+ like sda, sdb, ... can change between reboots! This is not a concern
+ if you have only one disk. One possibility is to make sure the
+ partition number is not present on additional disks or also swap there.
+ Another is to encapsulate the swap partition (by making it a 1-partition
+ RAID1 or by using LVM), as that gets a persistent identifier.
+ Specifying it directly by UUID does not work, unfortunately, as the UUID
+ is part of the swap signature and that is not visible from the outside
+ due to the encryption and in addition changes on each reboot with this
+ setup.
+
+ Note: Use /dev/random if you are paranoid or in a potential low-entropy
+ situation (embedded system, etc.). This may cause the operation to take
+ a long time during boot however. If you are in a "no entropy"
+ situation, you cannot encrypt swap securely. In this situation you
+ should find some entropy, also because nothing else using crypto will be
+ secure, like ssh, ssl or GnuPG.
+
+ Note: The "noearly" option makes sure things like LVM, RAID, etc. are
+ running. As swap is non-critical for boot, it is fine to start it late.
02) Add the swap partition to /etc/fstab. A line like the following
should do it:
- /dev/mapper/swap none swap sw 0 0
-
- That is it. Reboot or start it manually to activate encrypted swap.
+ /dev/mapper/swap none swap sw 0 0
+
+ That is it. Reboot or start it manually to activate encrypted swap.
Manual start would look like this:
- /etc/init.d/crypdisks start
- swapon /dev/mapper/swap
-
+ /etc/init.d/crypdisks start
+ swapon /dev/mapper/swap
- * 2.3 What is the difference between "plain" and LUKS format?
+
+ * 2.4 What is the difference between "plain" and LUKS format?
First, unless you happen to understand the cryptographic background
- well, you should use LUKS. It does protect the user from a lot of
- common mistakes. Plain dm-crypt is for experts.
+ well, you should use LUKS. It does protect the user from a lot of
+ common mistakes. Plain dm-crypt is for experts.
Plain format is just that: It has no metadata on disk, reads all
- parameters from the commandline (or the defaults), derives a
- master-key from the passphrase and then uses that to de-/encrypt
- the sectors of the device, with a direct 1:1 mapping between
- encrypted and decrypted sectors.
-
- Primary advantage is high resilience to damage, as one damaged
- encrypted sector results in exactly one damaged decrypted sector.
- Also, it is not readily apparent that there even is encrypted data
- on the device, as an overwrite with crypto-grade randomness (e.g.
- from /dev/urandom) looks exactly the same on disk.
-
- Side-note: That has limited value against the authorities. In
- civilized countries, they cannot force you to give up a crypto-key
- anyways. In quite a few countries around the world, they can force
- you to give up the keys (using imprisonment or worse to pressure
- you, sometimes without due process), and in the worst case, they
- only need a nebulous "suspicion" about the presence of encrypted
- data. Sometimes this applies to everybody, sometimes only when you
- are suspected of having "illicit data" (definition subject to
- change) and sometimes specifically when crossing a border. Note
- that this is going on in countries like the US and the UK, to
- different degrees and sometimes with courts restricting what the
- authorities can actually demand.
+ parameters from the commandline (or the defaults), derives a master-key
+ from the passphrase and then uses that to de-/encrypt the sectors of the
+ device, with a direct 1:1 mapping between encrypted and decrypted
+ sectors.
+
+ Primary advantage is high resilience to damage, as one damaged encrypted
+ sector results in exactly one damaged decrypted sector. Also, it is not
+ readily apparent that there even is encrypted data on the device, as an
+ overwrite with crypto-grade randomness (e.g. from
+ /dev/urandom) looks exactly the same on disk.
+
+ Side-note: That has limited value against the authorities. In civilized
+ countries, they cannot force you to give up a crypto-key anyways. In
+ quite a few countries around the world, they can force you to give up
+ the keys (using imprisonment or worse to pressure you, sometimes without
+ due process), and in the worst case, they only need a nebulous
+ "suspicion" about the presence of encrypted data. Sometimes this
+ applies to everybody, sometimes only when you are suspected of having
+ "illicit data" (definition subject to change) and sometimes specifically
+ when crossing a border. Note that this is going on in countries like
+ the US and the UK to different degrees and sometimes with courts
+ restricting what the authorities can actually demand.
My advice is to either be ready to give up the keys or to not have
encrypted data when traveling to those countries, especially when
- crossing the borders. The latter also means not having any
- high-entropy (random) data areas on your disk, unless you can
- explain them and demonstrate that explanation. Hence doing a
- zero-wipe of all free space, including unused space, may be a good
- idea.
-
- Disadvantages are that you do not have all the nice features that
- the LUKS metadata offers, like multiple passphrases that can be
- changed, the cipher being stored in the metadata, anti-forensic
- properties like key-slot diffusion and salts, etc..
-
- LUKS format uses a metadata header and 8 key-slot areas that are
- being placed at the beginning of the disk, see below under "What
- does the LUKS on-disk format looks like?". The passphrases are used
- to decrypt a single master key that is stored in the anti-forensic
- stripes.
+ crossing the borders. The latter also means not having any high-entropy
+ (random) data areas on your disk, unless you can explain them and
+ demonstrate that explanation. Hence doing a zero-wipe of all free
+ space, including unused space, may be a good idea.
+
+ Disadvantages are that you do not have all the nice features that the
+ LUKS metadata offers, like multiple passphrases that can be changed, the
+ cipher being stored in the metadata, anti-forensic properties like
+ key-slot diffusion and salts, etc..
+
+ LUKS format uses a metadata header and 8 key-slot areas that are being
+ placed at the beginning of the disk, see below under "What does the LUKS
+ on-disk format looks like?". The passphrases are used to decrypt a
+ single master key that is stored in the anti-forensic stripes. LUKS2
+ adds some more flexibility.
Advantages are a higher usability, automatic configuration of
- non-default crypto parameters, defenses against low-entropy
- passphrases like salting and iterated PBKDF2 passphrase hashing,
- the ability to change passphrases, and others.
+ non-default crypto parameters, defenses against low-entropy passphrases
+ like salting and iterated PBKDF2 or ARGON 2 passphrase hashing, the
+ ability to change passphrases, and others.
- Disadvantages are that it is readily obvious there is encrypted
- data on disk (but see side note above) and that damage to the
- header or key-slots usually results in permanent data-loss. See
- below under "6. Backup and Data Recovery" on how to reduce that
- risk. Also the sector numbers get shifted by the length of the
- header and key-slots and there is a loss of that size in capacity
- (1MB+4096B for defaults and 2MB for the most commonly used
- non-default XTS mode).
+ Disadvantages are that it is readily obvious there is encrypted data on
+ disk (but see side note above) and that damage to the header or
+ key-slots usually results in permanent data-loss. See below under "6.
+ Backup and Data Recovery" on how to reduce that risk. Also the sector
+ numbers get shifted by the length of the header and key-slots and there
+ is a loss of that size in capacity. Unless you have a specific need,
+ use LUKS2.
- * 2.4 Can I encrypt an already existing, non-empty partition to use
- LUKS?
+ * 2.5 Can I encrypt an existing, non-empty partition to use LUKS?
- There is no converter, and it is not really needed. The way to do
- this is to make a backup of the device in question, securely wipe
- the device (as LUKS device initialization does not clear away old
- data), do a luksFormat, optionally overwrite the encrypted device,
- create a new filesystem and restore your backup on the now
- encrypted device. Also refer to sections "Security Aspects" and
- "Backup and Data Recovery".
+ There is no converter, and it is not really needed. The way to do this
+ is to make a backup of the device in question, securely wipe the device
+ (as LUKS device initialization does not clear away old data), do a
+ luksFormat, optionally overwrite the encrypted device, create a new
+ filesystem and restore your backup on the now encrypted device. Also
+ refer to sections "Security Aspects" and "Backup and Data Recovery".
- For backup, plain GNU tar works well and backs up anything likely
- to be in a filesystem.
+ For backup, plain GNU tar works well and backs up anything likely to be
+ in a filesystem.
- * 2.5 How do I use LUKS with a loop-device?
+ * 2.6 How do I use LUKS with a loop-device?
- This can be very handy for experiments. Setup is just the same as
- with any block device. If you want, for example, to use a 100MiB
- file as LUKS container, do something like this:
+ This can be very handy for experiments. Setup is just the same as with
+ any block device. If you want, for example, to use a 100MiB file as
+ LUKS container, do something like this:
+
+ head -c 100M /dev/zero > luksfile # create empty file
+ losetup /dev/loop0 luksfile # map file to /dev/loop0
+ cryptsetup luksFormat --type luks2 /dev/loop0 # create LUKS2 container
- head -c 100M /dev/zero > luksfile # create empty file
- losetup /dev/loop0 luksfile # map luksfile to /dev/loop0
- cryptsetup luksFormat /dev/loop0 # create LUKS on loop device
-
Afterwards just use /dev/loop0 as a you would use a LUKS partition.
To unmap the file when done, use "losetup -d /dev/loop0".
- * 2.6 When I add a new key-slot to LUKS, it asks for a passphrase but
- then complains about there not being a key-slot with that
- passphrase?
+ * 2.7 When I add a new key-slot to LUKS, it asks for a passphrase
+ but then complains about there not being a key-slot with that
+ passphrase?
- That is as intended. You are asked a passphrase of an existing
- key-slot first, before you can enter the passphrase for the new
- key-slot. Otherwise you could break the encryption by just adding a
- new key-slot. This way, you have to know the passphrase of one of
- the already configured key-slots in order to be able to configure a
- new key-slot.
+ That is as intended. You are asked a passphrase of an existing key-slot
+ first, before you can enter the passphrase for the new key-slot.
+ Otherwise you could break the encryption by just adding a new key-slot.
+ This way, you have to know the passphrase of one of the already
+ configured key-slots in order to be able to configure a new key-slot.
- * 2.7 Encryption on top of RAID or the other way round?
+ * 2.8 Encryption on top of RAID or the other way round?
+ Also see Item 2.2.
Unless you have special needs, place encryption between RAID and
- filesystem, i.e. encryption on top of RAID. You can do it the other
+ filesystem, i.e. encryption on top of RAID. You can do it the other
way round, but you have to be aware that you then need to give the
- passphrase for each individual disk and RAID autodetection will
- not work anymore. Therefore it is better to encrypt the RAID
- device, e.g. /dev/dm0 .
+ passphrase for each individual disk and RAID auto-detection will not
+ work anymore. Therefore it is better to encrypt the RAID device, e.g.
+ /dev/dm0 .
This means that the typical layering looks like this:
Filesystem <- top
|
- Encryption
+ Encryption (LUKS)
|
RAID
|
- Raw partitions
+ Raw partitions (optional)
|
Raw disks <- bottom
-
- The big advantage is that you can manage the RAID container just
- like any RAID container, it does not care that what is in it is
- encrypted.
+
+ The big advantage of this is that you can manage the RAID container just
+ like any other regular RAID container, it does not care that its content
+ is encrypted. This strongly cuts down on complexity, something very
+ valuable with storage encryption.
- * 2.8 How do I read a dm-crypt key from file?
+ * 2.9 How do I read a dm-crypt key from file?
Use the --key-file option, like this:
- cryptsetup create --key-file keyfile e1 /dev/loop0
-
- This will read the binary key from file, i.e. no hashing or
- transformation will be applied to the keyfile before its bits are
- used as key. Extra bits (beyond the length of the key) at the end
- are ignored. Note that if you read from STDIN, the data will still
- be hashed, just as a key read interactively from the terminal. See
- the man-page sections "NOTES ON PASSPHRASE PROCESSING..." for more
- detail.
+ cryptsetup create --key-file keyfile e1 /dev/loop0
+
+ This will read the binary key from file, i.e. no hashing or
+ transformation will be applied to the keyfile before its bits are used
+ as key. Extra bits (beyond the length of the key) at the end are
+ ignored. Note that if you read from STDIN, the data will be hashed,
+ just as a key read interactively from the terminal. See the man-page
+ sections "NOTES ON PASSPHRASE PROCESSING..." for more detail.
- * 2.9 How do I read a LUKS slot key from file?
+ * 2.10 How do I read a LUKS slot key from file?
- What you really do here is to read a passphrase from file, just as
- you would with manual entry of a passphrase for a key-slot. You can
- add a new passphrase to a free key-slot, set the passphrase of an
- specific key-slot or put an already configured passphrase into a
- file. In the last case make sure no trailing newline (0x0a) is
- contained in the key file, or the passphrase will not work because
- the whole file is used as input.
+ What you really do here is to read a passphrase from file, just as you
+ would with manual entry of a passphrase for a key-slot. You can add a
+ new passphrase to a free key-slot, set the passphrase of an specific
+ key-slot or put an already configured passphrase into a file. Make sure
+ no trailing newline (0x0a) is contained in the input key file, or the
+ passphrase will not work because the whole file is used as input.
To add a new passphrase to a free key slot from file, use something
like this:
- cryptsetup luksAddKey /dev/loop0 keyfile
-
- To add a new passphrase to a specific key-slot, use something like
- this:
+ cryptsetup luksAddKey /dev/loop0 keyfile
+
+ To add a new passphrase to a specific key-slot, use something
+ like this:
+
+ cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
- cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
-
To supply a key from file to any LUKS command, use the --key-file
option, e.g. like this:
- cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
-
+ cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
- * 2.10 How do I read the LUKS master key from file?
- The question you should ask yourself first is why you would want to
- do this. The only legitimate reason I can think of is if you want
- to have two LUKS devices with the same master key. Even then, I
- think it would be preferable to just use key-slots with the same
- passphrase, or to use plain dm-crypt instead. If you really have a
- good reason, please tell me. If I am convinced, I will add how to
- do this here.
+ * 2.11 How do I read the LUKS master key from file?
- * 2.11 What are the security requirements for a key read from file?
+ The question you should ask yourself first is why you would want to do
+ this. The only legitimate reason I can think of is if you want to have
+ two LUKS devices with the same master key. Even then, I think it would
+ be preferable to just use key-slots with the same passphrase, or to use
+ plain dm-crypt instead. If you really have a good reason, please tell
+ me. If I am convinced, I will add how to do this here.
+
+
+ * 2.12 What are the security requirements for a key read from file?
+
+ A file-stored key or passphrase has the same security requirements as
+ one entered interactively, however you can use random bytes and thereby
+ use bytes you cannot type on the keyboard. You can use any file you
+ like as key file, for example a plain text file with a human readable
+ passphrase. To generate a file with random bytes, use something like
+ this:
+
+ head -c 256 /dev/random > keyfile
- A file-stored key or passphrase has the same security requirements
- as one entered interactively, however you can use random bytes and
- thereby use bytes you cannot type on the keyboard. You can use any
- file you like as key file, for example a plain text file with a
- human readable passphrase. To generate a file with random bytes,
- use something like this:
- head -c 256 /dev/random > keyfile
-
- * 2.12 If I map a journaled file system using dm-crypt/LUKS, does it
- still provide its usual transactional guarantees?
+ * 2.13 If I map a journaled file system using dm-crypt/LUKS, does
+ it still provide its usual transactional guarantees?
- Yes, it does, unless a very old kernel is used. The required flags
- come from the filesystem layer and are processed and passed onwards
- by dm-crypt. A bit more information on the process by which
- transactional guarantees are implemented can be found here:
+ Yes, it does, unless a very old kernel is used. The required flags come
+ from the filesystem layer and are processed and passed onward by
+ dm-crypt (regardless of direct key management or LUKS key management).
+ A bit more information on the process by which transactional guarantees
+ are implemented can be found here:
http://lwn.net/Articles/400541/
- Please note that these "guarantees" are weaker than they appear to
- be. One problem is that quite a few disks lie to the OS about
- having flushed their buffers. Some other things can go wrong as
- well. The filesystem developers are aware of these problems and
- typically can make it work anyways. That said, dm-crypt/LUKS will
- not make things worse.
-
- One specific problem you can run into though is that you can get
- short freezes and other slowdowns due to the encryption layer.
- Encryption takes time and forced flushes will block for that time.
- For example, I did run into frequent small freezes (1-2 sec) when
- putting a vmware image on ext3 over dm-crypt. When I went back to
- ext2, the problem went away. This seems to have gotten better with
- kernel 2.6.36 and the reworking of filesystem flush locking
- mechanism (less blocking of CPU activity during flushes). It
- should improve further and eventually the problem should go away.
-
-
- * 2.13 Can I use LUKS or cryptsetup with a more secure (external)
- medium for key storage, e.g. TPM or a smartcard?
-
- Yes, see the answers on using a file-supplied key. You do have to
- write the glue-logic yourself though. Basically you can have
- cryptsetup read the key from STDIN and write it there with your
- own tool that in turn gets the key from the more secure key
- storage.
+ Please note that these "guarantees" are weaker than they appear to be.
+ One problem is that quite a few disks lie to the OS about having flushed
+ their buffers. This is likely still true with SSDs. Some other things
+ can go wrong as well. The filesystem developers are aware of these
+ problems and typically can make it work anyways. That said,
+ dm-crypt/LUKS will not make things worse.
+
+ One specific problem you can run into is that you can get short freezes
+ and other slowdowns due to the encryption layer. Encryption takes time
+ and forced flushes will block for that time. For example, I did run
+ into frequent small freezes (1-2 sec) when putting a vmware image on
+ ext3 over dm-crypt. When I went back to ext2, the problem went away.
+ This seems to have gotten better with kernel 2.6.36 and the reworking of
+ filesystem flush locking mechanism (less blocking of CPU activity during
+ flushes). This should improve further and eventually the problem should
+ go away.
+
+
+ * 2.14 Can I use LUKS or cryptsetup with a more secure (external)
+ medium for key storage, e.g. TPM or a smartcard?
+
+ Yes, see the answers on using a file-supplied key. You do have to write
+ the glue-logic yourself though. Basically you can have cryptsetup read
+ the key from STDIN and write it there with your own tool that in turn
+ gets the key from the more secure key storage.
For TPM support, you may want to have a look at tpm-luks at
- https://github.com/shpedoikal/tpm-luks. Note that tpm-luks is not
+ https://github.com/shpedoikal/tpm-luks. Note that tpm-luks is not
related to the cryptsetup project.
- * 2.14 Can I resize a dm-crypt or LUKS partition?
+ * 2.15 Can I resize a dm-crypt or LUKS container?
+
+ Yes, you can, as neither dm-crypt nor LUKS1 stores partition size and
+ LUKS2 uses a generic "whole device" size as default. Note that LUKS2
+ can use specified data-area sizes as a non-standard case and that these
+ may cause issues when resizing a LUKS2 container if set to a specific
+ value.
- Yes, you can, as neither dm-crypt nor LUKS stores partition size.
- Whether you should is a different question. Personally I recommend
- backup, recreation of the encrypted partition with new size,
- recreation of the filesystem and restore. This gets around the
- tricky business of resizing the filesystem. Resizing a dm-crypt or
- LUKS container does not resize the filesystem in it. The backup is
- really non-optional here, as a lot can go wrong, resulting in
- partial or complete data loss. Using something like gparted to
- resize an encrypted partition is slow, but typically works. This
- will not change the size of the filesystem hidden under the
- encryption though.
+ Whether you should do this is a different question. Personally I
+ recommend backup, recreation of the dm-crypt or LUKS container with new
+ size, recreation of the filesystem and restore. This gets around the
+ tricky business of resizing the filesystem. Resizing a dm-crypt or LUKS
+ container does not resize the filesystem in it. A backup is really
+ non-optional here, as a lot can go wrong, resulting in partial or
+ complete data loss. But if you have that backup, you can also just
+ recreate everything.
- You also need to be aware of size-based limitations. The one
- currently relevant is that aes-xts-plain should not be used for
- encrypted container sizes larger than 2TiB. Use aes-xts-plain64
- for that.
+ You also need to be aware of size-based limitations. The one currently
+ relevant is that aes-xts-plain should not be used for encrypted
+ container sizes larger than 2TiB. Use aes-xts-plain64 for that.
