6. Backup and Data Recovery
7. Interoperability with other Disk Encryption Tools
8. Issues with Specific Versions of cryptsetup
+9. References and Further Reading
A. Contributors
1. General Questions
- * What is this?
+ * 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,
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
+ https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions
- * WARNINGS
+ * 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
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 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
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
PASSPHRASE CHARACTER SET: Some people have had difficulties with
this when upgrading distributions. It is highly advisable to only
- use the 94 printable characters from the first 128 characters of
+ 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 caracters can, e.g. be found on
+ 128 ASCII characters can, e.g. be found on
http://en.wikipedia.org/wiki/ASCII
- * System Specific warnings
+ * 1.3 System specific warnings
- Ubuntu as of 4/2011: It seems the installer offers to create
LUKS partitions in a way that several people mistook for an offer
This issue has been acknowledged by the Ubuntu dev team, see here:
http://launchpad.net/bugs/420080
+ 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.
+
+
+ * 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
+ done only after the initial problem.
+
+ - Do not reboot. The keys mays 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.
+
+ - Relax some more.
+
+ - Read section 6 of this FAQ.
- * Who wrote this?
+ - Ask on the mailing-list if you need more help.
- Current FAQ maintainer is Arno Wagner <arno@wagner.name>. 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.
+
+ * 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.
+
+ 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
least problems.
- * Where is the project website?
+ * 1.6 Where is the project website?
- There is the project website at http://code.google.com/p/cryptsetup/
+ 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.
- * 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
http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
+ * 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.
+
+ 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.
+
+ 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):
+
+ 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
- * What is the difference between "plain" and LUKS format?
+ * 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.
+ encrypted disk, partition or loop-file.
+
+ 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
+ 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.
+
+ To just quickly wipe file systems (old data may remain), use
+
+ 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 | 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.
+
+ Alternatively, plain cm-crypt can be used for a very fast wipe with
+ crypto-grade randomness, see Item 2.19
+
+ 04) Create the LUKS container:
+ cryptsetup luksFormat <target device>
+
+ Just follow the on-screen instructions.
+
+ 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
+
+ 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
+
+ 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.
+
+ 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.
+
+
+ * 2.2 LUKS on partitions or raw disks?
+
+ 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 do so 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 permanently 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. Be aware that you
+ lose autodetection with them and have to fall back to some
+ user-space script to do it.
+
+ 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 move that to
+ additional encrypted partitions. If you really are concerned your
+ root partition may be sabotaged by somebody with physical access
+ (that 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
+ 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.
+
+
+ * 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, if 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-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.
+
+ 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.
+ Manual start would look like this:
+
+ /etc/init.d/crypdisks start
+ swapon /dev/mapper/swap
+
+
+ * 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.
Plain format is just that: It has no metadata on disk, reads all
- paramters from the commandline (or the defaults), derives a
+ 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.
Side-note: That has limited value against the authorities. In
civilized countries, they cannot force you to give up a crypto-key
- anyways. In the US, the UK and dictatorships around the world,
- they can force you to give up the keys (using imprisonment or worse
- to pressure you), and in the worst case, they only need a
- nebulous "suspicion" about the presence of encrypted data. My
- advice is to either be ready to give up the keys or to not have
+ 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.
+ 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
properties like key-slot diffusion and salts, etc..
LUKS format uses a metadata header and 8 key-slot areas that are
- being placed ath the begining of the disk, see below under "What
+ 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 decryt a single master key that is stored in the anti-forensic
+ to decrypt a single master key that is stored in the anti-forensic
stripes.
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 passhrases, and others.
+ 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
non-default XTS mode).
- * Can I encrypt an already existing, non-empty partition to use LUKS?
+ * 2.5 Can I encrypt an already 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
to be in a filesystem.
- * 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
To unmap the file when done, use "losetup -d /dev/loop0".
- * When I add a new key-slot to LUKS, it asks for a passphrase but
+ * 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?
new key-slot.
- * Encrytion on top of RAID or the other way round?
+ * 2.8 Encryption on top of RAID or the other way round?
Unless you have special needs, place encryption between RAID and
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
- pasphrase for each individual disk and RAID autotetection will not
- work anymore. Therefore it is better to encrypt the RAID device,
- e.g. /dev/dm0 .
