4. Troubleshooting
5. Security Aspects
6. Backup and Data Recovery
-7. Issues with Specific Versions of cryptsetup
+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,
- no management, no descriptor on disk) and LUKS (multiple user keys
- with one master key, anti-forensics, descriptor block at start of
- device, ...). The latest version should usually be available at
+ 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 just managed to
- somehow format or overwrite the start of their LUKS partitions.
- Usually, 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!
+ 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.
+
+ 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
+
+
+ * 1.3 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
+ 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
+
+ Update 7/2012: I am unsure whether this has been fixed by now, best
+ be careful.
+
+
+ * 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.
+
+ - Ask on the mailing-list if you need more help.
+
-
- * Who wrote this?
+ * 1.5 Who wrote this?
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. Please note that by contributing to this FAQ, you accept
- the license described below.
+ 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.
+ * 1.5 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.
+
+
+ * 1.6 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
+ 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.
+
+ 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
+
+
+ * 1.7 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
- * Can I encrypt an already existing, non-empty partition to use
- LUKS?
+ * 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.
+
+ 04) Create the LUKS container:
+ cryptsetup luksFormat <target device>
+
+ Just follow the on-screen instructions.
+
+ 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 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
+ 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 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
+ encrypted data when traveling to those countries, especially when
+ crossing the borders.
+
+ 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.
+
+ 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.
+
+ 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).
+
+
+ * 2.3 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?
+ * 2.4 How do I use LUKS with a loop-device?
- 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 luksfile to /dev/loop0
- cryptsetup luksFormat /dev/loop0 # create LUKS on the loop device
+ 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".
- * 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?
+ * 2.5 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
new key-slot.
- * How do I read a dm-crypt key from file?
+ * 2.6 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
+ 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 .
+
+
+ * 2.7 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:
cryptsetup create --key-file keyfile e1 /dev/loop0
- * How do I read a LUKS slot key from file?
+ * 2.8 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?
+ * 2.9 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
+ 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.
- * What are the security requirements for a key read from file?
+ * 2.10 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?
+ * 2.11 If I map a journaled file system using dm-crypt/LUKS, does it
+ still provide its usual transactional guarantees?
- As far as I know you do (but I may be wrong), but please note that
- these "guarantees" are far weaker than they appear to be. For
- example, you not 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 issues so far.
+ 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.
+ 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?
+
+ * 2.12 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
storage.
- * Can I resize a dm-crypt or LUKS partition?
+ * 2.13 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
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. 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 pretty safe and should be fine.
- This will not change the size of the filesystem hidden under 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.
You also need to be aware of size-based limitations. The one
for that.
+ * 2.14 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.
+
+
3. Common Problems
- * My dm-crypt/LUKS mapping does not work! What general steps are
- there to investigate the problem?
+ * 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 "/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.
+ - 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.
- - 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.
- * 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
have no older version of cryptsetup on your system that then gets
- called by cron/CGI.For example some distributions install
+ 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 you have the
- right version.
+ from cron/CGI or the non-shell mechanism to be sure the right
+ 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 every key-slot (iteration is needed to
- prevent dictionary attacks) is calculated during the luksFormat
- operation. By default it is 1 second on the machine where the
- format operation is done. If you format a device on a fast machine
- and then unlock it on a slow machine, the unlocking time can be
- much more longer. Also take into account that up to 8 key-slots
- have to be tried in order to find the right one.
+ 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. In order to change that, you will have to backup
- the data in the LUKS container, luksFormat on the slow machine and
- restore the data.
+ 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.
- * "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 luksHeaderBackup --header-backup-file <file> <device>
cryptsetup luksHeaderRestore --header-backup-file <file> <device>
- If you cannot use a 1.1.x cryptsetup, you can also do a manual wipe
- of the area in question with the command below. Be very, VERY,
- careful and make sure to do a backup of the header before. If you
- get this wrong, your device may become permanently inaccessible.
-
- dd if=/dev/zero of=<device> bs=512 seek=2 count=6
-
- * 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 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 +
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 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
+ 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
find . -type f -exec md5sum \{\} \; > checksum-file
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 various obscure errors.
+ flipped single bit and can result in various obscure errors.