- * 2.15 How do I Benchmark the Ciphers, Hashes and Modes?
+ * 2.16 How do I Benchmark the Ciphers, Hashes and Modes?
- Since version 1.60 cryptsetup supports the "benchmark" command.
+ Since version 1.60 cryptsetup supports the "benchmark" command.
Simply run as root:
- cryptsetup benchmark
-
- It will output first iterations/second for the key-derivation
- function PBKDF2 parameterized with different hash-functions, and
- then the raw encryption speed of ciphers with different modes and
- key-sizes. You can get more than the default benchmarks, see the
- man-page for the relevant parameters. Note that XTS mode takes two
- keys, hence the listed key sizes are double that for other modes
- and half of it is the cipher key, the other half is the XTS key.
+ cryptsetup benchmark
+
+ You can get more than the default benchmarks, see the man-page for the
+ relevant parameters. Note that XTS mode takes two keys, hence the
+ listed key sizes are double that for other modes and half of it is the
+ cipher key, the other half is the XTS key.
+
+
+ * 2.17 How do I Verify I have an Authentic cryptsetup Source Package?
+ Current maintainer is Milan Broz and he signs the release packages with
+ his PGP key. The key he currently uses is the "RSA key ID D93E98FC",
+ fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B D93E 98FC. While I
+ have every confidence this really is his key and that he is who he
+ claims to be, don't depend on it if your life is at stake. For that
+ matter, if your life is at stake, don't depend on me being who I claim
+ to be either.
- * 2.16 How do I Verify I have an Authentic cryptsetup Source Package?
+ That said, as cryptsetup is under good version control and a malicious
+ change should be noticed sooner or later, but it may take a while.
+ Also, the attacker model makes compromising the sources in a non-obvious
+ way pretty hard. Sure, you could put the master-key somewhere on disk,
+ but that is rather obvious as soon as somebody looks as there would be
+ data in an empty LUKS container in a place it should not be. Doing this
+ in a more nefarious way, for example hiding the master-key in the salts,
+ would need a look at the sources to be discovered, but I think that
+ somebody would find that sooner or later as well.
- Current maintainer is Milan Broz and he signs the release packages
- with his PGP key. The key he currently uses is the "RSA key ID
- D93E98FC", fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B
- D93E 98FC. While I have every confidence this really is his key and
- that he is who he claims to be, don't depend on it if your life is
- at stake. For that matter, if your life is at stake, don't depend
- on me being who I claim to be either.
+ That said, this discussion is really a lot more complicated and longer
+ as an FAQ can sustain. If in doubt, ask on the mailing list.
- That said, as cryptsetup is under good version control, a malicious
- change should be noticed sooner or later, but it may take a while.
- Also, the attacker model makes compromising the sources in a
- non-obvious way pretty hard. Sure, you could put the master-key
- somewhere on disk, but that is rather obvious as soon as somebody
- looks as there would be data in an empty LUKS container in a place
- it should not be. Doing this in a more nefarious way, for example
- hiding the master-key in the salts, would need a look at the
- sources to be discovered, but I think that somebody would find that
- sooner or later as well.
- That said, this discussion is really a lot more complicated and
- longer as an FAQ can sustain. If in doubt, ask on the mailing list.
+ * 2.18 Is there a concern with 4k Sectors?
+ Not from dm-crypt itself. Encryption will be done in 512B blocks, but
+ if the partition and filesystem are aligned correctly and the filesystem
+ uses multiples of 4kiB as block size, the dm-crypt layer will just
+ process 8 x 512B = 4096B at a time with negligible overhead. LUKS does
+ place data at an offset, which is 2MiB per default and will not break
+ alignment. See also Item 6.12 of this FAQ for more details. Note that
+ if your partition or filesystem is misaligned, dm-crypt can make the
+ effect worse though. Also note that SSDs typically have much larger
+ blocks internally (e.g. 128kB or even larger).
-3. Common Problems
+ * 2.19 How can I wipe a device with crypto-grade randomness?
- * 3.1 My dm-crypt/LUKS mapping does not work! What general steps are
- there to investigate the problem?
+ The conventional recommendation if you want to do more than just a
+ zero-wipe is to use something like
- If you get a specific error message, investigate what it claims
- first. If not, you may want to check the following things.
+ cat /dev/urandom > <taget-device>
- - Check that "/dev", including "/dev/mapper/control" is there. If it
- is missing, you may have a problem with the "/dev" tree itself or
- you may have broken udev rules.
+ That used to very slow and painful at 10-20MB/s on a fast computer, but
+ newer kernels can give you > 200MB/s (depending on hardware). An
+ alternative is using cryptsetup and a plain dm-crypt device with a
+ random key, which is fast and on the same level of security. The
+ defaults are quite enough.
+
+ For device set-up, do the following:
+
+ cryptsetup open --type plain -d /dev/urandom /dev/<device> target
+
+ This maps the container as plain under /dev/mapper/target with a random
+ password. For the actual wipe you have several options. Basically, you
+ pipe zeroes into the opened container that then get encrypted. Simple
+ wipe without progress-indicator:
+
+ cat /dev/zero > /dev/mapper/to_be_wiped
+
+ Progress-indicator by dd_rescue:
+
+ dd_rescue -w /dev/zero /dev/mapper/to_be_wiped
+
+ Progress-indicator by my "wcs" stream meter (available from
+ http://www.tansi.org/tools/index.html ):
+
+ cat /dev/zero | wcs > /dev/mapper/to_be_wiped
+
+ Or use plain "dd", which gives you the progress when sent a SIGUSR1, see
+ the dd man page.
+
+ Remove the mapping at the end and you are done.
+
+
+ * 2.20 How to I wipe only the LUKS header?
+
+ This does _not_ describe an emergency wipe procedure, see Item 5.4 for
+ that. This procedure here is intended to be used when the data should
+ stay intact, e.g. when you change your LUKS container to use a detached
+ header and want to remove the old one. Please only do this if you have
+ a current backup.
+
+ LUKS1:
+ 01) Determine header size in 512 Byte sectors with luksDump:
+
+ cryptsetup luksDump <device with LUKS container>
+
+-> ...
+ Payload offset: <number>
+ ...
+
+ 02) Take the result number, multiply by 512 zeros and write to
+ the start of the device, e.g. like this:
+
+ dd bs=512 count=<number> if=/dev/zero of=<device>
+
+
+ LUKS2: (warning, untested! Remember that backup?) This assumes the
+ LUKS2 container uses the defaults, in particular there is only one data
+ segment. 01) Determine the data-segment offset using luksDump, same
+ as above for LUKS1:
+
+-> ...
+ Data segments:
+ 0: crypt
+ offset: <number> [bytes]
+ ...
+
+ 02) Overwrite the stated number of bytes from the start of the device.
+ Just to give yet another way to get a defined number of zeros:
+
+ head -c /dev/zero > /dev/<device>
+
+
+3. Common Problems
+
+
+ * 3.1 My dm-crypt/LUKS mapping does not work! What general steps
+ are there to investigate the problem?
+
+ If you get a specific error message, investigate what it claims first.
+ If not, you may want to check the following things.
+
+ - Check that "/dev", including "/dev/mapper/control" is there. If it is
+ missing, you may have a problem with the "/dev" tree itself or you may
+ have broken udev rules.
- Check that you have the device mapper and the crypt target in your
- kernel. The output of "dmsetup targets" should list a "crypt"
- target. If it is not there or the command fails, add device mapper
- and crypt-target to the kernel.
+ kernel. The output of "dmsetup targets" should list a "crypt" target.
+ If it is not there or the command fails, add device mapper and
+ crypt-target to the kernel.
- - Check that the hash-functions and ciphers you want to use are in
- the kernel. The output of "cat /proc/crypto" needs to list them.
+ - Check that the hash-functions and ciphers you want to use are in the
+ kernel. The output of "cat /proc/crypto" needs to list them.
- * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
+ * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
- The default cipher, hash or mode may have changed (the mode changed
- from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
+ The default cipher, hash or mode may have changed (the mode changed from
+ 1.0.x to 1.1.x). See under "Issues With Specific Versions of
cryptsetup".
- * 3.3 When I call cryptsetup from cron/CGI, I get errors about
- unknown features?
+ * 3.3 When I call cryptsetup from cron/CGI, I get errors about
+ unknown features?
If you get errors about unknown parameters or the like that are not
- present when cryptsetup is called from the shell, make sure you
- have no older version of cryptsetup on your system that then gets
- called by cron/CGI. For example some distributions install
- cryptsetup into /usr/sbin, while a manual install could go to
- /usr/local/sbin. As a debugging aid, call "cryptsetup --version"
- from cron/CGI or the non-shell mechanism to be sure the right
- version gets called.
-
-
- * 3.4 Unlocking a LUKS device takes very long. Why?
-
- The iteration time for a key-slot (see Section 5 for an explanation
- what iteration does) is calculated when setting a passphrase. By
- default it is 1 second on the machine where the passphrase is set.
- If you set a passphrase on a fast machine and then unlock it on a
- slow machine, the unlocking time can be much longer. Also take into
- account that up to 8 key-slots have to be tried in order to find the
- right one.
-
- If this is problem, you can add another key-slot using the slow
- machine with the same passphrase and then remove the old key-slot.
- The new key-slot will have an iteration count adjusted to 1 second
- on the slow machine. Use luksKeyAdd and then luksKillSlot or
- luksRemoveKey.
-
- However, this operation will not change volume key iteration count
- (MK iterations in output of "cryptsetup luksDump"). In order to
- change that, you will have to backup the data in the LUKS
- container (i.e. your encrypted data), luksFormat on the slow
- machine and restore the data. Note that in the original LUKS
- specification this value was fixed to 10, but it is now derived
- from the PBKDF2 benchmark as well and set to iterations in 0.125
- sec or 1000, whichever is larger. Also note that MK iterations
- are not very security relevant. But as each key-slot already takes
- 1 second, spending the additional 0.125 seconds really does not
- matter.
-
-
- * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
- device. What is wrong?
-
- Some old versions of cryptsetup have a bug where the header does
- not get completely wiped during LUKS format and an older ext2/swap
- signature remains on the device. This confuses blkid.
+ present when cryptsetup is called from the shell, make sure you have no
+ older version of cryptsetup on your system that then gets called by
+ cron/CGI. For example some distributions install cryptsetup into
+ /usr/sbin, while a manual install could go to /usr/local/sbin. As a
+ debugging aid, call "cryptsetup --version" from cron/CGI or the
+ non-shell mechanism to be sure the right version gets called.
+
+
+ * 3.4 Unlocking a LUKS device takes very long. Why?
+
+ The unlock time for a key-slot (see Section 5 for an explanation what
+ iteration does) is calculated when setting a passphrase. By default it
+ is 1 second (2 seconds for LUKS2). If you set a passphrase on a fast
+ machine and then unlock it on a slow machine, the unlocking time can be
+ much longer. Also take into account that up to 8 key-slots (LUKS2: up
+ to 32 key-slots) have to be tried in order to find the right one.
+
+ If this is problem, you can add another key-slot using the slow machine
+ with the same passphrase and then remove the old key-slot. The new
+ key-slot will have the unlock time adjusted to the slow machine. Use
+ luksKeyAdd and then luksKillSlot or luksRemoveKey. You can also use
+ the -i option to reduce iteration time (and security level) when setting
+ a passphrase. Default is 1000 (1 sec) for LUKS1 and 2000 (2sec) for
+ LUKS2.
+
+ However, this operation will not change volume key iteration count ("MK
+ iterations" for LUKS1, "Iterations" under "Digests" for LUKS2). In
+ order to change that, you will have to backup the data in the LUKS
+ container (i.e. your encrypted data), luksFormat on the slow machine
+ and restore the data. Note that MK iterations are not very security
+ relevant.
+
+
+ * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
+ device. What is wrong?
+
+ Some old versions of cryptsetup have a bug where the header does not get
+ completely wiped during LUKS format and an older ext2/swap signature
+ remains on the device. This confuses blkid.
Fix: Wipe the unused header areas by doing a backup and restore of
- the header with cryptsetup 1.1.x:
+ the header with cryptsetup 1.1.x or later:
- cryptsetup luksHeaderBackup --header-backup-file <file> <device>
- cryptsetup luksHeaderRestore --header-backup-file <file> <device>
-
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
+ cryptsetup luksHeaderRestore --header-backup-file <file> <device>
- * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
- There seems to be some interference between the hardening and and
- the way cryptsetup benchmarks PBKDF2. The solution to this is
- currently not quite clear for an encrypted root filesystem. For
- other uses, you can apparently specify USE="dynamic" as compile
- flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470
-
-4. Troubleshooting
+4. Troubleshooting
- * 4.1 I get the error "LUKS keyslot x is invalid." What does that
- mean?
+ * 4.1 I get the error "LUKS keyslot x is invalid." What does that mean?
- This means that the given keyslot has an offset that points
- outside the valid keyslot area. Typically, the reason is a
- corrupted LUKS header because something was written to the start of
- the device the LUKS container is on. Refer to Section "Backup and
- Data Recovery" and ask on the mailing list if you have trouble
- diagnosing and (if still possible) repairing this.
+ For LUKS1, this means that the given keyslot has an offset that points
+ outside the valid keyslot area. Typically, the reason is a corrupted
+ LUKS1 header because something was written to the start of the device
+ the LUKS1 container is on. For LUKS2, I do not know when this error can
+ happen, but I expect it will be something similar. Refer to Section
+ "Backup and Data Recovery" and ask on the mailing list if you have
+ trouble diagnosing and (if still possible) repairing this.
- * 4.2 I cannot unlock my LUKS container! What could be the problem?
+ * 4.2 I cannot unlock my LUKS container! What could be the problem?
- First, make sure you have a correct passphrase. Then make sure you
- have the correct key-map and correct keyboard. And then make sure
- you have the correct character set and encoding, see also
- "PASSPHRASE CHARACTER SET" under Section 1.2.
+ First, make sure you have a correct passphrase. Then make sure you have
+ the correct key-map and correct keyboard. And then make sure you have
+ the correct character set and encoding, see also "PASSPHRASE CHARACTER
+ SET" under Section 1.2.
If you are sure you are entering the passphrase right, there is the
- possibility that the respective key-slot has been damaged. There
- is no way to recover a damaged key-slot, except from a header
- backup (see Section 6). For security reasons, there is also no
- checksum in the key-slots that could tell you whether a key-slot has
- been damaged. The only checksum present allows recognition of a
- correct passphrase, but that only works if the passphrase is
- correct and the respective key-slot is intact.
-
- In order to find out whether a key-slot is damaged one has to look
- for "non-random looking" data in it. There is a tool that
- automatizes this in the cryptsetup distribution from version 1.6.0
- onwards. It is located in misc/keyslot_checker/. Instructions how
- to use and how to interpret results are in the README file. Note
- that this tool requires a libcryptsetup from cryptsetup 1.6.0 or
- later (which means libcryptsetup.so.4.5.0 or later). If the tool
- complains about missing functions in libcryptsetup, you likely
- have an earlier version from your distribution still installed. You
- can either point the symbolic link(s) from libcryptsetup.so.4 to
- the new version manually, or you can uninstall the distribution
- version of cryptsetup and re-install that from cryptsetup >= 1.6.0
- again to fix this.
-
-
- * 4.3 Can a bad RAM module cause problems?
+ possibility that the respective key-slot has been damaged. There is no
+ way to recover a damaged key-slot, except from a header backup (see
+ Section 6). For security reasons, there is also no checksum in the
+ key-slots that could tell you whether a key-slot has been damaged. The
+ only checksum present allows recognition of a correct passphrase, but
+ that only works with that correct passphrase and a respective key-slot
+ that is intact.
+
+ In order to find out whether a key-slot is damaged one has to look for
+ "non-random looking" data in it. There is a tool that automatizes this
+ for LUKS1 in the cryptsetup distribution from version 1.6.0 onwards. It
+ is located in misc/keyslot_checker/. Instructions how to use and how to
+ interpret results are in the README file. Note that this tool requires
+ a libcryptsetup from cryptsetup 1.6.0 or later (which means
+ libcryptsetup.so.4.5.0 or later). If the tool complains about missing
+ functions in libcryptsetup, you likely have an earlier version from your
+ distribution still installed. You can either point the symbolic link(s)
+ from libcryptsetup.so.4 to the new version manually, or you can
+ uninstall the distribution version of cryptsetup and re-install that
+ from cryptsetup >= 1.6.0 again to fix this.
+
+
+ * 4.3 Can a bad RAM module cause problems?
LUKS and dm-crypt can give the RAM quite a workout, especially when
- combined with software RAID. In particular the combination RAID5 +
- LUKS + XFS seems to uncover RAM problems that never caused obvious
- problems before. Symptoms vary, but often the problem manifest
+ combined with software RAID. In particular the combination RAID5 +
+ LUKS1 + XFS seems to uncover RAM problems that do not cause obvious
+ problems otherwise. Symptoms vary, but often the problem manifest
itself when copying large amounts of data, typically several times
larger than your main memory.
- Side note: One thing you should always do on large data
- copy/movements is to run a verify, for example with the "-d"
- option of "tar" or by doing a set of MD5 checksums on the source
- or target with
+ Note: One thing you should always do on large data copying or movements
+ is to run a verify, for example with the "-d" option of "tar" or by
+ doing a set of MD5 checksums on the source or target with
- find . -type f -exec md5sum \{\} \; > checksum-file
-
- and then a "md5sum -c checksum-file" on the other side. If you get
- mismatches here, RAM is the primary suspect. A lesser suspect is
- an overclocked CPU. I have found countless hardware problems in
- verify runs after copying or making backups. Bit errors are much
- more common than most people think.
+ find . -type f -exec md5sum \{\} \; > checksum-file
+
+ and then a "md5sum -c checksum-file" on the other side. If you get
+ mismatches here, RAM is the primary suspect. A lesser suspect is an
+ overclocked CPU. I have found countless hardware problems in verify
+ runs after copying data or making backups. Bit errors are much more
+ common than most people think.
Some RAM issues are even worse and corrupt structures in one of the
- layers. This typically results in lockups, CPU state dumps in the
- system logs, kernel panic or other things. It is quite possible to
- have the problem with an encrypted device, but not with an
- otherwise the same unencrypted device. The reason for that is that
- encryption has an error amplification property: You flip one bit
- in an encrypted data block, and the decrypted version has half of
- its bits flipped. This is an important security property for modern
- ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
- get up to a completely changed 512 byte block per bit error. A
- corrupt block causes a lot more havoc than the occasionally
- flipped single bit and can result in various obscure errors.
-
- Note, that a verify run on copying between encrypted or
- unencrypted devices will reliably detect corruption, even when the
- copying itself did not report any problems. If you find defect
- RAM, assume all backups and copied data to be suspect, unless you
- did a verify.
-
-
- * 4.4 How do I test RAM?
-
- First you should know that overclocking often makes memory
- problems worse. So if you overclock (which I strongly recommend
- against in a system holding data that has some worth), run the
- tests with the overclocking active.
-
- There are two good options. One is Memtest86+ and the other is
- "memtester" by Charles Cazabon. Memtest86+ requires a reboot and
- then takes over the machine, while memtester runs from a
- root-shell. Both use different testing methods and I have found
- problems fast with each one that the other needed long to find. I
- recommend running the following procedure until the first error is
- found:
+ layers. This typically results in lockups, CPU state dumps in the
+ system logs, kernel panic or other things. It is quite possible to have
+ a problem with an encrypted device, but not with an otherwise the same
+ unencrypted device. The reason for that is that encryption has an error
+ amplification property: If you flip one bit in an encrypted data block,
+ the decrypted version has half of its bits flipped. This is actually an
+ important security property for modern ciphers. With the usual modes in
+ cryptsetup (CBC, ESSIV, XTS), you can get a completely changed 512 byte
+ block for a bit error. A corrupt block causes a lot more havoc than the
+ occasionally flipped single bit and can result in various obscure
+ errors.
