[dm-crypt] Updated: Monthly FAQ posting 12/2010

Arno Wagner arno at wagner.name
Sat Dec 4 04:07:25 CET 2010


Since I just added sections about salts and why the header is
critical for LUKS, and extended comments on low entropy 
passphrases and passphrase iteration count (all in Section 5),
here is an updated repost. Also added a comment about the
mailing-list to Section 1.

In addition, I fixed some formatting issues. In the process I 
am learning that ASCII text formatting is a lot more difficult 
that I thought ;-)

Feel free to comment, cricicize and propose improvements.

Arno
---

Sections 

1. General Questions
2. Setup
3. Common Problems
4. Troubleshooting
5. Security Aspects
6. Backup and Data Recovery
7. Issues with Specific Versions of cryptsetup
A. Contributors


1. General Questions 


 * 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
  http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions

  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. 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!


 * Who wrote this?

  Current FAQ maintainer is Arno Wagner <arno at 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.

  This work is under the "Attribution-Share Alike 3.0 Unported"
  license, which means distribution is unlimited, you may create
  derived works, but attributions to original authors and this
  license statement must be retained and the derived work must be
  under the same license. See
  http://creativecommons.org/licenses/by-sa/3.0/ for more details of
  the license.

  Side note: I did text license research some time ago and I think
  this license is best suited for the purpose at hand and creates the
  least problems.


 * 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.


 * 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 at saout.de
  with just the word "help" in the subject or message body. Make sure
  to send it from your list address.


2. Setup 


 * 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
  the device (as LUKS device initialization does not clear away old
  data), do a luksFormat, optionally overwrite the encrypted device,
  create a new filesystem and restore your backup on the now
  encrypted device. Also refer to sections "Security Aspects" and
  "Backup and Data Recovery".

  For backup, plain GNU tar works well and backs up anything likely
  to be in a filesystem.


 * 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:

      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?

  That is as intended. You are asked a passphrase of an existing
  key-slot first, before you can enter the passphrase for the new
  key-slot. Otherwise you could break the encryption by just adding a
  new key-slot. This way, you have to know the passphrase of one of
  the already configured key-slots in order to be able to configure a
  new key-slot.


 * 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?

  What you really do here is to read a passphrase from file, just as
  you would with manual entry of a passphrase for a key-slot. You can
  add a new passphrase to a free key-slot, set the passphrase of an
  specific key-slot or put an already configured passphrase into a
  file. In the last case make sure no trailing newline (0x0a) is
  contained in the key file, or the passphrase will not work because
  the whole file is used as input.

  To add a new passphrase to a free key slot from file, use something
  like this:

      cryptsetup luksAddKey /dev/loop0 keyfile
 
  To add a new passphrase to a specific key-slot, use something like
  this:

      cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
 
  To supply a key from file to any LUKS command, use the --key-file
  option, e.g. like this:

      cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
 

 * How do I read the LUKS master key from file?

  The question you should ask yourself first, is why you would want
  to do this. The only legitimate reason I can think of is if you
  want to have two LUKS devices with the same master key. Even then,
  I think it would be preferable to just use key-slots with the same
  passphrase, or to use plain dm-crypt instead. If you really have a
  good reason, please tell me. If I am convinced, I will add how to
  do this here.


 * What are the security requirements for a key read from file?

  A file-stored key or passphrase has the same security requirements
  as one entered interactively, however you can use random bytes and
  thereby use bytes you cannot type on the keyboard. You can use any
  file you like as key file, for example a plain text file with a
  human readable passphrase. To generate a file with random bytes,
  use something like this:

      head -c 256 /dev/random > keyfile
 

 * 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 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.

  Personally, I have several instances of ext3 on dm-crypt and have
  not noticed any specific problems.

  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 2.6.37 is expected to improve this even further.


 * Can I use LUKS or cryptsetup with a more secure (external) medium
   for key storage, e.g. TPM or a smartcard?

  Yes, see the answers on using a file-supplied key. You do have to
  write the glue-logic yourself though. Basically you can have
  cryptsetup read the key from STDIN and write it there with your
  own tool that in turn gets the key from the more secure key
  storage.


 * 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
  encryption though.

  You also need to be aware of size-based limitations. The one
  currently relevant is that aes-xts-plain should not be used for
  encrypted container sizes larger than 2TiB. Use aes-xts-plain64
  for that.