- Note however that a verify run on copying between encrypted or
- unencrypted devices can also show you corruption 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.
+ 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.
- * How do I test RAM?
+ * 4.3 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.
+ 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
recommend running the following procedure until the first error is
found:
- - Run Memtest86+ for one cycle
- - Run memterster for one cycle (shut down as many other applications as possible)
+ - Run Memtest86+ for one cycle
+
+ - Run memtester for one cycle (shut down as many other applications
+ as possible)
+
- Run Memtest86+ for 24h or more
- - Run memtester 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
5. Security Aspects
- * Should I initialize (overwrite) a new LUKS/dm-crypt partition?
+ * 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 hasing 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
+ 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
dd_rescue -w /dev/zero /dev/mapper/e1
- or alternatively with one of the following more standard commands:
+ or alternatively with one of the following more standard commands:
cat /dev/zero > /dev/mapper/e1
dd if=/dev/zero of=/dev/mapper/e1
- * How do I securely erase a LUKS (or other) partition?
-
- For LUKS, if you are in a desperate hurry, overwrite the first few
- kilobytes of the LUKS partition. This erases the salts and makes
- access impossible. However a LUKS header backup or full backup will
- still grant access to most or all data.
-
- To do this right, overwrite the whole LUKS partition with a single
- pass of zeros. This is enough for current HDDs. For SDDs you may
- want to erase the whole drive several times to be sure data is not
- retained by wear leveling. This is possibly insecure as SDD
- 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 could be enough. If in doubt, use physical
- destruction in addition. Keep in mind to also erase all backups.
-
- Example for a zero-overwrite erase of partition sda10 done with
+ * 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:
+ 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
dd_rescue:
- dd_rescue -w /dev/zero /dev/sda10
+ 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, you may want to overwrite the
- complete SSD several times and use physical destruction in addition,
- see last item. Treat USB flash drives the same as SSDs. 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. 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.
-
-
- * What about backup? Does it compromise security?
-
- That depends. See next section.
-
-
- * Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
+ 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.
+
+
+ * 5.6 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.
+
+ 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.
+
+ 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?
+
+ 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?
+
+ 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 safe to use with CBC, see
- previous FAQ item.
+ 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
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
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 or Flash 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-adressable 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 if it finishes after a few seconds, it was
+ possibly faked by the SSD.
+
+ If you can do without password management and are fine with doing
+ physical destruction for permenently deleting data (allways 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.
+
+ If you are concerned about your laptop being stolen, you are likely
+ fine using LUKS on an SSD. 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 targetted laptop theft to get at your
+ data, you should be fine.
+
+
6. Backup and Data Recovery
- * Does a backup compromise security?
+ * 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.
+
+ 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.
+
+ 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:
+
+
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
+
+ To restore, use the inverse command, i.e.
+
+ cryptsetup luksHeaderRestore --header-backup-file <file> <device>
+
+
+ * 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
+ 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:
+
+ 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.
+
+ 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.
+
+ 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.
- Depends on how you do it. First, a backup is non-optional with
- encrypted data just the same way it is with non-encrypted data.
- Disks do break and they do not care whether they make plain or
- encrypted data inaccessible.
+ 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.
+
+ 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.
+
+
+ * 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.
- However there are risks introduced by backups. For example if you
+ 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, or use a
- different set-up. One option is to have a different passphrase on
- the backup and to make the backup with both containers open.
- Another one is to make a backup of the original, opened container to
- a single file, e.g. with tar, and to encrypt that file with
- public-key-cryptography, e.g. with GnuPG. You can then keep the
- secret key in a safe place, because it is only used to decrypt a
- backup. The key the backup is encrypted with can be stored without
- special security measures, as long as an attacker cannot replace
- it with his own key.
+ 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
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: An
- attacker can see how many sectors and which ones have been changed
- since the backup. This is not possible with the public-key method
- though.
+ 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 date) or three
- disks (high value data) in rotating order for backups, and either
- use different passphrases or keep them easily accessible in case
- you need to disable a key-slot. If you do network-backup or
- tape-backup, I strongly recommend to go the public-key path,
- especially as you typically cannot reliably delete data in these
- scenarios. (Well, you can burn the tape if it is under your
- control...)