+
+ Note that a verify run on copying between encrypted or unencrypted
+ devices will reliably detect corruption, even when the copying itself
+ did not report any problems. If you find defect RAM, assume all backups
+ and copied data to be suspect, unless you did a verify.
+
+
+ * 4.4 How do I test RAM?
+
+ First you should know that overclocking often makes memory problems
+ worse. So if you overclock (which I strongly recommend against in a
+ system holding data that has any worth), run the tests with the
+ overclocking active.
+
+ There are two good options. One is Memtest86+ and the other is
+ "memtester" by Charles Cazabon. Memtest86+ requires a reboot and then
+ takes over the machine, while memtester runs from a root-shell. Both
+ use different testing methods and I have found problems fast with either
+ one that the other needed long to find. I recommend running the
+ following procedure until the first error is found:
- Run Memtest86+ for one cycle
- Run memtester for one cycle (shut down as many other applications
- as possible)
+ as possible and use the largest memory area you can get)
- Run Memtest86+ for 24h or more
- Run memtester for 24h or more
If all that does not produce error messages, your RAM may be sound,
- but I have had one weak bit that Memtest86+ needed around 60 hours
- to find. If you can reproduce the original problem reliably, a good
- additional test may be to remove half of the RAM (if you have more
- than one module) and try whether the problem is still there and if
- so, try with the other half. If you just have one module, get a
- different one and try with that. If you do overclocking, reduce
- the settings to the most conservative ones available and try with
- that.
+ but I have had one weak bit in the past that Memtest86+ needed around
+ 60 hours to find. If you can reproduce the original problem reliably,
+ a good additional test may be to remove half of the RAM (if you have
+ more than one module) and try whether the problem is still there and if
+ so, try with the other half. If you just have one module, get a
+ different one and try with that. If you do overclocking, reduce the
+ settings to the most conservative ones available and try with that.
-5. Security Aspects
+ * 4.5 Is there a risk using debugging tools like strace?
+ There most definitely is. A dump from strace and friends can contain
+ all data entered, including the full passphrase. Example with strace
+ and passphrase "test":
- * 5.1 How long is a secure passphrase ?
+ > strace cryptsetup luksOpen /dev/sda10 c1
+ ...
+ read(6, "test\n", 512) = 5
+ ...
- This is just the short answer. For more info and explanation of
- some of the terms used in this item, read the rest of Section 5.
- The actual recommendation is at the end of this item.
+ Depending on different factors and the tool used, the passphrase may
+ also be encoded and not plainly visible. Hence it is never a good idea
+ to give such a trace from a live container to anybody. Recreate the
+ problem with a test container or set a temporary passphrase like "test"
+ and use that for the trace generation. Item 2.6 explains how to create
+ a loop-file backed LUKS container that may come in handy for this
+ purpose.
- First, passphrase length is not really the right measure,
- passphrase entropy is. For example, a random lowercase letter (a-z)
- gives you 4.7 bit of entropy, one element of a-z0-9 gives you 5.2
- bits of entropy, an element of a-zA-Z0-9 gives you 5.9 bits and
- a-zA-Z0-9!@#$%^&:-+ gives you 6.2 bits. On the other hand, a random
- English word only gives you 0.6...1.3 bits of entropy per
- character. Using sentences that make sense gives lower entropy,
- series of random words gives higher entropy. Do not use sentences
- that can be tied to you or found on your computer. This type of
- attack is done routinely today.
+ See also Item 6.10 for another set of data you should not give to
+ others.
- That said, it does not matter too much what scheme you use, but it
- does matter how much entropy your passphrase contains, because an
- attacker has to try on average
- 1/2 * 2^(bits of entropy in passphrase)
-
+5. Security Aspects
+
+
+ * 5.1 How long is a secure passphrase ?
+
+ This is just the short answer. For more info and explanation of some of
+ the terms used in this item, read the rest of Section 5. The actual
+ recommendation is at the end of this item.
+
+ First, passphrase length is not really the right measure, passphrase
+ entropy is. If your passphrase is 200 times the letter "a", it is long
+ but has very low entropy and is pretty insecure.
+
+ For example, a random lowercase letter (a-z) gives you 4.7 bit of
+ entropy, one element of a-z0-9 gives you 5.2 bits of entropy, an element
+ of a-zA-Z0-9 gives you 5.9 bits and a-zA-Z0-9!@#$%\^&:-+ gives you 6.2
+ bits. On the other hand, a random English word only gives you 0.6...1.3
+ bits of entropy per character. Using sentences that make sense gives
+ lower entropy, series of random words gives higher entropy. Do not use
+ sentences that can be tied to you or found on your computer. This type
+ of attack is done routinely today.
+
+ That said, it does not matter too much what scheme you use, but it does
+ matter how much entropy your passphrase contains, because an attacker
+ has to try on average
+
+ 1/2 * 2^(bits of entropy in passphrase)
+
different passphrases to guess correctly.
- Historically, estimations tended to use computing time estimates,
- but more modern approaches try to estimate cost of guessing a
- passphrase.
+ Historically, estimations tended to use computing time estimates, but
+ more modern approaches try to estimate cost of guessing a passphrase.
As an example, I will try to get an estimate from the numbers in
- http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
- More references can be found a the end of this document. Note that
- these are estimates from the defender side, so assuming something
- is easier than it actually is is fine. An attacker may still have
- vastly higher cost than estimated here.
-
- LUKS uses SHA1 for hashing per default. The claim in the reference
- is 63 billion tries/second for SHA1. We will leave aside the check
- whether a try actually decrypts a key-slot. Now, the machine has 25
- GPUs, which I will estimate at an overall lifetime cost of USD/EUR
- 1000 each, and an useful lifetime of 2 years. (This is on the low
- side.) Disregarding downtime, the machine can then break
-
- N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
-
- passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed
- once with SHA1 for EUR/USD 25k. Note that as this can be
- parallelized, it can be done faster than 2 years with several of
- these machines.
-
- For plain dm-crypt (no hash iteration) this is it. This gives (with
- SHA1, plain dm-crypt default is ripemd160 which seems to be
- slightly slower than SHA1):
-
- Passphrase entropy Cost to break
- 60 bit EUR/USD 6k
+ https://gist.github.com/epixoip/a83d38f412b4737e99bbef804a270c40 This
+ thing costs 23kUSD and does 68Ghashes/sec for SHA1. This is in 2017.
+
+ Incidentally, my older calculation for a machine around 1000 times
+ slower was off by a factor of about 1000, but in the right direction,
+ i.e. I estimated the attack to be too easy. Nobody noticed ;-) On the
+ plus side, the tables are now (2017) pretty much accurate.
+
+ More references can be found a the end of this document. Note that
+ these are estimates from the defender side, so assuming something is
+ easier than it actually is is fine. An attacker may still have
+ significantly higher cost than estimated here.
+
+ LUKS1 used SHA1 (since version 1.7.0 it uses SHA256) for hashing per
+ default. We will leave aside the check whether a try actually decrypts
+ a key-slot. I will assume a useful lifetime of the hardware of 2 years.
+ (This is on the low side.) Disregarding downtime, the machine can then
+ break
+
+ N = 68*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
+
+ passphrases for EUR/USD 23k. That is one 62 bit passphrase hashed once
+ with SHA1 for EUR/USD 23k. This can be parallelized, it can be done
+ faster than 2 years with several of these machines.
+
+ For LUKS2, things look a bit better, as the advantage of using graphics
+ cards is massively reduced. Using the recommendations below should
+ hence be fine for LUKS2 as well and give a better security margin.
+
+ For plain dm-crypt (no hash iteration) this is it. This gives (with
+ SHA1, plain dm-crypt default is ripemd160 which seems to be slightly
+ slower than SHA1):
+
+ Passphrase entropy Cost to break
+ 60 bit EUR/USD 6k
65 bit EUR/USD 200K
70 bit EUR/USD 6M
75 bit EUR/USD 200M
80 bit EUR/USD 6B
85 bit EUR/USD 200B
- ... ...
-
- For LUKS, you have to take into account hash iteration in PBKDF2.
- For a current CPU, there are about 100k iterations (as can be
- queried with ''cryptsetup luksDump''.
+ ... ...
+
+
+ For LUKS1, you have to take into account hash iteration in PBKDF2.
+ For a current CPU, there are about 100k iterations (as can be queried
+ with ''cryptsetup luksDump''.
The table above then becomes:
- Passphrase entropy Cost to break
- 50 bit EUR/USD 600k
+ Passphrase entropy Cost to break
+ 50 bit EUR/USD 600k
55 bit EUR/USD 20M
- 60 bit EUR/USD 600M
+ 60 bit EUR/USD 600M
65 bit EUR/USD 20B
70 bit EUR/USD 600B
75 bit EUR/USD 20T
- ... ...
-
+ ... ...
+
+
Recommendation:
- To get reasonable security for the next 10 years, it is a good idea
+ To get reasonable security for the next 10 years, it is a good idea
to overestimate by a factor of at least 1000.
- Then there is the question of how much the attacker is willing to
- spend. That is up to your own security evaluation. For general use,
- I will assume the attacker is willing to spend up to 1 million
- EUR/USD. Then we get the following recommendations:
+ Then there is the question of how much the attacker is willing to spend.
+ That is up to your own security evaluation. For general use, I will
+ assume the attacker is willing to spend up to 1 million EUR/USD. Then
+ we get the following recommendations:
Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
or a random English sentence of > 135 characters length.
- LUKS: Use > 65 bit. That is e.g. 14 random chars from a-z or a
- random English sentence of > 108 characters length.
+ LUKS1 and LUKS2: Use > 65 bit. That is e.g. 14 random chars from a-z
+ or a random English sentence of > 108 characters length.
If paranoid, add at least 20 bit. That is roughly four additional
characters for random passphrases and roughly 32 characters for a
- random English sentence.
-
-
- * 5.2 Is LUKS insecure? Everybody can see I have encrypted data!
-
- In practice it does not really matter. In most civilized countries
- you can just refuse to hand over the keys, no harm done. In some
- countries they can force you to hand over the keys, if they suspect
- encryption. However the suspicion is enough, they do not have to
- prove anything. This is for practical reasons, as even the presence
- of a header (like the LUKS header) is not enough to prove that you
- have any keys. It might have been an experiment, for example. Or it
- was used as encrypted swap with a key from /dev/random. So they
- make you prove you do not have encrypted data. Of course that is
- just as impossible as the other way round.
-
- This means that if you have a large set of random-looking data,
- they can already lock you up. Hidden containers (encryption hidden
- within encryption), as possible with Truecrypt, do not help
- either. They will just assume the hidden container is there and
- unless you hand over the key, you will stay locked up. Don't have
- a hidden container? Though luck. Anybody could claim that.
-
- Still, if you are concerned about the LUKS header, use plain
- dm-crypt with a good passphrase. See also Section 2, "What is the
- difference between "plain" and LUKS format?"
-
-
- * 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
-
- If you just create a filesystem on it, most of the old data will
- still be there. If the old data is sensitive, you should overwrite
- it before encrypting. In any case, not initializing will leave the
- old data there until the specific sector gets written. That may
- enable an attacker to determine how much and where on the
- partition data was written. If you think this is a risk, you can
- prevent this by overwriting the encrypted device (here assumed to
- be named "e1") with zeros like this:
-
- dd_rescue -w /dev/zero /dev/mapper/e1
-
+ random English sentence.
+
+
+ * 5.2 Is LUKS insecure? Everybody can see I have encrypted data!
+
+ In practice it does not really matter. In most civilized countries you
+ can just refuse to hand over the keys, no harm done. In some countries
+ they can force you to hand over the keys if they suspect encryption.
+ The suspicion is enough, they do not have to prove anything. This is
+ for practical reasons, as even the presence of a header (like the LUKS
+ header) is not enough to prove that you have any keys. It might have
+ been an experiment, for example. Or it was used as encrypted swap with
+ a key from /dev/random. So they make you prove you do not have
+ encrypted data. Of course, if true, that is impossible and hence the
+ whole idea is not compatible with fair laws. Note that in this context,
+ countries like the US or the UK are not civilized and do not have fair
+ laws.
+
+ This means that if you have a large set of random-looking data, they can
+ already lock you up. Hidden containers (encryption hidden within
+ encryption), as possible with Truecrypt, do not help either. They will
+ just assume the hidden container is there and unless you hand over the
+ key, you will stay locked up. Don't have a hidden container? Though
+ luck. Anybody could claim that.
+
+ Still, if you are concerned about the LUKS header, use plain dm-crypt
+ with a good passphrase. See also Section 2, "What is the difference
+ between "plain" and LUKS format?"
+
+
+ * 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
+
+ If you just create a filesystem on it, most of the old data will still
+ be there. If the old data is sensitive, you should overwrite it before
+ encrypting. In any case, not initializing will leave the old data there
+ until the specific sector gets written. That may enable an attacker to
+ determine how much and where on the partition data was written. If you
+ think this is a risk, you can prevent this by overwriting the encrypted
+ device (here assumed to be named "e1") with zeros like this:
+
+ dd_rescue -w /dev/zero /dev/mapper/e1
+
or alternatively with one of the following more standard commands:
- cat /dev/zero > /dev/mapper/e1
- dd if=/dev/zero of=/dev/mapper/e1
-
-
- * 5.4 How do I securely erase a LUKS (or other) partition?
-
- For LUKS, if you are in a desperate hurry, overwrite the LUKS
- header and key-slot area. This means overwriting the first
- (keyslots x stripes x keysize) + offset bytes. For the default
- parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the
- LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
- 512 bit key) this is 2MiB. (The different offset stems from
- differences in the sector alignment of the key-slots.) If in doubt,
- just be generous and overwrite the first 10MB or so, it will likely
- still be fast enough. A single overwrite with zeros should be
- enough. If you anticipate being in a desperate hurry, prepare the
- command beforehand. Example with /dev/sde1 as the LUKS partition
- and default parameters:
-
- head -c 1052672 /dev/zero > /dev/sde1; sync
-
- A LUKS header backup or full backup will still grant access to
- most or all data, so make sure that an attacker does not have
- access to backups or destroy them as well.
-
- If you have time, overwrite the whole LUKS partition with a single
- pass of zeros. This is enough for current HDDs. For SSDs or FLASH
- (USB sticks) you may want to overwrite the whole drive several
- times to be sure data is not retained by wear leveling. This is
- possibly still insecure as SSD technology is not fully understood
- in this regard. Still, due to the anti-forensic properties of the
- LUKS key-slots, a single overwrite of an SSD or FLASH drive could
- be enough. If in doubt, use physical destruction in addition. Here
- is a link to some current research results on erasing SSDs and
- FLASH drives:
+ cat /dev/zero > /dev/mapper/e1
+ dd if=/dev/zero of=/dev/mapper/e1
+
+
+
+ * 5.4 How do I securely erase a LUKS container?
+
+ For LUKS, if you are in a desperate hurry, overwrite the LUKS header and
+ key-slot area. For LUKS1 and LUKS2, just be generous and overwrite the
+ first 100MB. A single overwrite with zeros should be enough. If you
+ anticipate being in a desperate hurry, prepare the command beforehand.
+ Example with /dev/sde1 as the LUKS partition and default parameters:
+
+ head -c 100000000 /dev/zero > /dev/sde1; sync
+
+ A LUKS header backup or full backup will still grant access to most or
+ all data, so make sure that an attacker does not have access to backups
+ or destroy them as well.
+
+ Also note that SSDs and also some HDDs (SMR and hybrid HDDs, for
+ example) may not actually overwrite the header and only do that an
+ unspecified and possibly very long time later. The only way to be sure
+ there is physical destruction. If the situation permits, do both
+ overwrite and physical destruction.
+
+ If you have time, overwrite the whole drive with a single pass of random
+ data. This is enough for most HDDs. For SSDs or FLASH (USB sticks) or
+ SMR or hybrid drives, you may want to overwrite the whole drive several
+ times to be sure data is not retained. This is possibly still insecure
+ as the respective technologies are not fully understood in this regard.
+ Still, due to the anti-forensic properties of the LUKS key-slots, a
+ single overwrite could be enough. If in doubt, use physical destruction
+ in addition. Here is a link to some current research results on erasing
+ SSDs and FLASH drives:
http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
Keep in mind to also erase all backups.
- Example for a zero-overwrite erase of partition sde1 done with
+ Example for a random-overwrite erase of partition sde1 done with
dd_rescue:
- dd_rescue -w /dev/zero /dev/sde1
-
+ dd_rescue -w /dev/urandom /dev/sde1
- * 5.5 How do I securely erase a backup of a LUKS partition or header?
- That depends on the medium it is stored on. For HDD and SSD, use
- overwrite with zeros. For an SSD or FLASH drive (USB stick), you
- may want to overwrite the complete SSD several times and use
- physical destruction in addition, see last item. For re-writable
- CD/DVD, a single overwrite should also be enough, due to the
- anti-forensic properties of the LUKS keyslots. For write-once
- media, use physical destruction. For low security requirements,
- just cut the CD/DVD into several parts. For high security needs,
- shred or burn the medium. If your backup is on magnetic tape, I
- advise physical destruction by shredding or burning, after
- overwriting . The problem with magnetic tape is that it has a
- higher dynamic range than HDDs and older data may well be
- recoverable after overwrites. Also write-head alignment issues can
- lead to data not actually being deleted at all during overwrites.
+ * 5.5 How do I securely erase a backup of a LUKS partition or header?
- * 5.6 What about backup? Does it compromise security?
+ That depends on the medium it is stored on. For HDD and SSD, use
+ overwrite with random data. For an SSD, FLASH drive (USB stick) hybrid
+ HDD or SMR HDD, you may want to overwrite the complete drive several
+ times and use physical destruction in addition, see last item. For
+ re-writable CD/DVD, a single overwrite should be enough, due to the
+ anti-forensic properties of the LUKS keyslots. For write-once media,
+ use physical destruction. For low security requirements, just cut the
+ CD/DVD into several parts. For high security needs, shred or burn the
+ medium.
+
+ If your backup is on magnetic tape, I advise physical destruction by
+ shredding or burning, after (!) overwriting . The problem with magnetic
+ tape is that it has a higher dynamic range than HDDs and older data may
+ well be recoverable after overwrites. Also write-head alignment issues
+ can lead to data not actually being deleted during overwrites.
+
+ The best option is to actually encrypt the backup, for example with
+ PGP/GnuPG and then just destroy all copies of the encryption key if
+ needed. Best keep them on paper, as that has excellent durability and
+ secure destruction is easy, for example by burning and then crushing the
+ ashes to a fine powder. A blender and water also works nicely.
+
+
+ * 5.6 What about backup? Does it compromise security?
That depends. See item 6.7.
- * 5.7 Why is all my data permanently gone if I overwrite the LUKS
- header?
-
- Overwriting the LUKS header in part or in full is the most common
- reason why access to LUKS containers is lost permanently.
- Overwriting can be done in a number of fashions, like creating a
- new filesystem on the raw LUKS partition, making the raw partition
- part of a raid array and just writing to the raw partition.
-
- The LUKS header contains a 256 bit "salt" value and without that no
- decryption is possible. While the salt is not secret, it is
- key-grade material and cannot be reconstructed. This is a
- cryptographically strong "cannot". From observations on the
- cryptsetup mailing-list, people typically go though the usual
- stages of grief (Denial, Anger, Bargaining, Depression, Acceptance)
- when this happens to them. Observed times vary between 1 day and 2
- weeks to complete the cycle. Seeking help on the mailing-list is
- fine. Even if we usually cannot help with getting back your data,
- most people found the feedback comforting.
-
- If your header does not contain an intact salt, best go directly
- to the last stage ("Acceptance") and think about what to do now.
- There is one exception that I know of: If your LUKS container is
- still open, then it may be possible to extract the master key from
- the running system. See Item "How do I recover the master key from
- a mapped LUKS container?" in Section "Backup and Data Recovery".
-
-
- * 5.8 What is a "salt"?
-
- A salt is a random key-grade value added to the passphrase before
- it is processed. It is not kept secret. The reason for using salts
- is as follows: If an attacker wants to crack the password for a
- single LUKS container, then every possible passphrase has to be
- tried. Typically an attacker will not try every binary value, but
- will try words and sentences from a dictionary.