+ 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 .
+
+ This means that the typical layering looks like this:
+
+ Filesystem <- top
+ |
+ Encryption
+ |
+ RAID
+ |
+ Raw partitions
+ |
+ 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.
- * How do I read a dm-crypt key from file?
+ * 2.9 How do I read a dm-crypt key from file?
- Note that the file will still be hashed first, just like keyboard
- input. Use the --key-file option, like this:
+ 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.
+
- * 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
cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
- * How do I read the LUKS master key from file?
+ * 2.11 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
do this here.
- * What are the security requirements for a key read from file?
+ * 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
head -c 256 /dev/random > keyfile
- * 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?
- As far as I know it does (but I may be wrong), but please note that
- these "guarantees" are far weaker than they appear to be. For
- example, you may not get a hard flush to disk surface even on a
- call to fsync. In addition, the HDD itself may do independent
- write reordering. 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 should not make
- things worse.
+ 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:
- Personally, I have several instances of ext3 on dm-crypt and have
- not noticed any specific problems.
+ http://lwn.net/Articles/400541/
- Update: I did run into frequent small freezes (1-2 sec) when putting
- a vmware image on ext3 over dm-crypt. This does indicate that the
- transactional guarantees are in place, but at a cost. When I went
- back to ext2, the problem went away. This also seems to have gotten
- better with kernel 2.6.36 and the reworking of filesystem flush
- locking. Kernel 2.6.38 is expected to have more improvements here.
+ 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.
- * Can I use LUKS or cryptsetup with a more secure (external) medium
- for key storage, e.g. TPM or a smartcard?
+
+ * 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
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
+ related to the cryptsetup project.
+
- * Can I resize a dm-crypt or LUKS partition?
+ * 2.15 Can I resize a dm-crypt or LUKS partition?
Yes, you can, as neither dm-crypt nor LUKS stores partition size.
Whether you should is a different question. Personally I recommend
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 typicaly works. This
+ resize an encrypted partition is slow, but typically works. This
will not change the size of the filesystem hidden under the
encryption though.
for that.
+ * 2.16 How do I Benchmark the Ciphers, Hashes and Modes?
+
+ 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.
+
+
+ * 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.
+
+ 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.
+
+
+ * 2.19 How can I wipe a device with crypto-grade randomness?
+
+ The conventional recommendation if you want to not just do a
+ zero-wipe is to use something like
+
+ cat /dev/urandom > <taget-device>
+
+ That is very slow and painful at 10-20MB/s on a fast computer.
+ Using cryptsetup and a plain dm-crypt device with a random key, it
+ is much faster and gives you 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/<block-device> to_be_wiped
+
+ Then you have several options. 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
+
+ Remove the mapping at the end and you are done.
+
+
3. Common Problems
- * My dm-crypt/LUKS mapping does not work! What general steps are
+ * 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
the kernel. The output of "cat /proc/crypto" needs to list them.
- * 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
cryptsetup".
- * 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
version gets called.
- * Unlocking a LUKS device takes very long. Why?
+ * 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
matter.
- * "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device.
- What is wrong?
+ * 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
cryptsetup luksHeaderRestore --header-backup-file <file> <device>
- * cryptsetup segfaults on Gentoo amd64 hardened ...
+ * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
- There seems to be some inteference between the hardening and and
+ 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
4. Troubleshooting
- * Can a bad RAM module cause problems?
+ * 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.
+
+
+ * 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.
+
+ 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?
LUKS and dm-crypt can give the RAM quite a workout, especially when
combined with software RAID. In particular the combination RAID5 +
did a verify.
- * How do I test RAM?
+ * 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
- Run Memtest86+ for one cycle
- - Run memterster for one cycle (shut down as many other applications
+ - Run memtester for one cycle (shut down as many other applications
as possible)
- Run Memtest86+ for 24h or more
5. Security Aspects
- * Is LUKS insecure? Everybody can see I have encrypted data!
+ * 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. 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.
+
+ 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
+ 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''.
+
+ The table above then becomes:
+
+ Passphrase entropy Cost to break
+ 50 bit EUR/USD 600k
+ 55 bit EUR/USD 20M
+ 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 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:
+
+ 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.
+
+ 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
difference between "plain" and LUKS format?"