+ 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...)
- * 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
+ 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. You may also damage the key-slots
- in part or in full. See also last item.
+ 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.
+
+
+ * 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. 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.
+
+ 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/misc/luks-header-from-active
+ You can also do this manually. Here is how:
- * What does the on-disk structure of dm-crypt look like?
+ - 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
+ 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:
+
+ echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
+
+ - Do a luksFormat to create a new LUKS 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>
+
+ 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.
+
+ 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
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
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 and on key block alignment.
+ number of anti-forensic stripes, key material offset and master
+ key size.
- With 4000 stripes (the default), each key-slot is a bit less than
+ 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
+ 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. Start of
- bulk data (with the default 4000 stripes and 8 key-slots) 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" dumps exactly
- the first 1'052'672 bytes to file and "luksHeaderRestore" restores
+ "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.
+
The exact specification of the format is here:
http://code.google.com/p/cryptsetup/wiki/Specification
- * How do I backup a LUKS header?
+ * 6.13 What is the smallest possible LUKS container?
- 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:
+ 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:
+ cryptsetup luksFormat -s 128 --align-payload=8 <device>
- cryptsetup luksHeaderBackup --header-backup-file h_bak /dev/mapper/c1
+ 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.
- * How do I backup a LUKS partition?
- You do a sector-image of the whole partition. This will contain the
- LUKS header, the keys-slots and the data ares. It can be done
- under Linux e.g. with dd_rescue (for a direct image copy) and with
- "cat" or "dd". Example:
+ * 6.14 I think this is overly complicated. Is there an alternative?
- cat /dev/sda10 > sda10.img
- dd_rescue /dev/sda10 sda10.img
+ 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).
+
+
+ * 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
+
+
+ * 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.
+
+ 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.
+
+
+ * 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 specified in bytes (cryptsetup: 512 byte sectors)
+
+ - The need to specify an IV offset
+
+ - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
+
+ - Key size needs specifying (e.g. "-s 128" for 128 bit keys)
+
+ - 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.
+
+
+ * 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 -e twofish -o 2560 /dev/loop0 /dev/sdb1
+ mount /dev/loop0 mount-point
- 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 sense to use it.
+ translates to
+ cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
+ mount /dev/mapper/e1 mount-point
+
- * Do I need a backup of the full partition? Would the header and
- key-slots not be enough?
+ * 7.4 loop-aes with 160 bit key
- 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.
+ 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
- For this case, a header+key-slot backup would often be enough. But
- keep in mind that a HDD has roughly a failure risk of 5% per year.
- It is highly advisable to have a complete backup to protect against
- this case.
+ --cipher twofish-cbc-null -s 192 -h ripemd160:20
+
+ * 7.5 loop-aes v1 format OpenSUSE
- * Are there security risks from a backup of the LUKS header or a
- whole LUKS partition?
+ 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:
- Yes. One risk is that if you remove access rights for specific
- key-slots by deleting their contents, the data can still be
- accessed with invalidated passphrase and the backup. The other risk
- is that if you erase a LUKS partition, a backup could still grant
- access, especially if you only erased the LUKS header and not the
- whole partition.
+ cryptsetup create <target> <device> -c aes -s 128 -h sha256
+
+ * 7.6 Kernel encrypted loop device (cryptoloop)
- * I think this is overly complicated. Is there an alternative?
+ There are a number of different losetup implementations for using
+ encrypted loop devices so getting this to work may need a bit of
+ experimentation.
- Yes, you can use plain dm-crypt. 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).
+ 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:
-7. Issues with Specific Versions of cryptsetup
+ 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.
- * 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. 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!
With cryptsetup 1.1.x, the distro maintainer can define different
default encryption modes for LUKS and plain devices. You can check
compatibility with cryptsetup 1.0.x defaults, simple use the
following:
- cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <device>
+ cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
- LUKS stores cipher and mode in the metadata on disk, avoiding this
+ LUKS stores cipher and mode in the metadata on disk, avoiding this
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.
+
+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:
+ http://code.google.com/p/cryptsetup/wiki/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
+
- Milan Broz
+
projects / platform / upstream / cryptsetup.git / blobdiff