-
- If an attacker wants to attack several LUKS containers with the
- same dictionary, then a different approach makes sense: Compute the
- resulting slot-key for each dictionary element and store it on
- disk. Then the test for each entry is just the slow unlocking with
- the slot key (say 0.00001 sec) instead of calculating the slot-key
- first (1 sec). For a single attack, this does not help. But if you
- have more than one container to attack, this helps tremendously,
- also because you can prepare your table before you even have the
- container to attack! The calculation is also very simple to
- parallelize. You could, for example, use the night-time unused CPU
- power of your desktop PCs for this.
-
- This is where the salt comes in. If the salt is combined with the
- passphrase (in the simplest form, just appended to it), you
- suddenly need a separate table for each salt value. With a
- reasonably-sized salt value (256 bit, e.g.) this is quite
- infeasible.
-
-
- * 5.9 Is LUKS secure with a low-entropy (bad) passphrase?
-
- Note: You should only use the 94 printable characters from 7 bit
- ASCII code to prevent your passphrase from failing when the
- character encoding changes, e.g. because of a system upgrade, see
- also the note at the very start of this FAQ under "WARNINGS".
-
- This needs a bit of theory. The quality of your passphrase is
- directly related to its entropy (information theoretic, not
- thermodynamic). The entropy says how many bits of "uncertainty" or
- "randomness" are in you passphrase. In other words, that is how
- difficult guessing the passphrase is.
+ * 5.7 Why is all my data permanently gone if I overwrite the LUKS header?
+
+ Overwriting the LUKS header in part or in full is the most common reason
+ why access to LUKS containers is lost permanently. Overwriting can be
+ done in a number of fashions, like creating a new filesystem on the raw
+ LUKS partition, making the raw partition part of a raid array and just
+ writing to the raw partition.
+
+ The LUKS1 header contains a 256 bit "salt" per key-slot and without that
+ no decryption is possible. While the salts are not secret, they are
+ key-grade material and cannot be reconstructed. This is a
+ cryptographically strong "cannot". From observations on the cryptsetup
+ mailing-list, people typically go though the usual stages of grief
+ (Denial, Anger, Bargaining, Depression, Acceptance) when this happens to
+ them. Observed times vary between 1 day and 2 weeks to complete the
+ cycle. Seeking help on the mailing-list is fine. Even if we usually
+ cannot help with getting back your data, most people found the feedback
+ comforting.
+
+ If your header does not contain an intact key-slot salt, best go
+ directly to the last stage ("Acceptance") and think about what to do
+ now. There is one exception that I know of: If your LUKS1 container is
+ still open, then it may be possible to extract the master key from the
+ running system. See Item "How do I recover the master key from a mapped
+ LUKS1 container?" in Section "Backup and Data Recovery".
+
+ For LUKS2, things are both better and worse. First, the salts are in a
+ less vulnerable position now. But, on the other hand, the keys of a
+ mapped (open) container are now stored in the kernel key-store, and
+ while there probably is some way to get them out of there, I am not sure
+ how much effort that needs.
+
+
+ * 5.8 What is a "salt"?
+
+ A salt is a random key-grade value added to the passphrase before it is
+ processed. It is not kept secret. The reason for using salts is as
+ follows: If an attacker wants to crack the password for a single LUKS
+ container, then every possible passphrase has to be tried. Typically an
+ attacker will not try every binary value, but will try words and
+ sentences from a dictionary.
+
+ If an attacker wants to attack several LUKS containers with the same
+ dictionary, then a different approach makes sense: Compute the resulting
+ slot-key for each dictionary element and store it on disk. Then the
+ test for each entry is just the slow unlocking with the slot key (say
+ 0.00001 sec) instead of calculating the slot-key first (1 sec). For a
+ single attack, this does not help. But if you have more than one
+ container to attack, this helps tremendously, also because you can
+ prepare your table before you even have the container to attack! The
+ calculation is also very simple to parallelize. You could, for example,
+ use the night-time unused CPU power of your desktop PCs for this.
+
+ This is where the salt comes in. If the salt is combined with the
+ passphrase (in the simplest form, just appended to it), you suddenly
+ need a separate table for each salt value. With a reasonably-sized salt
+ value (256 bit, e.g.) this is quite infeasible.
+
+
+ * 5.9 Is LUKS secure with a low-entropy (bad) passphrase?
+
+ Short answer: yes. Do not use a low-entropy passphrase.
+
+ Note: For LUKS2, protection for bad passphrases is a bit better
+ due to the use of Argon2, but that is only a gradual improvement.
+
+ Longer answer:
+ This needs a bit of theory. The quality of your passphrase is directly
+ related to its entropy (information theoretic, not thermodynamic). The
+ entropy says how many bits of "uncertainty" or "randomness" are in you
+ passphrase. In other words, that is how difficult guessing the
+ passphrase is.
Example: A random English sentence has about 1 bit of entropy per
- character. A random lowercase (or uppercase) character has about
- 4.7 bit of entropy.
+ character. A random lowercase (or uppercase) character has about 4.7
+ bit of entropy.
Now, if n is the number of bits of entropy in your passphrase and t
is the time it takes to process a passphrase in order to open the
LUKS container, then an attacker has to spend at maximum
- attack_time_max = 2^n * t
-
- time for a successful attack and on average half that. There is no
- way getting around that relationship. However, there is one thing
- that does help, namely increasing t, the time it takes to use a
- passphrase, see next FAQ item.
-
- Still, if you want good security, a high-entropy passphrase is the
- only option. For example, a low-entropy passphrase can never be
- considered secure against a TLA-level (Three Letter Agency level,
- i.e. government-level) attacker, no matter what tricks are used in
- the key-derivation function. Use at least 64 bits for secret stuff.
- That is 64 characters of English text (but only if randomly chosen)
- or a combination of 12 truly random letters and digits.
-
- For passphrase generation, do not use lines from very well-known
- texts (religious texts, Harry potter, etc.) as they are to easy to
- guess. For example, the total Harry Potter has about 1'500'000
- words (my estimation). Trying every 64 character sequence starting
- and ending at a word boundary would take only something like 20
- days on a single CPU and is entirely feasible. To put that into
- perspective, using a number of Amazon EC2 High-CPU Extra Large
- instances (each gives about 8 real cores), this test costs
- currently about 50USD/EUR, but can be made to run arbitrarily fast.
-
- On the other hand, choosing 1.5 lines from, say, the Wheel of Time
- is in itself not more secure, but the book selection adds quite a
- bit of entropy. (Now that I have mentioned it here, don't use tWoT
- either!) If you add 2 or 3 typos or switch some words around, then
- this is good passphrase material.
-
-
- * 5.10 What is "iteration count" and why is decreasing it a bad idea?
-
- Iteration count is the number of PBKDF2 iterations a passphrase is
- put through before it is used to unlock a key-slot. Iterations are
- done with the explicit purpose to increase the time that it takes
- to unlock a key-slot. This provides some protection against use of
- low-entropy passphrases.
-
- The idea is that an attacker has to try all possible passphrases.
- Even if the attacker knows the passphrase is low-entropy (see last
- item), it is possible to make each individual try take longer. The
- way to do this is to repeatedly hash the passphrase for a certain
- time. The attacker then has to spend the same time (given the same
- computing power) as the user per try. With LUKS, the default is 1
- second of PBKDF2 hashing.
-
- Example 1: Lets assume we have a really bad passphrase (e.g. a
- girlfriends name) with 10 bits of entropy. With the same CPU, an
- attacker would need to spend around 500 seconds on average to
- break that passphrase. Without iteration, it would be more like
- 0.0001 seconds on a modern CPU.
-
- Example 2: The user did a bit better and has 32 chars of English
- text. That would be about 32 bits of entropy. With 1 second
- iteration, that means an attacker on the same CPU needs around 136
- years. That is pretty impressive for such a weak passphrase.
- Without the iterations, it would be more like 50 days on a modern
- CPU, and possibly far less.
-
- In addition, the attacker can both parallelize and use special
- hardware like GPUs or FPGAs to speed up the attack. The attack can
- also happen quite some time after the luksFormat operation and CPUs
- can have become faster and cheaper. For that reason you want a
- bit of extra security. Anyways, in Example 1 your are screwed.
- In example 2, not necessarily. Even if the attack is faster, it
- still has a certain cost associated with it, say 10000 EUR/USD
- with iteration and 1 EUR/USD without iteration. The first can be
- prohibitively expensive, while the second is something you try
- even without solid proof that the decryption will yield something
- useful.
-
- The numbers above are mostly made up, but show the idea. Of course
- the best thing is to have a high-entropy passphrase.
-
- Would a 100 sec iteration time be even better? Yes and no.
- Cryptographically it would be a lot better, namely 100 times better.
- However, usability is a very important factor for security
- technology and one that gets overlooked surprisingly often. For
- LUKS, if you have to wait 2 minutes to unlock the LUKS container,
- most people will not bother and use less secure storage instead. It
- is better to have less protection against low-entropy passphrases
- and people actually use LUKS, than having them do without
- encryption altogether.
-
- Now, what about decreasing the iteration time? This is generally a
- very bad idea, unless you know and can enforce that the users only
- use high-entropy passphrases. If you decrease the iteration time
- without ensuring that, then you put your users at increased risk,
- and considering how rarely LUKS containers are unlocked in a
- typical work-flow, you do so without a good reason. Don't do it.
- The iteration time is already low enough that users with entropy
- low passphrases are vulnerable. Lowering it even further increases
- this danger significantly.
-
-
- * 5.11 Some people say PBKDF2 is insecure?
+ attack_time_max = 2^n * t
+
+ time for a successful attack and on average half that. There is no way
+ getting around that relationship. However, there is one thing that does
+ help, namely increasing t, the time it takes to use a passphrase, see
+ next FAQ item.
+
+ Still, if you want good security, a high-entropy passphrase is the only
+ option. For example, a low-entropy passphrase can never be considered
+ secure against a TLA-level (Three Letter Agency level, i.e.
+ government-level) attacker, no matter what tricks are used in the
+ key-derivation function. Use at least 64 bits for secret stuff. That
+ is 64 characters of English text (but only if randomly chosen) or a
+ combination of 12 truly random letters and digits.
+
+ For passphrase generation, do not use lines from very well-known texts
+ (religious texts, Harry potter, etc.) as they are too easy to guess.
+ For example, the total Harry Potter has about 1'500'000 words (my
+ estimation). Trying every 64 character sequence starting and ending at
+ a word boundary would take only something like 20 days on a single CPU
+ and is entirely feasible. To put that into perspective, using a number
+ of Amazon EC2 High-CPU Extra Large instances (each gives about 8 real
+ cores), this test costs currently about 50USD/EUR, but can be made to
+ run arbitrarily fast.
+
+ On the other hand, choosing 1.5 lines from, say, the Wheel of Time, is
+ in itself not more secure, but the book selection adds quite a bit of
+ entropy. (Now that I have mentioned it here, don't use tWoT either!) If
+ you add 2 or 3 typos and switch some words around, then this is good
+ passphrase material.
+
+
+ * 5.10 What is "iteration count" and why is decreasing it a bad idea?
+
+ LUKS1:
+ Iteration count is the number of PBKDF2 iterations a passphrase is put
+ through before it is used to unlock a key-slot. Iterations are done
+ with the explicit purpose to increase the time that it takes to unlock a
+ key-slot. This provides some protection against use of low-entropy
+ passphrases.
+
+ The idea is that an attacker has to try all possible passphrases. Even
+ if the attacker knows the passphrase is low-entropy (see last item), it
+ is possible to make each individual try take longer. The way to do this
+ is to repeatedly hash the passphrase for a certain time. The attacker
+ then has to spend the same time (given the same computing power) as the
+ user per try. With LUKS1, the default is 1 second of PBKDF2 hashing.
+
+ Example 1: Lets assume we have a really bad passphrase (e.g. a
+ girlfriends name) with 10 bits of entropy. With the same CPU, an
+ attacker would need to spend around 500 seconds on average to break that
+ passphrase. Without iteration, it would be more like 0.0001 seconds on
+ a modern CPU.
+
+ Example 2: The user did a bit better and has 32 chars of English text.
+ That would be about 32 bits of entropy. With 1 second iteration, that
+ means an attacker on the same CPU needs around 136 years. That is
+ pretty impressive for such a weak passphrase. Without the iterations,
+ it would be more like 50 days on a modern CPU, and possibly far less.
+
+ In addition, the attacker can both parallelize and use special hardware
+ like GPUs or FPGAs to speed up the attack. The attack can also happen
+ quite some time after the luksFormat operation and CPUs can have become
+ faster and cheaper. For that reason you want a bit of extra security.
+ Anyways, in Example 1 your are screwed. In example 2, not necessarily.
+ Even if the attack is faster, it still has a certain cost associated
+ with it, say 10000 EUR/USD with iteration and 1 EUR/USD without
+ iteration. The first can be prohibitively expensive, while the second
+ is something you try even without solid proof that the decryption will
+ yield something useful.
+
+ The numbers above are mostly made up, but show the idea. Of course the
+ best thing is to have a high-entropy passphrase.
+
+ Would a 100 sec iteration time be even better? Yes and no.
+ Cryptographically it would be a lot better, namely 100 times better.
+ However, usability is a very important factor for security technology
+ and one that gets overlooked surprisingly often. For LUKS, if you have
+ to wait 2 minutes to unlock the LUKS container, most people will not
+ bother and use less secure storage instead. It is better to have less
+ protection against low-entropy passphrases and people actually use LUKS,
+ than having them do without encryption altogether.
+
+ Now, what about decreasing the iteration time? This is generally a very
+ bad idea, unless you know and can enforce that the users only use
+ high-entropy passphrases. If you decrease the iteration time without
+ ensuring that, then you put your users at increased risk, and
+ considering how rarely LUKS containers are unlocked in a typical
+ work-flow, you do so without a good reason. Don't do it. The iteration
+ time is already low enough that users with low entropy passphrases are
+ vulnerable. Lowering it even further increases this danger
+ significantly.
+
+ LUKS2: Pretty much the same reasoning applies. The advantages of using
+ GPUs or FPGAs in an attack have been significantly reduced, but that
+ is the only main difference.
+
+
+ * 5.11 Some people say PBKDF2 is insecure?
There is some discussion that a hash-function should have a "large
- memory" property, i.e. that it should require a lot of memory to be
- computed. This serves to prevent attacks using special programmable
- circuits, like FPGAs, and attacks using graphics cards. PBKDF2
- does not need a lot of memory and is vulnerable to these attacks.
- However, the publication usually referred in these discussions is
- not very convincing in proving that the presented hash really is
- "large memory" (that may change, email the FAQ maintainer when it
- does) and it is of limited usefulness anyways. Attackers that use
- clusters of normal PCs will not be affected at all by a "large
- memory" property. For example the US Secret Service is known to
- use the off-hour time of all the office PCs of the Treasury for
- password breaking. The Treasury has about 110'000 employees.
- Assuming every one has an office PC, that is significant computing
- power, all of it with plenty of memory for computing "large
- memory" hashes. Bot-net operators also have all the memory they
- want. The only protection against a resourceful attacker is a
- high-entropy passphrase, see items 5.9 and 5.10.
-
-
- * 5.12 What about iteration count with plain dm-crypt?
-
- Simple: There is none. There is also no salting. If you use plain
- dm-crypt, the only way to be secure is to use a high entropy
- passphrase. If in doubt, use LUKS instead.
-
-
- * 5.13 Is LUKS with default parameters less secure on a slow CPU?
-
- Unfortunately, yes. However the only aspect affected is the
- protection for low-entropy passphrase or master-key. All other
- security aspects are independent of CPU speed.
-
- The master key is less critical, as you really have to work at it
- to give it low entropy. One possibility is to supply the master key
- yourself. If that key is low-entropy, then you get what you
- deserve. The other known possibility is to use /dev/urandom for
- key generation in an entropy-starved situation (e.g. automatic
- installation on an embedded device without network and other entropy
- sources).
-
- For the passphrase, don't use a low-entropy passphrase. If your
- passphrase is good, then a slow CPU will not matter. If you insist
- on a low-entropy passphrase on a slow CPU, use something like
- "--iter-time=10" or higher and wait a long time on each LUKS unlock
- and pray that the attacker does not find out in which way exactly
- your passphrase is low entropy. This also applies to low-entropy
- passphrases on fast CPUs. Technology can do only so much to
- compensate for problems in front of the keyboard.
-
-
- * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
+ memory" property, i.e. that it should require a lot of memory to be
+ computed. This serves to prevent attacks using special programmable
+ circuits, like FPGAs, and attacks using graphics cards. PBKDF2 does not
+ need a lot of memory and is vulnerable to these attacks. However, the
+ publication usually referred in these discussions is not very convincing
+ in proving that the presented hash really is "large memory" (that may
+ change, email the FAQ maintainer when it does) and it is of limited
+ usefulness anyways. Attackers that use clusters of normal PCs will not
+ be affected at all by a "large memory" property. For example the US
+ Secret Service is known to use the off-hour time of all the office PCs
+ of the Treasury for password breaking. The Treasury has about 110'000
+ employees. Assuming every one has an office PC, that is significant
+ computing power, all of it with plenty of memory for computing "large
+ memory" hashes. Bot-net operators also have all the memory they want.
+ The only protection against a resourceful attacker is a high-entropy
+ passphrase, see items 5.9 and 5.10.
+
+ That said, LUKS2 defaults to Argon2, which has a large-memory property
+ and massively reduces the advantages of GPUs and FPGAs.
+
+
+ * 5.12 What about iteration count with plain dm-crypt?
+
+ Simple: There is none. There is also no salting. If you use plain
+ dm-crypt, the only way to be secure is to use a high entropy passphrase.
+ If in doubt, use LUKS instead.
+
+
+ * 5.13 Is LUKS with default parameters less secure on a slow CPU?
+
+ Unfortunately, yes. However the only aspect affected is the protection
+ for low-entropy passphrase or master-key. All other security aspects
+ are independent of CPU speed.
+
+ The master key is less critical, as you really have to work at it to
+ give it low entropy. One possibility to mess this up is to supply the
+ master key yourself. If that key is low-entropy, then you get what you
+ deserve. The other known possibility to create a LUKS container with a
+ bad master key is to use /dev/urandom for key generation in an
+ entropy-starved situation (e.g. automatic installation on an embedded
+ device without network and other entropy sources or installation in a VM
+ under certain circumstances).
+
+ For the passphrase, don't use a low-entropy passphrase. If your
+ passphrase is good, then a slow CPU will not matter. If you insist on a
+ low-entropy passphrase on a slow CPU, use something like
+ "--iter-time=10000" or higher and wait a long time on each LUKS unlock
+ and pray that the attacker does not find out in which way exactly your
+ passphrase is low entropy. This also applies to low-entropy passphrases
+ on fast CPUs. Technology can do only so much to compensate for problems
+ in front of the keyboard.
+
+ Also note that power-saving modes will make your CPU slower. This will
+ reduce iteration count on LUKS container creation. It will keep unlock
+ times at the expected values though at this CPU speed.
+
+
+ * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
Note: This item applies both to plain dm-crypt and to LUKS
- The problem is that cbc-plain has a fingerprint vulnerability, where
- a specially crafted file placed into the crypto-container can be
- recognized from the outside. The issue here is that for cbc-plain
- the initialization vector (IV) is the sector number. The IV gets
- XORed to the first data chunk of the sector to be encrypted. If you
- make sure that the first data block to be stored in a sector
- contains the sector number as well, the first data block to be
- encrypted is all zeros and always encrypted to the same ciphertext.
- This also works if the first data chunk just has a constant XOR
- with the sector number. By having several shifted patterns you can
- take care of the case of a non-power-of-two start sector number of
- the file.
-
- This mechanism allows you to create a pattern of sectors that have
- the same first ciphertext block and signal one bit per sector to the
- outside, allowing you to e.g. mark media files that way for
- recognition without decryption. For large files this is a
- practical attack. For small ones, you do not have enough blocks to
- signal and take care of different file starting offsets.