- * Should I initialize (overwrite) a new LUKS/dm-crypt partition?
+ * 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
dd if=/dev/zero of=/dev/mapper/e1
- * How do I securely erase a LUKS (or other) partition?
+ * 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 diferent offset stems from
+ 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
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 reseach results on erasing SSDs and FLASH
- drives:
+ 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.
dd_rescue -w /dev/zero /dev/sde1
- * How do I securely erase a backup of a LUKS partition or header?
+ * 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
lead to data not actually being deleted at all during overwrites.
- * What about backup? Does it compromise security?
+ * 5.6 What about backup? Does it compromise security?
- That depends. See next section.
+ That depends. See item 6.7.
- * Why is all my data permanently gone if I overwrite the LUKS header?
+ * 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.
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
+ The LUKS 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)
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".
+ 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 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".
- * What is a "salt"?
+ * 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
infeasible.
- * Is LUKS secure with a low-entropy (bad) passphrase?
+ * 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
passphrase, see next FAQ item.
Still, if you want good security, a high-entropy passphrase is the
- only option. 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.
+ 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
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 tests costs
+ 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
this is good passphrase material.
- * What is "iteration count" and why is decreasing it a bad idea?
+ * 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
CPU, and possibly far less.
In addition, the attacker can both parallelize and use special
- hardware like GPUs 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
+ 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
+ even without solid proof that the decryption will yield something
useful.
The numbers above are mostly made up, but show the idea. Of course
this danger significantly.
- * What about iteration count with plain dm-crypt?
+ * 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.
- * Is LUKS with default parameters less secure on a slow CPU?
+ * 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
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-startved situation (e.g. automatic
+ key generation in an entropy-starved situation (e.g. automatic
installation on an embedded device without network and other entropy
sources).
compensate for problems in front of the keyboard.
- * Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
+ * 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
knowing the encryption key and the watermarking attack fails.
- * Are there any problems with "plain" IV? What is "plain64"?
+ * 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.
does not cause any performance penalty compared to "plain".
- * What about XTS mode?
+ * 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
+ 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>
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
+ 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
+ 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.
+
+ 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.
+
+ 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 truly erase the magnetic part,
+ but only mark the SSD/Flash part as erased while data is still in
+ there.
+
+ 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 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 disk, but otherwise you get LUKS with the same
+ security as on a magnetic disk.
+
+ 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 LUKS 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 LUKS
+ 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.
+
+
+ * 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 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.
+
+
6. Backup and Data Recovery
- * Why do I need Backup?
+ * 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
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 occuurence. For example an
+ 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 inacessible. See more below on LUKS
+ LUKS partition permanently inaccessible. See more below on LUKS
header damage.
So, data-backup in some form is non-optional. For LUKS, you may
only needs an update if you change passphrases.
- * How do I backup a LUKS header?
+ * 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
partition creation. Example:
- cryptsetup luksHeaderBackup --header-backup-file h /dev/mapper/c1
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
To restore, use the inverse command, i.e.
- cryptsetup luksHeaderRestore --header-backup-file h /dev/mapper/c1
+ 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.
- * How do I backup a LUKS or dm-crypt partition?
+
+ * 6.3 How do I test 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
+
+ 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?
There are two options, a sector-image and a plain file or
filesystem backup of the contents of the partition. The sector
cat backup.tbz2.gpg | gpg - | tar djf -
- Note: Allways verify backups, especially encrypted ones.
+ Note: Always verify backups, especially encrypted ones!
+
+ 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
unencrypted, turn off swap if it is not encrypted before doing the
backup.
+ 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.
+
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 filestem-level backup that can be used when
+ 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.
- * Do I need a backup of the full partition? Would the header and
+ * 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
backup to protect against this case.
- * *What do I need to backup if I use "decrypt_derived"?
+ * *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.
device with the new LUKS header.
- * 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.
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 methid that encrypts the whole backup in
+ 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
is under your control...)
- * 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.
+ in the key-slot descriptors of the header 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.
- * 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
- irretrivable, unless you have a LUKS header backup. You may also
+ irretrievable, unless you have a LUKS header backup. You may also
damage the key-slots in part or in full. See also last item.
- * How do I recover the master key from a mapped LUKS container?
+ * 6.10 How do I recover the master key from a mapped LUKS container?