-
- In order to prevent this attack, the default was changed to
- cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
- encryption key as key. This makes the IV unpredictable without
- knowing the encryption key and the watermarking attack fails.
-
-
- * 5.15 Are there any problems with "plain" IV? What is "plain64"?
-
- First, "plain" and "plain64" are both not secure to use with CBC,
- see previous FAQ item.
-
- However there are modes, like XTS, that are secure with "plain" IV.
- The next limit is that "plain" is 64 bit, with the upper 32 bit set
- to zero. This means that on volumes larger than 2TiB, the IV
- repeats, creating a vulnerability that potentially leaks some
- data. To avoid this, use "plain64", which uses the full sector
- number up to 64 bit. Note that "plain64" requires a kernel >=
- 2.6.33. Also note that "plain64" is backwards compatible for
- volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
- does not cause any performance penalty compared to "plain".
-
-
- * 5.16 What about XTS mode?
+ The problem is that cbc-plain has a fingerprint vulnerability, where a
+ specially crafted file placed into the crypto-container can be
+ recognized from the outside. The issue here is that for cbc-plain the
+ initialization vector (IV) is the sector number. The IV gets XORed to
+ the first data chunk of the sector to be encrypted. If you make sure
+ that the first data block to be stored in a sector contains the sector
+ number as well, the first data block to be encrypted is all zeros and
+ always encrypted to the same ciphertext. This also works if the first
+ data chunk just has a constant XOR with the sector number. By having
+ several shifted patterns you can take care of the case of a
+ non-power-of-two start sector number of the file.
+
+ This mechanism allows you to create a pattern of sectors that have the
+ same first ciphertext block and signal one bit per sector to the
+ outside, allowing you to e.g. mark media files that way for recognition
+ without decryption. For large files this is a practical attack. For
+ small ones, you do not have enough blocks to signal and take care of
+ different file starting offsets.
+
+ In order to prevent this attack, the default was changed to cbc-essiv.
+ ESSIV uses a keyed hash of the sector number, with the encryption key as
+ key. This makes the IV unpredictable without knowing the encryption key
+ and the watermarking attack fails.
+
+
+ * 5.15 Are there any problems with "plain" IV? What is "plain64"?
+
+ First, "plain" and "plain64" are both not secure to use with CBC, see
+ previous FAQ item.
+
+ However there are modes, like XTS, that are secure with "plain" IV. The
+ next limit is that "plain" is 64 bit, with the upper 32 bit set to zero.
+ This means that on volumes larger than 2TiB, the IV repeats, creating a
+ vulnerability that potentially leaks some data. To avoid this, use
+ "plain64", which uses the full sector number up to 64 bit. Note that
+ "plain64" requires a kernel 2.6.33 or more recent. Also note that
+ "plain64" is backwards compatible for volume sizes of maximum size 2TiB,
+ but not for those > 2TiB. Finally, "plain64" does not cause any
+ performance penalty compared to "plain".
+
+
+ * 5.16 What about XTS mode?
XTS mode is potentially even more secure than cbc-essiv (but only if
- cbc-essiv is insecure in your scenario). It is a NIST standard and
- used, e.g. in Truecrypt. At the moment, if you want to use it, you
- have to specify it manually as "aes-xts-plain", i.e.
+ cbc-essiv is insecure in your scenario). It is a NIST standard and
+ used, e.g. in Truecrypt. From version 1.6.0 of cryptsetup onwards,
+ aes-xts-plain64 is the default for LUKS. If you want to use it with a
+ cryptsetup before version 1.6.0 or with plain dm-crypt, you have to
+ specify it manually as "aes-xts-plain", i.e.
- cryptsetup -c aes-xts-plain luksFormat <device>
-
- For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ
- item on "plain" and "plain64"):
+ cryptsetup -c aes-xts-plain luksFormat <device>
- cryptsetup -c aes-xts-plain64 luksFormat <device>
-
- There is a potential security issue with XTS mode and large blocks.
- LUKS and dm-crypt always use 512B blocks and the issue does not
- apply.
+ For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ item
+ on "plain" and "plain64"):
+
+ cryptsetup -c aes-xts-plain64 luksFormat <device>
+ There is a potential security issue with XTS mode and large blocks.
+ LUKS and dm-crypt always use 512B blocks and the issue does not apply.
- * 5.17 Is LUKS FIPS-140-2 certified?
- No. But that is more a problem of FIPS-140-2 than of LUKS. From a
+ * 5.17 Is LUKS FIPS-140-2 certified?
+
+ No. But that is more a problem of FIPS-140-2 than of LUKS. From a
technical point-of-view, LUKS with the right parameters would be
- FIPS-140-2 compliant, but in order to make it certified, somebody
- has to pay real money for that. And then, whenever cryptsetup is
- changed or extended, the certification lapses and has to be
- obtained again.
-
- From the aspect of actual security, LUKS with default parameters
- should be as good as most things that are FIPS-140-2 certified,
- although you may want to make sure to use /dev/random (by
- specifying --use-random on luksFormat) as randomness source for
- the master key to avoid being potentially insecure in an
- entropy-starved situation.
-
-
- * 5.18 What about Plausible Deniability?
-
- First let me attempt a definition for the case of encrypted
- filesystems: Plausible deniability is when you hide encrypted data
- inside an encrypted container and it is not possible to prove it is
- there. The idea is compelling and on first glance it seems
- possible to do it. And from a cryptographic point of view, it
- actually is possible.
-
- So, does it work in practice? No, unfortunately. The reasoning used
- by its proponents is fundamentally flawed in several ways and the
+ FIPS-140-2 compliant, but in order to make it certified, somebody has to
+ pay real money for that. And then, whenever cryptsetup is changed or
+ extended, the certification lapses and has to be obtained again.
+
+ From the aspect of actual security, LUKS with default parameters should
+ be as good as most things that are FIPS-140-2 certified, although you
+ may want to make sure to use /dev/random (by specifying --use-random on
+ luksFormat) as randomness source for the master key to avoid being
+ potentially insecure in an entropy-starved situation.
+
+
+ * 5.18 What about Plausible Deniability?
+
+ First let me attempt a definition for the case of encrypted filesystems:
+ Plausible deniability is when you store data inside an encrypted
+ container and it is not possible to prove it is there without having a
+ special passphrase. And at the same time it must be "plausible" that
+ there actually is no hidden data there.
+
+ As a simple entropy-analysis will show that here may be data there, the
+ second part is what makes it tricky.
+
+ There seem to be a lot of misunderstandings about this idea, so let me
+ make it clear that this refers to the situation where the attackers can
+ prove that there is data that either may be random or may be part of a
+ plausible-deniability scheme, they just cannot prove which one it is.
+ Hence a plausible-deniability scheme must hold up when the attackers
+ know there is something potentially fishy. If you just hide data and
+ rely on it not being found, that is just simple deniability, not
+ "plausible" deniability and I am not talking about that in the
+ following. Simple deniability against a low-competence attacker may be
+ as simple as renaming a file or putting data into an unused part of a
+ disk. Simple deniability against a high-skill attacker with time to
+ invest is usually pointless unless you go for advanced steganographic
+ techniques, which have their own drawbacks, such as low data capacity.
+
+ Now, the idea of plausible deniability is compelling and on a first
+ glance it seems possible to do it. And from a cryptographic point of
+ view, it actually is possible.
+
+ So, does the idea work in practice? No, unfortunately. The reasoning
+ used by its proponents is fundamentally flawed in several ways and the
cryptographic properties fail fatally when colliding with the real
world.
First, why should "I do not have a hidden partition" be any more
- plausible than "I forgot my crypto key" or "I wiped that partition
- with random data, nothing in there"? I do not see any reason.
-
- Second, there are two types of situations: Either they cannot force
- you to give them the key (then you simply do not) or the can. In
- the second case, they can always do bad things to you, because they
- cannot prove that you have the key in the first place! This means
- they do not have to prove you have the key, or that this random
- looking data on your disk is actually encrypted data. So the
- situation will allow them to waterboard/lock-up/deport you
- anyways, regardless of how "plausible" your deniability is. Do not
- have a hidden partition you could show to them, but there are
- indications you may? Too bad for you. Unfortunately "plausible
- deniability" also means you cannot prove there is no hidden data.
-
- Third, hidden partitions are not that hidden. There are basically
- just two possibilities: a) Make a large crypto container, but put a
- smaller filesystem in there and put the hidden partition into the
- free space. Unfortunately this is glaringly obvious and can be
- detected in an automated fashion. This means that the initial
- suspicion to put you under duress in order to make you reveal you
- hidden data is given. b) Make a filesystem that spans the whole
- encrypted partition, and put the hidden partition into space not
- currently used by that filesystem. Unfortunately that is also
- glaringly obvious, as you then cannot write to the filesystem
- without a high risk of destroying data in the hidden container.
- Have not written anything to the encrypted filesystem in a while?
- Too bad, they have the suspicion they need to do unpleasant things
- to you.
-
- To be fair, if you prepare option b) carefully and directly before
- going into danger, it may work. But then, the mere presence of
- encrypted data may already be enough to get you into trouble in
- those places were they can demand encryption keys.
+ plausible than "I forgot my crypto key" or "I wiped that partition with
+ random data, nothing in there"? I do not see any reason.
+
+ Second, there are two types of situations: Either they cannot force you
+ to give them the key (then you simply do not) or they can. In the
+ second case, they can always do bad things to you, because they cannot
+ prove that you have the key in the first place! This means they do not
+ have to prove you have the key, or that this random looking data on your
+ disk is actually encrypted data. So the situation will allow them to
+ waterboard/lock-up/deport you anyways, regardless of how "plausible"
+ your deniability is. Do not have a hidden partition you could show to
+ them, but there are indications you may? Too bad for you.
+ Unfortunately "plausible deniability" also means you cannot prove there
+ is no hidden data.
+
+ Third, hidden partitions are not that hidden. There are basically just
+ two possibilities: a) Make a large crypto container, but put a smaller
+ filesystem in there and put the hidden partition into the free space.
+ Unfortunately this is glaringly obvious and can be detected in an
+ automated fashion. This means that the initial suspicion to put you
+ under duress in order to make you reveal your hidden data is given. b)
+ Make a filesystem that spans the whole encrypted partition, and put the
+ hidden partition into space not currently used by that filesystem.
+ Unfortunately that is also glaringly obvious, as you then cannot write
+ to the filesystem without a high risk of destroying data in the hidden
+ container. Have not written anything to the encrypted filesystem in a
+ while? Too bad, they have the suspicion they need to do unpleasant
+ things to you.
+
+ To be fair, if you prepare option b) carefully and directly before going
+ into danger, it may work. But then, the mere presence of encrypted data
+ may already be enough to get you into trouble in those places were they
+ can demand encryption keys.
Here is an additional reference for some problems with plausible
- deniability: http://www.schneier.com/paper-truecrypt-dfs.pdf I
- strongly suggest you read it.
-
- So, no, I will not provide any instructions on how to do it with
- plain dm-crypt or LUKS. If you insist on shooting yourself in the
- foot, you can figure out how to do it yourself.
-
-
- * 5.19 What about SSDs, Flash and Hybrid Drives?
-
- The problem is that you cannot reliably erase parts of these
- devices, mainly due to wear-leveling and possibly defect
- management.
-
- Basically, when overwriting a sector (of 512B), what the device
- does is to move an internal sector (may be 128kB or even larger) to
- some pool of discarded, not-yet erased unused sectors, take a
- fresh empty sector from the empty-sector pool and copy the old
- sector over with the changes to the small part you wrote. This is
- done in some fashion so that larger writes do not cause a lot of
- small internal updates.
-
- The thing is that the mappings between outside-addressable sectors
- and inside sectors is arbitrary (and the vendors are not talking).
- Also the discarded sectors are not necessarily erased immediately.
- They may linger a long time.
-
- For plain dm-crypt, the consequences are that older encrypted data
- may be lying around in some internal pools of the device. Thus may
- or may not be a problem and depends on the application. Remember
- the same can happen with a filesystem if consecutive writes to the
- same area of a file can go to different sectors.
-
- However, for LUKS, the worst case is that key-slots and LUKS
- header may end up in these internal pools. This means that password
- management functionality is compromised (the old passwords may
- still be around, potentially for a very long time) and that fast
- erase by overwriting the header and key-slot area is insecure.
-
- Also keep in mind that the discarded/used pool may be large. For
- example, a 240GB SSD has about 16GB of spare area in the chips that
- it is free to do with as it likes. You would need to make each
- individual key-slot larger than that to allow reliable overwriting.
- And that assumes the disk thinks all other space is in use.
- Reading the internal pools using forensic tools is not that hard,
- but may involve some soldering.
+ deniability: http://www.schneier.com/paper-truecrypt-dfs.pdf I strongly
+ suggest you read it.
+
+ So, no, I will not provide any instructions on how to do it with plain
+ dm-crypt or LUKS. If you insist on shooting yourself in the foot, you
+ can figure out how to do it yourself.
+
+
+ * 5.19 What about SSDs, Flash, Hybrid and SMR Drives?
+
+ The problem is that you cannot reliably erase parts of these devices,
+ mainly due to wear-leveling and possibly defect management and delayed
+ writes to the main data area.
+
+ For example for SSDs, when overwriting a sector, what the device does is
+ to move an internal sector (may be 128kB or even larger) to some pool of
+ discarded, not-yet erased unused sectors, take a fresh empty sector from
+ the empty-sector pool and copy the old sector over with the changes to
+ the small part you wrote. This is done in some fashion so that larger
+ writes do not cause a lot of small internal updates.
+
+ The thing is that the mappings between outside-addressable sectors and
+ inside sectors is arbitrary (and the vendors are not talking). Also the
+ discarded sectors are not necessarily erased immediately. They may
+ linger a long time.
+
+ For plain dm-crypt, the consequences are that older encrypted data may
+ be lying around in some internal pools of the device. Thus may or may
+ not be a problem and depends on the application. Remember the same can
+ happen with a filesystem if consecutive writes to the same area of a
+ file can go to different sectors.
+
+ However, for LUKS, the worst case is that key-slots and LUKS header may
+ end up in these internal pools. This means that password management
+ functionality is compromised (the old passwords may still be around,
+ potentially for a very long time) and that fast erase by overwriting the
+ header and key-slot area is insecure.
+
+ Also keep in mind that the discarded/used pool may be large. For
+ example, a 240GB SSD has about 16GB of spare area in the chips that it
+ is free to do with as it likes. You would need to make each individual
+ key-slot larger than that to allow reliable overwriting. And that
+ assumes the disk thinks all other space is in use. Reading the internal
+ pools using forensic tools is not that hard, but may involve some
+ soldering.
What to do?
- If you trust the device vendor (you probably should not...) you can
- try an ATA "secure erase" command for SSDs. That does not work for
- USB keys though and may or may not be secure for a hybrid drive. If
- it finishes on an SSD after a few seconds, it was possibly faked.
- UNfortunately, for hybrid drives that indicator does not work, as
- the drive may well take the time to dully erase the magnetic part,
- but only mark the SSD/Flash part as erased while data is still in
- there.
+ If you trust the device vendor (you probably should not...) you can try
+ an ATA "secure erase" command. That is not present in USB keys though
+ and may or may not be secure for a hybrid drive.
If you can do without password management and are fine with doing
- physical destruction for permanently deleting data (always after
- one or several full overwrites!), you can use plain dm-crypt or
- LUKS.
-
- If you want or need the original LUKS security features to work,
- you can use a detached LUKS header and put that on a conventional,
- magnetic disk. That leaves potentially old encrypted data in the
- pools on the disk, but otherwise you get LUKS with the same
- security as on a magnetic disk.
+ physical destruction for permanently deleting data (always after one or
+ several full overwrites!), you can use plain dm-crypt.
+
+ If you want or need all the original LUKS security features to work, you
+ can use a detached LUKS header and put that on a conventional, magnetic
+ disk. That leaves potentially old encrypted data in the pools on the
+ main disk, but otherwise you get LUKS with the same security as on a
+ traditional magnetic disk. Note however that storage vendors are prone
+ to lying to their customers. For example, it recently came out that
+ HDDs sold without any warning or mentioning in the data-sheets were
+ actually using SMR and that will write data first to a faster area and
+ only overwrite the original data area some time later when things are
+ quiet.
+
+ If you are concerned about your laptop being stolen, you are likely fine
+ using LUKS on an SSD or hybrid drive. An attacker would need to have
+ access to an old passphrase (and the key-slot for this old passphrase
+ would actually need to still be somewhere in the SSD) for your data to
+ be at risk. So unless you pasted your old passphrase all over the
+ Internet or the attacker has knowledge of it from some other source and
+ does a targeted laptop theft to get at your data, you should be fine.
+
+
+ * 5.20 LUKS1 is broken! It uses SHA-1!
+
+ No, it is not. SHA-1 is (academically) broken for finding collisions,
+ but not for using it in a key-derivation function. And that collision
+ vulnerability is for non-iterated use only. And you need the hash-value
+ in verbatim.
+
+ This basically means that if you already have a slot-key, and you have
+ set the PBKDF2 iteration count to 1 (it is > 10'000 normally), you could
+ (maybe) derive a different passphrase that gives you the the same
+ slot-key. But if you have the slot-key, you can already unlock the
+ key-slot and get the master key, breaking everything. So basically,
+ this SHA-1 vulnerability allows you to open a LUKS1 container with high
+ effort when you already have it open.
+
+ The real problem here is people that do not understand crypto and claim
+ things are broken just because some mechanism is used that has been
+ broken for a specific different use. The way the mechanism is used
+ matters very much. A hash that is broken for one use can be completely
+ secure for other uses and here it is.
+
+ Since version 1.7.0, cryptsetup uses SHA-256 as default to ensure that
+ it will be compatible in the future. There are already some systems
+ where SHA-1 is completely phased out or disabled by a security policy.
+
+
+ * 5.21 Why is there no "Nuke-Option"?
+
+ A "Nuke-Option" or "Kill-switch" is a password that when entered upon
+ unlocking instead wipes the header and all passwords. So when somebody
+ forces you to enter your password, you can destroy the data instead.
+
+ While this sounds attractive at first glance, it does not make sense
+ once a real security analysis is done. One problem is that you have to
+ have some kind of HSM (Hardware Security Module) in order to implement
+ it securely. In the movies, a HSM starts to smoke and melt once the
+ Nuke-Option has been activated. In actual reality, it just wipes some
+ battery-backed RAM cells. A proper HSM costs something like
+ 20'000...100'000 EUR/USD and there a Nuke-Option may make some sense.
+ BTW, a chipcard or a TPM is not a HSM, although some vendors are
+ promoting that myth.
+
+ Now, a proper HSMs will have a wipe option but not a Nuke-Option, i.e.
+ you can explicitly wipe the HSM, but by a different process than
+ unlocking it takes. Why is that? Simple: If somebody can force you to
+ reveal passwords, then they can also do bad things to you if you do not
+ or if you enter a nuke password instead. Think locking you up for a few
+ years for "destroying evidence" or for far longer and without trial for
+ being a "terrorist suspect". No HSM maker will want to expose its
+ customers to that risk.
+
+ Now think of the typical LUKS application scenario, i.e. disk
+ encryption. Usually the ones forcing you to hand over your password
+ will have access to the disk as well, and, if they have any real
+ suspicion, they will mirror your disk before entering anything supplied
+ by you. This neatly negates any Nuke-Option. If they have no suspicion
+ (just harassing people that cross some border for example), the
+ Nuke-Option would work, but see above about likely negative consequences
+ and remember that a Nuke-Option may not work reliably on SSD and hybrid
+ drives anyways.
+
+ Hence my advice is to never take data that you do not want to reveal
+ into any such situation in the first place. There is no need to
+ transfer data on physical carriers today. The Internet makes it quite
+ possible to transfer data between arbitrary places and modern encryption
+ makes it secure. If you do it right, nobody will even be able to
+ identify source or destination. (How to do that is out of scope of this
+ document. It does require advanced skills in this age of pervasive
+ surveillance.)