This is typically only needed if you managed to damage your LUKS
- header, but the container is still mapped, i.e. "luksOpen"ed.
+ 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
key afterwards. Changing the master key requires a full data
backup, luksFormat and then restore of the backup.
- First, there is a script by Milan that tries to automatize the
- whole process, including generating a new LUKS header with the old
- master key:
+ 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):
-http://code.google.com/p/cryptsetup/source/browse/trunk/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:
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
+ echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
- - Do a luksFormat to create a new LUKS header. Unmapthe device
- before you do that (luksClose). Replace \dev\dsa10 with the device
- the LUKS container is on:
+ - Do a luksFormat to create a new LUKS header.
- cryptsetup luksFormat --master-key-file=master_key \dev\sda10
+ 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>
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, just
- do the first step, keep the whole result safe and try with 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.
+ - 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.
- * What does the on-disk structure of dm-crypt look like?
+
+ * 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
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
+ dm-crypt device, the damage will be limited to the area you
overwrote.
- * What does the on-disk structure of LUKS look like?
+ * 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
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. Start of
- bulk data is at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB
- + 4096 bytes from the start of the partition. This is also the
- value given by command "luksDump" with "Payload offset: 2056",
- just multiply by the sector size (512 bytes). Incidentally,
- "luksHeaderBackup" for a LUKS container created with default
- parameters dumps exactly the first 1'052'672 bytes to file and
- "luksHeaderRestore" restores them.
+ "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. 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.
+ 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
+ https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
+
+ For your convenience, here is the LUKS 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 LUKS On-Disk Format Specification Version 1.2.1
+LUKS 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 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.
+
+ 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.
+ Example 1 - AES 128 bit with CBC:
- * I think this is overly complicated. Is there an alternative?
+ 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
+ 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.
+
+
+ * 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
borders).
+ * 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. 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.
+
+ 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.
+
+
7. Interoperability with other Disk Encryption Tools
- * 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 somtimes works and sometimes does not. This section
- collects insights into what works, what does not and where more
- information is required.
+ 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 intertested, please email the FAQ maintainer.
+ audience may be interested, please email the FAQ maintainer.
- * 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
from cryptsetup option syntax include:
- - Offsets specifed in bytes (cryptsetup: 512 byte sectors)
+ - Offsets specified in bytes (cryptsetup: 512 byte sectors)
- The need to specify an IV offset
- Passphrase hash algorithm needs specifying
Also note that because plain dm-crypt and loop-aes format does not
- have metadata, autodetection, while feasible in most cases, would
- be a lot of work that nobody really wants to do. 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.
+ 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.
- * 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 losetupp
- command
+ takes it in sectors of 512 bytes each. Example: The losetup command
losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
- mount /dev/loop0 mountpoint
+ mount /dev/loop0 mount-point
translates to
cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
- mount /dev/mapper/e1 mountpoint
+ mount /dev/mapper/e1 mount-point
- * loop-aes with 160 bit key
+ * 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
--cipher twofish-cbc-null -s 192 -h ripemd160:20
+ * 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
+ experimentation.
+
+ NOTE: Do NOT use this for new containers! Some of the existing
+ implementations are insecure and future support is uncertain.
+
+ 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):
+
+ -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):
+
+ -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
- * 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
problem.
- * 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.
+
+ * 8.3 Gcrypt after 1.5.3 breaks Whirlpool
+
+ It is the other way round: In gcrypt 1.5.3 and before Whirlpool is
+ broken and it was fixed in the next version. 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.
+
+ The only two ways to deal with this are either to 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.
+ Versions of gcrypt between 1.5.4 and 1.6.0 cannot be used.
+
+ Steps:
+
+ - Make a 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:
+
+
+ cryptsetup luksDump <your luks device> --debug | grep backend
+
+ If gcrypt is at version 1.5.3 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.):
+
+ cryptsetup-reencrypt --keep-key --hash sha256 <your luks device>
+
+ 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. 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 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.
+
+ At this time I would like to limit the references to things that
+ are available on the web.
+
+
+ * Specifications
+
+ - LUKS on-disk format spec:
+ https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
+
+ * Code Examples
+
+ - Some code examples are in the source package under docs/examples
+
+
+ * 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
projects / platform / upstream / cryptsetup.git / blobdiff