+
+ Hence, LUKS has not kill option because it would do much more harm than
+ good.
+
+ Still, if you have a good use-case (i.e. non-abstract real-world
+ situation) where a Nuke-Option would actually be beneficial, please let
+ me know.
+
+
+ * 5.22 Does cryptsetup open network connections to websites, etc. ?
+
+ This question seems not to make much sense at first glance, but here is
+ an example form the real world: The TrueCrypt GUI has a "Donation"
+ button. Press it, and a web-connection to the TrueCrypt website is
+ opened via the default browser, telling everybody that listens that you
+ use TrueCrypt. In the worst case, things like this can get people
+ tortured or killed.
+
+ So: Cryptsetup will never open any network connections except the
+ local netlink socket it needs to talk to the kernel crypto API.
+
+ In addition, the installation package should contain all documentation,
+ including this FAQ, so that you do not have to go to a web-site to read
+ it. (If your distro cuts the docu, please complain to them.) In
+ security software, any connection initiated to anywhere outside your
+ machine should always be the result of an explicit request for such a
+ connection by the user and cryptsetup will stay true to that principle.
- If you are concerned about your laptop being stolen, you are likely
- fine using LUKS on an SSD or hybrid drive. An attacker would need
- to have access to an old passphrase (and the key-slot for this old
- passphrase would actually need to still be somewhere in the SSD)
- for your data to be at risk. So unless you pasted your old
- passphrase all over the Internet or the attacker has knowledge of
- it from some other source and does a targeted laptop theft to get
- at your data, you should be fine.
-
-6. Backup and Data Recovery
+6. Backup and Data Recovery
* 6.1 Why do I need Backup?
- First, disks die. The rate for well-treated (!) disk is about 5%
- per year, which is high enough to worry about. There is some
- indication that this may be even worse for some SSDs. This applies
- both to LUKS and plain dm-crypt partitions.
+ First, disks die. The rate for well-treated (!) disk is about 5% per
+ year, which is high enough to worry about. There is some indication
+ that this may be even worse for some SSDs. This applies both to LUKS
+ and plain dm-crypt partitions.
- Second, for LUKS, if anything damages the LUKS header or the
- key-stripe area then decrypting the LUKS device can become
- impossible. This is a frequent occurrence. For example an
- accidental format as FAT or some software overwriting the first
- sector where it suspects a partition boot sector typically makes a
- LUKS partition permanently inaccessible. See more below on LUKS
- header damage.
+ Second, for LUKS, if anything damages the LUKS header or the key-stripe
+ area then decrypting the LUKS device can become impossible. This is a
+ frequent occurrence. For example an accidental format as FAT or some
+ software overwriting the first sector where it suspects a partition boot
+ sector typically makes a LUKS1 partition permanently inaccessible. See
+ more below on LUKS header damage.
- So, data-backup in some form is non-optional. For LUKS, you may
- also want to store a header backup in some secure location. This
- only needs an update if you change passphrases.
+ So, data-backup in some form is non-optional. For LUKS, you may also
+ want to store a header backup in some secure location. This only needs
+ an update if you change passphrases.
* 6.2 How do I backup a LUKS header?
- While you could just copy the appropriate number of bytes from the
- start of the LUKS partition, the best way is to use command option
- "luksHeaderBackup" of cryptsetup. This protects also against
- errors when non-standard parameters have been used in LUKS
- partition creation. Example:
+ While you could just copy the appropriate number of bytes from the start
+ of the LUKS partition, the best way is to use command option
+ "luksHeaderBackup" of cryptsetup. This protects also against errors
+ when non-standard parameters have been used in LUKS partition creation.
+ Example:
+
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
-
- cryptsetup luksHeaderBackup --header-backup-file <file> <device>
-
To restore, use the inverse command, i.e.
- cryptsetup luksHeaderRestore --header-backup-file <file> <device>
-
+ cryptsetup luksHeaderRestore --header-backup-file <file> <device>
+
+ If you are unsure about a header to be restored, make a backup of the
+ current one first! You can also test the header-file without restoring
+ it by using the --header option for a detached header like this:
+
+ cryptsetup --header <file> luksOpen <device> </dev/mapper/name>
+
+ If that unlocks your keys-lot, you are good. Do not forget to close
+ the device again.
- * 6.3 How do I test a LUKS header?
+ Under some circumstances (damaged header), this fails. Then use the
+ following steps in case it is LUKS1:
+
+ First determine the master-key size:
+
+ cryptsetup luksDump <device>
+
+ gives a line of the form
+
+ MK bits: <bits>
+
+ with bits equal to 256 for the old defaults and 512 for the new
+ defaults. 256 bits equals a total header size of 1'052'672 Bytes and
+ 512 bits one of 2MiB. (See also Item 6.12) If luksDump fails, assume
+ 2MiB, but be aware that if you restore that, you may also restore the
+ first 1M or so of the filesystem. Do not change the filesystem if you
+ were unable to determine the header size! With that, restoring a
+ too-large header backup is still safe.
+
+ Second, dump the header to file. There are many ways to do it, I
+ prefer the following:
+
+ head -c 1052672 <device> > header_backup.dmp
+
+ or
+
+ head -c 2M <device> > header_backup.dmp
+
+ for a 2MiB header. Verify the size of the dump-file to be sure.
+
+ To restore such a backup, you can try luksHeaderRestore or do a more
+ basic
+
+ cat header_backup.dmp > <device>
+
+
+
+ * 6.3 How do I test for a LUKS header?
Use
- cryptsetup -v isLuks <device>
-
- on the device. Without the "-v" it just signals its result via
- exit-status. You can also use the more general test
+ cryptsetup -v isLuks <device>
- blkid -p <device>
-
- which will also detect other types and give some more info. Omit
+ on the device. Without the "-v" it just signals its result via
+ exit-status. You can also use the more general test
+
+ blkid -p <device>
+
+ which will also detect other types and give some more info. Omit
"-p" for old versions of blkid that do not support it.
- * 6.4 How do I backup a LUKS or dm-crypt partition?
+ * 6.4 How do I backup a LUKS or dm-crypt partition?
- There are two options, a sector-image and a plain file or
- filesystem backup of the contents of the partition. The sector
- image is already encrypted, but cannot be compressed and contains
- all empty space. The filesystem backup can be compressed, can
- contain only part of the encrypted device, but needs to be
- encrypted separately if so desired.
+ There are two options, a sector-image and a plain file or filesystem
+ backup of the contents of the partition. The sector image is already
+ encrypted, but cannot be compressed and contains all empty space. The
+ filesystem backup can be compressed, can contain only part of the
+ encrypted device, but needs to be encrypted separately if so desired.
- A sector-image will contain the whole partition in encrypted form,
- for LUKS the LUKS header, the keys-slots and the data area. It can
- be done under Linux e.g. with dd_rescue (for a direct image copy)
- and with "cat" or "dd". Example:
+ A sector-image will contain the whole partition in encrypted form, for
+ LUKS the LUKS header, the keys-slots and the data area. It can be done
+ under Linux e.g. with dd_rescue (for a direct image copy) and with
+ "cat" or "dd". Examples:
+
+ cat /dev/sda10 > sda10.img
+ dd_rescue /dev/sda10 sda10.img
+
+ You can also use any other backup software that is capable of making a
+ sector image of a partition. Note that compression is ineffective for
+ encrypted data, hence it does not make sense to use it.
+
+ For a filesystem backup, you decrypt and mount the encrypted partition
+ and back it up as you would a normal filesystem. In this case the
+ backup is not encrypted, unless your encryption method does that. For
+ example you can encrypt a backup with "tar" as follows with GnuPG:
+
+ tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
- cat /dev/sda10 > sda10.img
- dd_rescue /dev/sda10 sda10.img
-
- You can also use any other backup software that is capable of making
- a sector image of a partition. Note that compression is
- ineffective for encrypted data, hence it does not make sense to
- use it.
-
- For a filesystem backup, you decrypt and mount the encrypted
- partition and back it up as you would a normal filesystem. In this
- case the backup is not encrypted, unless your encryption method
- does that. For example you can encrypt a backup with "tar" as
- follows with GnuPG:
-
- tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
-
And verify the backup like this if you are at "path":
- cat backup.tbz2.gpg | gpg - | tar djf -
-
- Note: Always verify backups, especially encrypted ones.
+ cat backup.tbz2.gpg | gpg - | tar djf -
+
+ Note: Always verify backups, especially encrypted ones!
- In both cases GnuPG will ask you interactively for your symmetric
- key. The verify will only output errors. Use "tar dvjf -" to get
- all comparison results. To make sure no data is written to disk
+ There is one problem with verifying like this: The kernel may still have
+ some files cached and in fact verify them against RAM or may even verify
+ RAM against RAM, which defeats the purpose of the exercise. The
+ following command empties the kernel caches:
+
+ echo 3 > /proc/sys/vm/drop_caches
+
+ Run it after backup and before verify.
+
+ In both cases GnuPG will ask you interactively for your symmetric key.
+ The verify will only output errors. Use "tar dvjf -" to get all
+ comparison results. To make sure no data is written to disk
unencrypted, turn off swap if it is not encrypted before doing the
backup.
- You can of course use different or no compression and you can use
- an asymmetric key if you have one and have a backup of the secret
- key that belongs to it.
+ Restore works like certification with the 'd' ('difference') replaced
+ by 'x' ('eXtract'). Refer to the man-page of tar for more explanations
+ and instructions. Note that with default options tar will overwrite
+ already existing files without warning. If you are unsure about how
+ to use tar, experiment with it in a location where you cannot do damage.
- A second option for a filesystem-level backup that can be used when
- the backup is also on local disk (e.g. an external USB drive) is
- to use a LUKS container there and copy the files to be backed up
- between both mounted containers. Also see next item.
+ You can of course use different or no compression and you can use an
+ asymmetric key if you have one and have a backup of the secret key that
+ belongs to it.
+ A second option for a filesystem-level backup that can be used when the
+ backup is also on local disk (e.g. an external USB drive) is to use a
+ LUKS container there and copy the files to be backed up between both
+ mounted containers. Also see next item.
- * 6.5 Do I need a backup of the full partition? Would the header and
- key-slots not be enough?
- Backup protects you against two things: Disk loss or corruption
- and user error. By far the most questions on the dm-crypt mailing
- list about how to recover a damaged LUKS partition are related
- to user error. For example, if you create a new filesystem on a
- LUKS partition, chances are good that all data is lost
- permanently.
+ * 6.5 Do I need a backup of the full partition? Would the header
+ and key-slots not be enough?
- For this case, a header+key-slot backup would often be enough. But
- keep in mind that a well-treated (!) HDD has roughly a failure
- risk of 5% per year. It is highly advisable to have a complete
- backup to protect against this case.
+ Backup protects you against two things: Disk loss or corruption and user
+ error. By far the most questions on the dm-crypt mailing list about how
+ to recover a damaged LUKS partition are related to user error. For
+ example, if you create a new filesystem on a non-mapped LUKS container,
+ chances are good that all data is lost permanently.
+ For this case, a header+key-slot backup would often be enough. But keep
+ in mind that a well-treated (!) HDD has roughly a failure risk of 5% per
+ year. It is highly advisable to have a complete backup to protect
+ against this case.
- * *6.6 What do I need to backup if I use "decrypt_derived"?
- This is a script in Debian, intended for mounting /tmp or swap with
- a key derived from the master key of an already decrypted device.
- If you use this for an device with data that should be persistent,
- you need to make sure you either do not lose access to that master
- key or have a backup of the data. If you derive from a LUKS
- device, a header backup of that device would cover backing up the
- master key. Keep in mind that this does not protect against disk
- loss.
+ * 6.6 What do I need to backup if I use "decrypt_derived"?
+
+ This is a script in Debian, intended for mounting /tmp or swap with a
+ key derived from the master key of an already decrypted device. If you
+ use this for an device with data that should be persistent, you need to
+ make sure you either do not lose access to that master key or have a
+ backup of the data. If you derive from a LUKS device, a header backup
+ of that device would cover backing up the master key. Keep in mind that
+ this does not protect against disk loss.
Note: If you recreate the LUKS header of the device you derive from
(using luksFormat), the master key changes even if you use the same
- passphrase(s) and you will not be able to decrypt the derived
- device with the new LUKS header.
+ passphrase(s) and you will not be able to decrypt the derived device
+ with the new LUKS header.
- * 6.7 Does a backup compromise security?
+ * 6.7 Does a backup compromise security?
- Depends on how you do it. However if you do not have one, you are
- going to eventually lose your encrypted data.
+ Depends on how you do it. However if you do not have one, you are going
+ to eventually lose your encrypted data.
- There are risks introduced by backups. For example if you
- change/disable a key-slot in LUKS, a binary backup of the partition
- will still have the old key-slot. To deal with this, you have to
- be able to change the key-slot on the backup as well, securely
- erase the backup or do a filesystem-level backup instead of a binary
- one.
+ There are risks introduced by backups. For example if you
+ change/disable a key-slot in LUKS, a binary backup of the partition will
+ still have the old key-slot. To deal with this, you have to be able to
+ change the key-slot on the backup as well, securely erase the backup or
+ do a filesystem-level backup instead of a binary one.
- If you use dm-crypt, backup is simpler: As there is no key
- management, the main risk is that you cannot wipe the backup when
- wiping the original. However wiping the original for dm-crypt
- should consist of forgetting the passphrase and that you can do
- without actual access to the backup.
+ If you use dm-crypt, backup is simpler: As there is no key management,
+ the main risk is that you cannot wipe the backup when wiping the
+ original. However wiping the original for dm-crypt should consist of
+ forgetting the passphrase and that you can do without actual access to
+ the backup.
- In both cases, there is an additional (usually small) risk with
- binary backups: An attacker can see how many sectors and which
- ones have been changed since the backup. To prevent this, use a
- filesystem level backup method that encrypts the whole backup in
- one go, e.g. as described above with tar and GnuPG.
+ In both cases, there is an additional (usually small) risk with binary
+ backups: An attacker can see how many sectors and which ones have been
+ changed since the backup. To prevent this, use a filesystem level
+ backup method that encrypts the whole backup in one go, e.g. as
+ described above with tar and GnuPG.
- My personal advice is to use one USB disk (low value data) or
- three disks (high value data) in rotating order for backups, and
- either use independent LUKS partitions on them, or use encrypted
- backup with tar and GnuPG.
+ My personal advice is to use one USB disk (low value data) or three
+ disks (high value data) in rotating order for backups, and either use
+ independent LUKS partitions on them, or use encrypted backup with tar
+ and GnuPG.
- If you do network-backup or tape-backup, I strongly recommend to
- go the filesystem backup path with independent encryption, as you
- typically cannot reliably delete data in these scenarios,
- especially in a cloud setting. (Well, you can burn the tape if it
- is under your control...)
+ If you do network-backup or tape-backup, I strongly recommend to go
+ the filesystem backup path with independent encryption, as you
+ typically cannot reliably delete data in these scenarios, especially
+ in a cloud setting. (Well, you can burn the tape if it is under your
+ control...)
- * 6.8 What happens if I overwrite the start of a LUKS partition or
- damage the LUKS header or key-slots?
+ * 6.8 What happens if I overwrite the start of a LUKS partition or
+ damage the LUKS header or key-slots?
- There are two critical components for decryption: The salt values
- in the header itself and the key-slots. If the salt values are
- overwritten or changed, nothing (in the cryptographically strong
- sense) can be done to access the data, unless there is a backup
- of the LUKS header. If a key-slot is damaged, the data can still
- be read with a different key-slot, if there is a remaining
- undamaged and used key-slot. Note that in order to make a key-slot
- unrecoverable in a cryptographically strong sense, changing about
- 4-6 bits in random locations of its 128kiB size is quite enough.
+ There are two critical components for decryption: The salt values in the
+ key-slot descriptors of the header and the key-slots. For LUKS2 they
+ are a bit better protected. but for LUKS1, these are right in the first
+ sector. If the salt values are overwritten or changed, nothing (in the
+ cryptographically strong sense) can be done to access the data, unless
+ there is a backup of the LUKS header. If a key-slot is damaged, the
+ data can still be read with a different key-slot, if there is a
+ remaining undamaged and used key-slot. Note that in order to make a
+ key-slot completely unrecoverable, changing about 4-6 bits in random
+ locations of its 128kiB size is quite enough.
- * 6.9 What happens if I (quick) format a LUKS partition?
+ * 6.9 What happens if I (quick) format a LUKS partition?
- I have not tried the different ways to do this, but very likely you
- will have written a new boot-sector, which in turn overwrites the
- LUKS header, including the salts, making your data permanently
- irretrievable, unless you have a LUKS header backup. You may also
- damage the key-slots in part or in full. See also last item.
+ I have not tried the different ways to do this, but very likely you will
+ have written a new boot-sector, which in turn overwrites the LUKS
+ header, including the salts, making your data permanently irretrievable,
+ unless you have a LUKS header backup. For LUKS2 this may still be
+ recoverable without that header backup, for LUKS1 it is not. You may
+ also damage the key-slots in part or in full. See also last item.
- * 6.10 How do I recover the master key from a mapped LUKS container?
+ * 6.10 How do I recover the master key from a mapped LUKS1 container?
- This is typically only needed if you managed to damage your LUKS
- header, but the container is still mapped, i.e. "luksOpen"ed. It
- also helps if you have a mapped container that you forgot or do not
- know a passphrase for (e.g. on a long running server.)
+ Note: LUKS2 uses the kernel keyring to store keys and hence this
+ procedure does not work unless you have explicitly disabled the use of
+ the keyring with "--disable-keyring" on opening.
+
+ This is typically only needed if you managed to damage your LUKS1
+ header, but the container is still mapped, i.e. "luksOpen"ed. It also
+ helps if you have a mapped container that you forgot or do not know a
+ passphrase for (e.g. on a long running server.)
WARNING: Things go wrong, do a full backup before trying this!
- WARNING: This exposes the master key of the LUKS container. Note
- that both ways to recreate a LUKS header with the old master key
- described below will write the master key to disk. Unless you are
- sure you have securely erased it afterwards, e.g. by writing it to
- an encrypted partition, RAM disk or by erasing the filesystem you
- wrote it to by a complete overwrite, you should change the master
- key afterwards. Changing the master key requires a full data
- backup, luksFormat and then restore of the backup.
+ WARNING: This exposes the master key of the LUKS1 container. Note that
+ both ways to recreate a LUKS header with the old master key described
+ below will write the master key to disk. Unless you are sure you have
+ securely erased it afterwards, e.g. by writing it to an encrypted
+ partition, RAM disk or by erasing the filesystem you wrote it to by a
+ complete overwrite, you should change the master key afterwards.
+ Changing the master key requires a full data backup, luksFormat and then
+ restore of the backup. Alternatively the tool cryptsetup-reencrypt from
+ the cryptsetup package can be used to change the master key (see its
+ man-page), but a full backup is still highly recommended.
- First, there is a script by Milan that automates the whole
- process, except generating a new LUKS header with the old master
- key (it prints the command for that though):
+ First, there is a script by Milan that automates the whole process,
+ except generating a new LUKS1 header with the old master key (it prints
+ the command for that though):
-http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
+ https://gitlab.com/cryptsetup/cryptsetup/blob/master/misc/luks-header-from-active
You can also do this manually. Here is how:
- - Get the master key from the device mapper. This is done by the
- following command. Substitute c5 for whatever you mapped to:
+ - Get the master key from the device mapper. This is done by the
+ following command. Substitute c5 for whatever you mapped to:
- # dmsetup table --target crypt --showkey /dev/mapper/c5
- Result:
- 0 200704 crypt aes-cbc-essiv:sha256
- a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
- 0 7:0 4096
-
- The result is actually one line, wrapped here for clarity. The long
+ # dmsetup table --target crypt --showkey /dev/mapper/c5
+
+ Result:
+ 0 200704 crypt aes-cbc-essiv:sha256
+ a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
+ 0 7:0 4096
+
+ The result is actually one line, wrapped here for clarity. The long
hex string is the master key.
- - Convert the master key to a binary file representation. You can
- do this manually, e.g. with hexedit. You can also use the tool
- "xxd" from vim like this:
+ - Convert the master key to a binary file representation. You can do
+ this manually, e.g. with hexedit. You can also use the tool "xxd"
+ from vim like this:
- echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
-
- - Do a luksFormat to create a new LUKS header.
+ echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
- NOTE: If your header is intact and you just forgot the
- passphrase, you can just set a new passphrase, see next
- sub-item.
+
+ - Do a luksFormat to create a new LUKS1 header.
+
+ NOTE: If your header is intact and you just forgot the passphrase,
+ you can just set a new passphrase, see next sub-item.
Unmap the device before you do that (luksClose). Then do
- cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
-
+ cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
+
Note that if the container was created with other than the default
settings of the cryptsetup version you are using, you need to give
- additional parameters specifying the deviations. If in doubt, try
- the script by Milan. It does recover the other parameters as well.
+ additional parameters specifying the deviations. If in doubt, try the
+ script by Milan. It does recover the other parameters as well.
- Side note: This is the way the decrypt_derived script gets at the
- master key. It just omits the conversion and hashes the master key
- string.
+ Side note: This is the way the decrypt_derived script gets at the master
+ key. It just omits the conversion and hashes the master key string.
- If the header is intact and you just forgot the passphrase, just
set a new passphrase like this:
cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
-
+
You may want to disable the old one afterwards.
- * 6.11 What does the on-disk structure of dm-crypt look like?
-
- There is none. dm-crypt takes a block device and gives encrypted
- access to each of its blocks with a key derived from the passphrase
- given. If you use a cipher different than the default, you have to
- specify that as a parameter to cryptsetup too. If you want to
- change the password, you basically have to create a second
- encrypted device with the new passphrase and copy your data over.
- On the plus side, if you accidentally overwrite any part of a
- dm-crypt device, the damage will be limited to the are you
- overwrote.
-
-
- * 6.12 What does the on-disk structure of LUKS look like?
-
- A LUKS partition consists of a header, followed by 8 key-slot
- descriptors, followed by 8 key slots, followed by the encrypted
- data area.
-
- Header and key-slot descriptors fill the first 592 bytes. The
- key-slot size depends on the creation parameters, namely on the
- number of anti-forensic stripes, key material offset and master
- key size.
-
- With the default parameters, each key-slot is a bit less than
- 128kiB in size. Due to sector alignment of the key-slot start,
- that means the key block 0 is at offset 0x1000-0x20400, key
- block 1 at offset 0x21000-0x40400, and key block 7 at offset
- 0xc1000-0xe0400. The space to the next full sector address is
- padded with zeros. Never used key-slots are filled with what the
- disk originally contained there, a key-slot removed with
- "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Due to
- 2MiB default alignment, start of the data area for cryptsetup 1.3
- and later is at 2MiB, i.e. at 0x200000. For older versions, it is
- at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes
- from the start of the partition. Incidentally, "luksHeaderBackup"
- for a LUKS container created with default parameters dumps exactly
- the first 2MiB (or 1'052'672 bytes for headers created with
- cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
+ * 6.11 What does the on-disk structure of dm-crypt look like?
+
+ There is none. dm-crypt takes a block device and gives encrypted access
+ to each of its blocks with a key derived from the passphrase given. If
+ you use a cipher different than the default, you have to specify that as
+ a parameter to cryptsetup too. If you want to change the password, you
+ basically have to create a second encrypted device with the new
+ passphrase and copy your data over. On the plus side, if you
+ accidentally overwrite any part of a dm-crypt device, the damage will be
+ limited to the area you overwrote.
+
+
+ * 6.12 What does the on-disk structure of LUKS1 look like?
+
+ Note: For LUKS2, refer to the LUKS2 document referenced in Item 1.2
+
+ A LUKS1 partition consists of a header, followed by 8 key-slot
+ descriptors, followed by 8 key slots, followed by the encrypted data
+ area.
+
+ Header and key-slot descriptors fill the first 592 bytes. The key-slot
+ size depends on the creation parameters, namely on the number of
+ anti-forensic stripes, key material offset and master key size.
+
+ With the default parameters, each key-slot is a bit less than 128kiB in
+ size. Due to sector alignment of the key-slot start, that means the key
+ block 0 is at offset 0x1000-0x20400, key block 1 at offset
+ 0x21000-0x40400, and key block 7 at offset 0xc1000-0xe0400. The space
+ to the next full sector address is padded with zeros. Never used
+ key-slots are filled with what the disk originally contained there, a
+ key-slot removed with "luksRemoveKey" or "luksKillSlot" gets filled with
+ 0xff. Due to 2MiB default alignment, start of the data area for
+ cryptsetup 1.3 and later is at 2MiB, i.e. at 0x200000. For older
+ versions, it is at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB +
+ 4096 bytes from the start of the partition. Incidentally,
+ "luksHeaderBackup" for a LUKS container created with default parameters
+ dumps exactly the first 2MiB (or 1'052'672 bytes for headers created
+ with cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
them.
- For non-default parameters, you have to figure out placement
- yourself. "luksDump" helps. See also next item. For the most common
- non-default settings, namely aes-xts-plain with 512 bit key, the
- offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
- 0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, ..., and start of
- bulk data at 0x200000.
+ For non-default parameters, you have to figure out placement yourself.
+ "luksDump" helps. See also next item. For the most common non-default
+ settings, namely aes-xts-plain with 512 bit key, the offsets are: 1st
+ keyslot 0x1000-0x3f800, 2nd keyslot 0x40000-0x7e000, 3rd keyslot
+ 0x7e000-0xbd800, ..., and start of bulk data at 0x200000.
The exact specification of the format is here:
- http://code.google.com/p/cryptsetup/wiki/Specification
-
-
- * 6.13 What is the smallest possible LUKS container?
-
- Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
- modern Linux partitioning tools that also align to 1MB, this will
- result in alignment to 2k sectors and typical Flash/SSD sectors,
- which is highly desirable for a number of reasons. Changing the
- alignment is not recommended.
-
- That said, with default parameters, the data area starts at
- exactly 2MB offset (at 0x101000 for cryptsetup versions before
- 1.3). The smallest data area you can have is one sector of 512
- bytes. Data areas of 0 bytes can be created, but fail on mapping.
+ https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
+
+ For your convenience, here is the LUKS1 header with hex offsets.
+ NOTE:
+ The spec counts key-slots from 1 to 8, but the cryptsetup tool counts
+ from 0 to 7. The numbers here refer to the cryptsetup numbers.
+
+
+Refers to LUKS1 On-Disk Format Specification Version 1.2.3
+
+LUKS1 header:
+
+offset length name data type description
+-----------------------------------------------------------------------
+0x0000 0x06 magic byte[] 'L','U','K','S', 0xba, 0xbe
+ 0 6
+0x0006 0x02 version uint16_t LUKS version
+ 6 3
+0x0008 0x20 cipher-name char[] cipher name spec.
+ 8 32
+0x0028 0x20 cipher-mode char[] cipher mode spec.
+ 40 32
+0x0048 0x20 hash-spec char[] hash spec.
+ 72 32
+0x0068 0x04 payload-offset uint32_t bulk data offset in sectors
+ 104 4 (512 bytes per sector)
+0x006c 0x04 key-bytes uint32_t number of bytes in key
+ 108 4
+0x0070 0x14 mk-digest byte[] master key checksum
+ 112 20 calculated with PBKDF2
+0x0084 0x20 mk-digest-salt byte[] salt for PBKDF2 when
+ 132 32 calculating mk-digest
+0x00a4 0x04 mk-digest-iter uint32_t iteration count for PBKDF2
+ 164 4 when calculating mk-digest
+0x00a8 0x28 uuid char[] partition UUID
+ 168 40
+0x00d0 0x30 key-slot-0 key slot key slot 0
+ 208 48
+0x0100 0x30 key-slot-1 key slot key slot 1
+ 256 48
+0x0130 0x30 key-slot-2 key slot key slot 2
+ 304 48
+0x0160 0x30 key-slot-3 key slot key slot 3
+ 352 48
+0x0190 0x30 key-slot-4 key slot key slot 4
+ 400 48
+0x01c0 0x30 key-slot-5 key slot key slot 5
+ 448 48
+0x01f0 0x30 key-slot-6 key slot key slot 6
+ 496 48
+0x0220 0x30 key-slot-7 key slot key slot 7
+ 544 48
+
+
+Key slot:
+
+offset length name data type description
+-------------------------------------------------------------------------
+0x0000 0x04 active uint32_t key slot enabled/disabled
+ 0 4
+0x0004 0x04 iterations uint32_t PBKDF2 iteration count
+ 4 4
+0x0008 0x20 salt byte[] PBKDF2 salt
+ 8 32
+0x0028 0x04 key-material-offset uint32_t key start sector
+ 40 4 (512 bytes/sector)
+0x002c 0x04 stripes uint32_t number of anti-forensic
+ 44 4 stripes
+
+
+
+ * 6.13 What is the smallest possible LUKS1 container?
+
+ Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With modern
+ Linux partitioning tools that also align to 1MB, this will result in
+ alignment to 2k sectors and typical Flash/SSD sectors, which is highly
+ desirable for a number of reasons. Changing the alignment is not
+ recommended.
+
+ That said, with default parameters, the data area starts at exactly 2MB
+ offset (at 0x101000 for cryptsetup versions before 1.3). The smallest
+ data area you can have is one sector of 512 bytes. Data areas of 0
+ bytes can be created, but fail on mapping.
While you cannot put a filesystem into something this small, it may
- still be used to contain, for example, key. Note that with current
- formatting tools, a partition for a container this size will be
- 3MiB anyways. If you put the LUKS container into a file (via
- losetup and a loopback device), the file needs to be 2097664 bytes
- in size, i.e. 2MiB + 512B.
-
- There two ways to influence the start of the data area are key-size
- and alignment.
-
- For alignment, you can go down to 1 on the parameter. This will
- still leave you with a data-area starting at 0x101000, i.e.
- 1MiB+4096B (default parameters) as alignment will be rounded up to
- the next multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run
- on a larger file and dump the LUKS header to get actual
- information.
-
- For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
- (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
- You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
- 128 bit has to be considered insecure today.
+ still be used to contain, for example, key. Note that with current
+ formatting tools, a partition for a container this size will be 3MiB
+ anyways. If you put the LUKS container into a file (via losetup and a
+ loopback device), the file needs to be 2097664 bytes in size, i.e. 2MiB
+ + 512B.
+
+ The two ways to influence the start of the data area are key-size and
+ alignment.
+
+ For alignment, you can go down to 1 on the parameter. This will still
+ leave you with a data-area starting at 0x101000, i.e. 1MiB+4096B
+ (default parameters) as alignment will be rounded up to the next
+ multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run on a larger
+ file and dump the LUKS header to get actual information.
+
+ For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
+ (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode). You
+ can do 64 bit (e.g. blowfish-64 with CBC), but anything below 128 bit
+ has to be considered insecure today.
Example 1 - AES 128 bit with CBC:
cryptsetup luksFormat -s 128 --align-payload=8 <device>
-
+
This results in a data offset of 0x81000, i.e. 516KiB or 528384
- bytes. Add one 512 byte sector and the smallest LUKS container size
+ bytes. Add one 512 byte sector and the smallest LUKS container size
with these parameters is 516KiB + 512B or 528896 bytes.
Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
-
+
This results in a data offset of 0x41000, i.e. 260kiB or 266240
- bytes, with a minimal LUKS container size of 260kiB + 512B or
- 266752 bytes.
+ bytes, with a minimal LUKS1 container size of 260kiB + 512B or 266752
+ bytes.
- * 6.14 I think this is overly complicated. Is there an alternative?
+ * 6.14 I think this is overly complicated. Is there an alternative?
- Not really. Encryption comes at a price. You can use plain
- dm-crypt to simplify things a bit. It does not allow multiple
- passphrases, but on the plus side, it has zero on disk description
- and if you overwrite some part of a plain dm-crypt partition,
- exactly the overwritten parts are lost (rounded up to sector
- borders).
+ Not really. Encryption comes at a price. You can use plain dm-crypt to
+ simplify things a bit. It does not allow multiple passphrases, but on
+ the plus side, it has zero on disk description and if you overwrite some
+ part of a plain dm-crypt partition, exactly the overwritten parts are
+ lost (rounded up to full sectors).
+ * 6.15 Can I clone a LUKS container?
- * 6.15 Can I clone a LUKS container?
+ You can, but it breaks security, because the cloned container has the
+ same header and hence the same master key. Even if you change the
+ passphrase(s), the master key stays the same. That means whoever has
+ access to one of the clones can decrypt them all, completely bypassing
+ the passphrases.
- You can, but it breaks security, because the cloned container has
- the same header and hence the same master key. You cannot change
- the master key on a LUKS container, even if you change the
- passphrase(s), the master key stays the same. That means whoever
- has access to one of the clones can decrypt them all, completely
- bypassing the passphrases.
+ While you can use cryptsetup-reencrypt to change the master key,
+ this is probably more effort than to create separate LUKS containers
+ in the first place.
- The right way to do this is to first luksFormat the target
- container, then to clone the contents of the source container, with
- both containers mapped, i.e. decrypted. You can clone the decrypted
- contents of a LUKS container in binary mode, although you may run
- into secondary issues with GUIDs in filesystems, partition tables,
- RAID-components and the like. These are just the normal problems
- binary cloning causes.
+ The right way to do this is to first luksFormat the target container,
+ then to clone the contents of the source container, with both containers
+ mapped, i.e. decrypted. You can clone the decrypted contents of a LUKS
+ container in binary mode, although you may run into secondary issues
+ with GUIDs in filesystems, partition tables, RAID-components and the
+ like. These are just the normal problems binary cloning causes.
Note that if you need to ship (e.g.) cloned LUKS containers with a
default passphrase, that is fine as long as each container was
- individually created (and hence has its own master key). In this
- case, changing the default passphrase will make it secure again.
+ individually created (and hence has its own master key). In this case,
+ changing the default passphrase will make it secure again.
-7. Interoperability with other Disk Encryption Tools
+7. Interoperability with other Disk Encryption Tools
- * 7.1 What is this section about?
+ * 7.1 What is this section about?
- Cryptsetup for plain dm-crypt can be used to access a number of
- on-disk formats created by tools like loop-aes patched into
- losetup. This sometimes works and sometimes does not. This
- section collects insights into what works, what does not and where
- more information is required.
+ Cryptsetup for plain dm-crypt can be used to access a number of on-disk
+ formats created by tools like loop-aes patched into losetup. This
+ sometimes works and sometimes does not. This section collects insights
+ into what works, what does not and where more information is required.
Additional information may be found in the mailing-list archives,
- mentioned at the start of this FAQ document. If you have a
- solution working that is not yet documented here and think a wider
- audience may be interested, please email the FAQ maintainer.
+ mentioned at the start of this FAQ document. If you have a solution
+ working that is not yet documented here and think a wider audience may
+ be interested, please email the FAQ maintainer.
- * 7.2 loop-aes: General observations.
+ * 7.2 loop-aes: General observations.
- One problem is that there are different versions of losetup around.
- loop-aes is a patch for losetup. Possible problems and deviations
+ One problem is that there are different versions of losetup around.
+ loop-aes is a patch for losetup. Possible problems and deviations
from cryptsetup option syntax include:
- Offsets specified in bytes (cryptsetup: 512 byte sectors)
- Passphrase hash algorithm needs specifying
- Also note that because plain dm-crypt and loop-aes format does not
- have metadata, and while the loopAES extension for cryptsetup tries
- autodetection (see command loopaesOpen), it may not always work.
- If you still have the old set-up, using a verbosity option (-v)
- on mapping with the old tool or having a look into the system logs
- after setup could give you the information you need. Below, there
- are also some things that worked for somebody.
+ Also note that because plain dm-crypt and loop-aes format does not have
+ metadata, and while the loopAES extension for cryptsetup tries
+ autodetection (see command loopaesOpen), it may not always work. If you
+ still have the old set-up, using a verbosity option (-v) on mapping with
+ the old tool or having a look into the system logs after setup could
+ give you the information you need. Below, there are also some things
+ that worked for somebody.
- * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32
+ * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32
In this case, the main problem seems to be that this variant of
- losetup takes the offset (-o option) in bytes, while cryptsetup
- takes it in sectors of 512 bytes each. Example: The losetup command
+ losetup takes the offset (-o option) in bytes, while cryptsetup takes
+ it in sectors of 512 bytes each.
+
+ Example: The losetup command
+
+ losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
+ mount /dev/loop0 mount-point
- losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
- mount /dev/loop0 mount-point
-
translates to
- cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
- mount /dev/mapper/e1 mount-point
-
+ cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
+ mount /dev/mapper/e1 mount-point
- * 7.4 loop-aes with 160 bit key
- This seems to be sometimes used with twofish and blowfish and
- represents a 160 bit ripemed160 hash output padded to 196 bit key
- length. It seems the corresponding options for cryptsetup are
- --cipher twofish-cbc-null -s 192 -h ripemd160:20
-
+ * 7.4 loop-aes with 160 bit key
- * 7.5 loop-aes v1 format OpenSUSE
+ This seems to be sometimes used with twofish and blowfish and represents
+ a 160 bit ripemed160 hash output padded to 196 bit key length. It seems
+ the corresponding options for cryptsetup are
- Apparently this is done by older OpenSUSE distros and stopped
- working from OpenSUSE 12.1 to 12.2. One user had success with the
- following:
+ --cipher twofish-cbc-null -s 192 -h ripemd160:20
- cryptsetup create <target> <device> -c aes -s 128 -h sha256
-
- * 7.6 Kernel encrypted loop device (cryptoloop)
+
+ * 7.5 loop-aes v1 format OpenSUSE
+
+ Apparently this is done by older OpenSUSE distros and stopped working
+ from OpenSUSE 12.1 to 12.2. One user had success with the following:
+
+ cryptsetup create <target> <device> -c aes -s 128 -h sha256
+
+
+
+ * 7.6 Kernel encrypted loop device (cryptoloop)
There are a number of different losetup implementations for using
encrypted loop devices so getting this to work may need a bit of
Example for a compatible mapping:
losetup -e twofish -N /dev/loop0 /image.img
-
+
translates to
cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain
-
+
with the mapping being done to /dev/mapper/image_plain instead of
to /dev/loop0.
More details:
- Cipher, mode and pasword hash (or no hash):
+ Cipher, mode and password hash (or no hash):
-e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256]
-e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256]
-
- Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512
- bytes):
+
+
+ Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512 bytes):
-k 128 => -s 128
-o 2560 => -o 5 -p 5 # 2560/512 = 5
-
+
+
There is no replacement for --pass-fd, it has to be emulated using
keyfiles, see the cryptsetup man-page.
-8. Issues with Specific Versions of cryptsetup
+8. Issues with Specific Versions of cryptsetup
- * 8.1 When using the create command for plain dm-crypt with
- cryptsetup 1.1.x, the mapping is incompatible and my data is not
- accessible anymore!
+ * 8.1 When using the create command for plain dm-crypt with
+ cryptsetup 1.1.x, the mapping is incompatible and my data is not
+ accessible anymore!
With cryptsetup 1.1.x, the distro maintainer can define different
- default encryption modes for LUKS and plain devices. You can check
- these compiled-in defaults using "cryptsetup --help". Moreover, the
- plain device default changed because the old IV mode was
- vulnerable to a watermarking attack.
+ default encryption modes. You can check the compiled-in defaults using
+ "cryptsetup --help". Moreover, the plain device default changed because
+ the old IV mode was vulnerable to a watermarking attack.
If you are using a plain device and you need a compatible mode, just
- specify cipher, key size and hash algorithm explicitly. For
- compatibility with cryptsetup 1.0.x defaults, simple use the
- following:
+ specify cipher, key size and hash algorithm explicitly. For
+ compatibility with cryptsetup 1.0.x defaults, simple use the following:
cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
-
+
LUKS stores cipher and mode in the metadata on disk, avoiding this
problem.
- * 8.2 cryptsetup on SLED 10 has problems...
+ * 8.2 cryptsetup on SLED 10 has problems...
- SLED 10 is missing an essential kernel patch for dm-crypt, which
- is broken in its kernel as a result. There may be a very old
- version of cryptsetup (1.0.x) provided by SLED, which should also
- not be used anymore as well. My advice would be to drop SLED 10.
+ SLED 10 is missing an essential kernel patch for dm-crypt, which is
+ broken in its kernel as a result. There may be a very old version of
+ cryptsetup (1.0.x) provided by SLED, which should also not be used
+ anymore as well. My advice would be to drop SLED 10.
-9. References and Further Reading
+ * 8.3 Gcrypt 1.6.x and later break Whirlpool
+ It is the other way round: In gcrypt 1.5.x, Whirlpool is broken and it
+ was fixed in 1.6.0 and later. If you selected whirlpool as hash on
+ creation of a LUKS container, it does not work anymore with the fixed
+ library. This shows one serious risk of using rarely used settings.
- * Purpose of this Section
+ Note that at the time this FAQ item was written, 1.5.4 was the latest
+ 1.5.x version and it has the flaw, i.e. works with the old Whirlpool
+ version. Possibly later 1.5.x versions will work as well. If not,
+ please let me know.
- The purpose of this section is to collect references to all
- materials that do not fit the FAQ but are relevant in some fashion.
- This can be core topics like the LUKS spec or disk encryption, but
- it can also be more tangential, like secure storage management or
- cryptography used in LUKS. It should still have relevance to
- cryptsetup and its applications.
+ The only two ways to access older LUKS containers created with Whirlpool
+ are to either decrypt with an old gcrypt version that has the flaw or to
+ use a compatibility feature introduced in cryptsetup 1.6.4 and gcrypt
+ 1.6.1 or later. Version 1.6.0 cannot be used.
- If you wan to see something added here, send email to the
- maintainer (or the cryptsetup mailing list) giving an URL, a
- description (1-3 lines preferred) and a section to put it in. You
- can also propose new sections.
+ Steps:
- At this time I would like to limit the references to things that
- are available on the web.
+ - Make at least a header backup or better, refresh your full backup.
+ (You have a full backup, right? See Item 6.1 and following.)
+ - Make sure you have cryptsetup 1.6.4 or later and check the gcrypt
+ version:
- * Specifications
+ cryptsetup luksDump <your luks device> --debug | grep backend
- - LUKS on-disk format spec:
- http://code.google.com/p/cryptsetup/wiki/Specification
+ If gcrypt is at version 1.5.x or before:
+ - Reencrypt the LUKS header with a different hash. (Requires entering
+ all keyslot passphrases. If you do not have all, remove the ones you
+ do not have before.):
- * Code Examples
+ cryptsetup-reencrypt --keep-key --hash sha256 <your luks device>
- - Some code examples are in the source package under docs/examples
+ If gcrypt is at version 1.6.1 or later:
+
+ - Patch the hash name in the LUKS header from "whirlpool" to
+ "whirlpool_gcryptbug". This activates the broken implementation.
+ The detailed header layout is in Item 6.12 of this FAQ and in the
+ LUKS on-disk format specification. One way to change the hash is
+ with the following command:
+
+ echo -n -e 'whirlpool_gcryptbug\0' | dd of=<luks device> bs=1 seek=72 conv=notrunc
+
+ - You can now open the device again. It is highly advisable to change
+ the hash now with cryptsetup-reencrypt as described above. While you
+ can reencrypt to use the fixed whirlpool, that may not be a good idea
+ as almost nobody seems to use it and hence the long time until the
+ bug was discovered.
+
+
+9. The Initrd question
- * Brute-forcing passphrases
+ * 9.1 My initrd is broken with cryptsetup
- -
- http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
+ That is not nice! However the initrd is supplied by your distribution,
+ not by the cryptsetup project and hence you should complain to them. We
+ cannot really do anything about it.
- -
- http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
+ * 9.2 CVE-2016-4484 says cryptsetup is broken!
- * Tools
+ Not really. It says the initrd in some Debian versions have a behavior
+ that under some very special and unusual conditions may be considered
+ a vulnerability.
+ What happens is that you can trick the initrd to go to a rescue-shell if
+ you enter the LUKS password wrongly in a specific way. But falling back
+ to a rescue shell on initrd errors is a sensible default behavior in the
+ first place. It gives you about as much access as booting a rescue
+ system from CD or USB-Stick or as removing the disk would give you. So
+ this only applies when an attacker has physical access, but cannot boot
+ anything else or remove the disk. These will be rare circumstances
+ indeed, and if you rely on the default distribution initrd to keep you
+ safe under these circumstances, then you have bigger problems than this
+ somewhat expected behavior.
- * SSD and Flash Disk Related
+ The CVE was exagerrated and should not be assigned to upstream
+ cryptsetup in the first place (it is a distro specific initrd issue).
+ It was driven more by a try to make a splash for self-aggrandizement,
+ than by any actual security concerns. Ignore it.
- * Disk Encryption
+ * 9.3 How do I do my own initrd with cryptsetup?
+
+ Note: The instructions here apply to an initrd in initramfs format, not
+ to an initrd in initrd format. The latter is a filesystem image, not a
+ cpio-archive, and seems to not be widely used anymore.
+
+ It depends on the distribution. Below, I give a very simple example and
+ step-by-step instructions for Debian. With a bit of work, it should be
+ possible to adapt this to other distributions. Note that the
+ description is pretty general, so if you want to do other things with an
+ initrd it provides a useful starting point for that too.
+
+ 01) Unpacking an existing initrd to use as template
+
+ A Linux initrd is in gzip'ed cpio format. To unpack it, use something
+ like this:
+
+ md tmp; cd tmp; cat ../initrd | gunzip | cpio -id
+
+ After this, you have the full initrd content in tmp/
+
+ 02) Inspecting the init-script
+
+ The init-script is the only thing the kernel cares about. All activity
+ starts there. Its traditional location is /sbin/init on disk, but /init
+ in an initrd. In an initrd unpacked as above it is tmp/init.
+
+ While init can be a binary despite usually being called "init script",
+ in Debian the main init on the root partition is a binary, but the init
+ in the initrd (and only that one is called by the kernel) is a script
+ and starts like this:
+
+ #!/bin/sh
+ ....
+
+ The "sh" used here is in tmp/bin/sh as just unpacked, and in Debian it
+ currently is a busybox.
+
+ 03) Creating your own initrd
+
+ The two examples below should give you most of what is needed. This is
+ tested with LUKS1 and should work with LUKS2 as well. If not, please
+ let me know.
+
+ Here is a really minimal example. It does nothing but set up some
+ things and then drop to an interactive shell. It is perfect to try out
+ things that you want to go into the init-script.
+
+ #!/bin/sh
+ export PATH=/sbin:/bin
+ [ -d /sys ] || mkdir /sys
+ [ -d /proc ] || mkdir /proc
+ [ -d /tmp ] || mkdir /tmp
+ mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
+ mount -t proc -o nodev,noexec,nosuid proc /proc
+ echo "initrd is running, starting BusyBox..."
+ exec /bin/sh --login
+
+
+ Here is an example that opens the first LUKS-partition it finds with the
+ hard-coded password "test2" and then mounts it as root-filesystem. This
+ is intended to be used on an USB-stick that after boot goes into a safe,
+ as it contains the LUKS-passphrase in plain text and is not secure to be
+ left in the system. The script contains debug-output that should make it
+ easier to see what is going on. Note that the final hand-over to the init
+ on the encrypted root-partition is done by "exec switch_root /mnt/root
+ /sbin/init", after mounting the decrypted LUKS container with "mount
+ /dev/mapper/c1 /mnt/root". The second argument of switch_root is relative
+ to the first argument, i.e. the init started with this command is really
+ /mnt/sbin/init before switch_root runs.
+
+ #!/bin/sh
+ export PATH=/sbin:/bin
+ [ -d /sys ] || mkdir /sys
+ [ -d /proc ] || mkdir /proc
+ [ -d /tmp ] || mkdir /tmp
+ mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
+ mount -t proc -o nodev,noexec,nosuid proc /proc
+ echo "detecting LUKS containers in sda1-10, sdb1-10"; sleep 1
+ for i in a b
+ do
+ for j in 1 2 3 4 5 6 7 8 9 10
+ do
+ sleep 0.5
+ d="/dev/sd"$i""$j
+ echo -n $d
+ cryptsetup isLuks $d >/dev/null 2>&1
+ r=$?
+ echo -n " result: "$r""
+ # 0 = is LUKS, 1 = is not LUKS, 4 = other error
+ if expr $r = 0 > /dev/null
+ then
+ echo " is LUKS, attempting unlock"
+ echo -n "test2" | cryptsetup luksOpen --key-file=- $d c1
+ r=$?
+ echo " result of unlock attempt: "$r""
+ sleep 2
+ if expr $r = 0 > /dev/null
+ then
+ echo "*** LUKS partition unlocked, switching root ***
+ echo " (waiting 30 seconds before doing that)"
+ mount /dev/mapper/c1 /mnt/root
+ sleep 30
+ exec switch_root /mnt/root /sbin/init
+ fi
+ else
+ echo " is not LUKS"
+ fi
+ done
+ done
+ echo "FAIL finding root on LUKS, loading BusyBox..."; sleep 5
+ exec /bin/sh --login
+
+
+ 04) What if I want a binary in the initrd, but libraries are missing?
+
+ That is a bit tricky. One option is to compile statically, but that
+ does not work for everything. Debian puts some libraries into lib/ and
+ lib64/ which are usually enough. If you need more, you can add the
+ libraries you need there. That may or may not need a configuration
+ change for the dynamic linker "ld" as well. Refer to standard Linux
+ documentation on how to add a library to a Linux system. A running
+ initrd is just a running Linux system after all, it is not special in
+ any way.
+
+ 05) How do I repack the initrd?
+
+ Simply repack the changed directory. While in tmp/, do
+ the following:
+ ```
+ find . | cpio --create --format='newc' | gzip > ../new_initrd
+ ```
+ Rename "new_initrd" to however you want it called (the name of
+ the initrd is a kernel-parameter) and move to /boot. That is it.
+
+
+10. LUKS2 Questions
+
+
+ * 10.1 Is the cryptography of LUKS2 different?
+
+ Mostly not. The header has changed in its structure, but the
+ crytpgraphy is the same. The one exception is that PBKDF2 has been
+ replaced by Argon2 to give better resilience against attacks attacks by
+ graphics cards and other hardware with lots of computing power but
+ limited local memory per computing element.
+
+
+ * 10.2 What new features does LUKS2 have?
+
+ There are quite a few. I recommend reading the man-page and the on-disk
+ format specification, see Item 1.2.
+
+ To list just some:
+ - A lot of the metadata is JSON, allowing for easier extension
+ - Max 32 key-slots per default
+ - Better protection for bad passphrases now available with Argon2
+ - Authenticated encryption
+ - The LUKS2 header is less vulnerable to corruption and has a 2nd copy
+
+
+ * 10.3 Why does LUKS2 need so much memory?
+
+ LUKS2 uses Argon2 instead of PBKDF2. That causes the increase in memory.
+ See next item.
+
+
+ * 10.4 Why use Argon2 in LUKS 2 instead of PBKDF2?
+
+ LUKS tries to be secure with not-so-good passwords. Bad passwords need to
+ be protected in some way against an attacker that just tries all possible
+ combinations. (For good passwords, you can just wait for the attacker to
+ die of old age...) The situation with LUKS is not quite the same as with a
+ password stored in a database, but there are similarities.
+
+ LUKS does not store passwords on disk. Instead, the passwords are used to
+ decrypt the master-key with it and that one is stored on disk in encrypted
+ form. If you have a good password, with, say, more than 80 bits of
+ entropy, you could just put the password through a single crypto-hash (to
+ turn it into something that can be used as a key) and that would be secure.
+ This is what plain dm-crypt does.
+
+ If the password has lower entropy, you want to make this process cost some
+ effort, so that each try takes time and resources and slows the attacker
+ down. LUKS1 uses PBKDF2 for that, adding an iteration count and a salt.
+ The iteration count is per default set to that it takes 1 second per try on
+ the CPU of the device where the respective passphrase was set. The salt is
+ there to prevent precomputation.
+
+ The problem with that is that if you use a graphics card, you can massively
+ speed up these computations as PBKDF2 needs very little memeory to compute
+ it. A graphics card is (grossly simplified) a mass of small CPUs with some
+ small very fast local memory per CPU and a large slow memory (the 4/6/8 GB
+ a current card may have). If you can keep a computation in the small,
+ CPU-local memory, you can gain a speed factor of 1000 or more when trying
+ passwords with PBKDF2.
+
+ Argon2 was created to address this problem. It adds a "large memory
+ property" where computing the result with less memory than the memory
+ parameter requires is massively (exponentially) slowed down. That means,
+ if you set, for example, 4GB of memory, computing Argon2 on a graphics card
+ with around 100kB of memory per "CPU" makes no sense at all because it is
+ far too slow. An attacker has hence to use real CPUs and furthermore is
+ limited by main memory bandwith.
+
+ Hence the large amount of memory used is a security feature and should not
+ be turned off or reduced. If you really (!) understand what you are doing
+ and can assure good passwords, you can either go back to PBKDF2 or set a
+ low amount of memory used for Argon2 when creating the header.
+
+
+ * 10.5 LUKS2 is insecure! It uses less memory than the Argon2 RFC say!
+
+ Well, not really. The RFC recommends 6GiB of memory for use with disk
+ encryption. That is a bit insane and something clearly went wrong in the
+ standardization process here. First, that makes Argon2 unusable on any 32
+ bit Linux and that is clearly a bad thing. Second, there are many small
+ Linux devices around that do not have 6GiB of RAM in the first place. For
+ example, the current Raspberry Pi has 1GB, 2GB or 4GB of RAM, and with the
+ RFC recommendations, none of these could compute Argon2 hashes.
+
+ Hence LUKS2 uses a more real-world approach. Iteration is set to a
+ minimum of 4 because there are some theoretical attacks that work up to an
+ iteration count of 3. The thread parameter is set to 4. To achieve 2
+ second/slot unlock time, LUKS2 adjusts the memory parameter down if
+ needed. In the other direction, it will respect available memory and not
+ exceed it. On a current PC, the memory parameter will be somewhere around
+ 1GB, which should quite generous. The minimum I was able to set in an
+ experiment with "-i 1" was 400kB of memory and that is too low to be
+ secure. A Raspberry Pi would probably end up somewhere around 50MB (have
+ not tried it) and that should still be plenty.
+
+ That said, if you have a good, high-entropy passphrase, LUKS2 is secure
+ with any memory parameter.
+
+
+ * 10.6 How does re-encryption store data while it is running?
+
+ All metadata necessary to perform a recovery of said segment (in case of
+ crash) is stored in the LUKS2 metadata area. No matter if the LUKS2
+ reencryption was run in online or offline mode.
+
+
+ * 10.7 What do I do if re-encryption crashes?
+
+ In case of a reencryption application crash, try to close the original
+ device via following command first:
+
+ cryptsetup close <my_crypt_device>.
+ Cryptsetup assesses if it's safe to teardown the reencryption device stack
+ or not. It will also cut off I/O (via dm-error mapping) to current
+ hotzone segment (to make later recovery possible). If it can't be torn
+ down, i.e. due to a mounted fs, you must unmount the filesystem first.
+ Never try to tear down reencryption dm devices manually using e.g.
+ dmsetup tool, at least not unless cryptsetup says it's safe to do so. It
+ could damage the data beyond repair.
- * Attacks Against Disk Encryption
+ * 10.8 Do I need to enter two passphrases to recover a crashed
+ re-encryption?
- * Risk Management as Relevant for Disk Encryption
+ Cryptsetup (command line utility) expects the passphrases to be identical
+ for the keyslot containing old volume key and for the keyslot containing
+ new one. So the recovery happens during normal the "cryptsetup open"
+ operation or the equivalent during boot.
+ Re-encryption recovery can be also performed in offline mode by
+ the "cryptsetup repair" command.
- * Cryptography
+ * 10.9 What is an unbound keyslot and what is it used for?
- * Secure Storage
+ Quite simply, an 'unbound key' is an independent 'key' stored in a luks2
+ keyslot that cannot be used to unlock a LUKS2 data device. More specifically,
+ an 'unbound key' or 'unbound luks2 keyslot' contains a secret that is not
+ currently associated with any data/crypt segment (encrypted area) in the
+ LUKS2 'Segments' section (displayed by luksDump).
- A. Contributors In no particular order:
+ This is a bit of a more general idea. It basically allows to use a keyslot
+ as a container for a key to be used in other things than decrypting a
+ data segment.
+
+ As of April 2020, the following uses are defined:
+
+ 1) LUKS2 re-encryption. The new volume key is stored in an unbound keyslot
+ which becomes a regular LUKS2 keyslot later when re-encryption is
+ finished.
+
+ 2) Somewhat similar is the use with a wrapped key scheme (e.g. with the
+ paes cipher). In this case, the VK (Volume Key) stored in a keyslot
+ is an encrypted binary binary blob. The KEK (Key Encryption Key) for
+ that blob may be refreshed (Note that this KEK is not managed by
+ cryptsetup!) and the binary blob gets changed. The KEK refresh process
+ uses an 'unbound keyslot'. First the future effective VK is placed
+ in the unbound keyslot and later it gets turned into the new real VK
+ (and bound to the respective crypt segment).
+
+
+ * 10.10 What about the size of the LUKS2 header?
+
+ While the LUKS1 header has a fixed size that is determined by the cipher
+ spec (see Item 6.12), LUKS2 is more variable. The default size is 16MB,
+ but it can be adjusted on creation by using the --luks2-metadata-size
+ and --luks2-keyslots-size options. Refer to the man-page for details.
+ While adjusting the size in an existing LUKS2 container is possible,
+ it is somewhat complicated and risky. My advice is to do a backup,
+ recreate the container with changed parameters and restore that backup.
+
+
+ * 10.11 Does LUKS2 store metadata anywhere except in the header?
+
+ It does not. But note that if you use the experimental integrity support,
+ there will be an integrity header as well at the start of the data area
+ and things get a bit more complicated. All metadata will still be at the
+ start of the device, nothing gets stored somewhere in the middle or at
+ the end.
+
+
+11. References and Further Reading
+
+ * Purpose of this Section
+
+ The purpose of this section is to collect references to all materials
+ that do not fit the FAQ but are relevant in some fashion. This can be
+ core topics like the LUKS spec or disk encryption, but it can also be
+ more tangential, like secure storage management or cryptography used in
+ LUKS. It should still have relevance to cryptsetup and its
+ applications.
+
+ If you want to see something added here, send email to the maintainer
+ (or the cryptsetup mailing list) giving an URL, a description (1-3 lines
+ preferred) and a section to put it in. You can also propose new
+ sections.
+
+ At this time I would like to limit the references to things that are
+ available on the web.
+
+ * Specifications
+
+ - LUKS on-disk format spec: See Item 1.2
+
+ * Other Documentation
+
+ - Arch Linux on LUKS, LVM and full-disk encryption:
+ https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system
+
+ * Code Examples
+
+ - Some code examples are in the source package under docs/examples
+
+ - LUKS AF Splitter in Ruby by John Lane: https://rubygems.org/gems/afsplitter
+
+ * Brute-forcing passphrases
+
+ - http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
+
+ - http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
+
+ * Tools
+
+ * SSD and Flash Disk Related
+
+ * Disk Encryption
+
+ * Attacks Against Disk Encryption
+
+ * Risk Management as Relevant for Disk Encryption
+
+ * Cryptography
+
+ * Secure Storage
+
+
+A. Contributors
+In no particular order:
- Arno Wagner
- Milan Broz
+___
projects / platform / upstream / cryptsetup.git / blobdiff