3. Common Problems 


 * My dm-crypt/LUKS mapping does not work! What general steps are
   there to investigate the problem?

  If you get a specific error message, investigate what it claims
  first. If not, you may want to check the following things.

  - Check that "/dev", including "/dev/mapper/control" is there. If it
  is missing, you may have a problem with the "/dev" tree itself or
  you may have broken udev rules.

  - Check that you have the device mapper and the crypt target in your
  kernel. The output of "dmsetup targets" should list a "crypt"
  target. If it is not there or the command fails, add device mapper
  and crypt-target to the kernel.

  - 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.

  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?

  If you get errors about unknown parameters or the like that are not
  present when cryptsetup is called from the shell, make sure you
  have no older version of cryptsetup on your system that then gets
  called by cron/CGI.For example some distributions install
  cryptsetup into /usr/sbin, while a manual install could go to
  /usr/local/sbin. As a debugging aid, call "cryptsetup --version"
  from cron/CGI or the non-shell mechanism to be sure you have the
  right version.


 * Unlocking a LUKS device takes very long. Why?

  The iteration time for a key-slot (see Section 5 for an explanation
  what iteration does) is calculated when setting a passphrase. By
  default it is 1 second on the machine where the passphrase is set.
  If you set a passphrase on a fast machine and then unlock it on a
  slow machine, the unlocking time can be much longer. Also take into
  account that up to 8 key-slots have to be tried in order to find the
  right one.

  If this is problem, you can add another key-slot using the slow
  machine with the same passphrase and then remove the old key-slot.
  The new key-slot will have an iteration count adjusted to 1 second
  on the slow machine. Use luksKeyAdd and then luksKillSlot or
  luksRemoveKey.

  However, this operation will not change volume key iteration count
  (MK iterations in output of "cryptsetup luksDump"). In order to
  change that, you will have to backup the data in the LUKS
  container, 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.


 * "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device.
   What is wrong?

  Some old versions of cryptsetup have a bug where the header does
  not get completely wiped during LUKS format and an older ext2/swap
  signature remains on the device. This confuses blkid.

  Fix: Wipe the unused header areas by doing a backup and restore of
  the header with cryptsetup 1.1.x:

      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 ...

  There seems to be some inteference between the hardening and and
  the way cryptsetup benchmarks PBKDF2. The solution to this is
  currently not quite clear for an encrypted root filesystem.     For
  other uses, you can apparently specify USE="dynamic" as compile
  flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470


4. Troubleshooting 


 * Can a bad RAM module cause problems?

  LUKS and dm-crypt can give the RAM quite a workout, especially when
  combined with software RAID. In particular the combination RAID5 +
  LUKS + XFS seems to uncover RAM problems that never caused obvious
  problems before. Symptoms vary, but often the problem manifest
  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

      find . -type f -exec md5sum \{\} \; > checksum-file
 
  and then a "md5sum -c checksum-file" on the other side. If you get
  mismatches here, RAM is the primary suspect. A lesser suspect is
  an overclocked CPU. I have found countless hardware problems in
  verify runs after copying or making backups. Bit errors are much
  more common than most people think.

  Some RAM issues are even worse and corrupt structures in one of the
  layers. This typically results in lockups, CPU state dumps in the
  system logs, kernel panic or other things. It is quite possible to
  have the problem with an encrypted device, but not with an
  otherwise the same unencrypted device. The reason for that is that
  encryption has an error amplification property: You flip one bit
  in an encrypted data block, and the decrypted version has half of
  its bits flipped. This is an important security property for modern
  ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
  get up to a completely changed 512 byte block per bit error. A
  corrupt block causes a lot more havoc than the occasionally
  flipped single bit and can result 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.


 * How do I test RAM?

  First you should know that overclocking often makes memory problems
  worse. So if you overclock (which I strongly recommend against in a
  system holding data that has some worth), run the tests with the
  overclocking active.

  There are two good options. One is Memtest86+ and the other is
  "memtester" by Charles Cazabon. Memtest86+ requires a reboot and
  then takes over the machine, while memtester runs from a
  root-shell. Both use different testing methods and I have found
  problems fast with each one that the other needed long to find. I
  recommend running the following procedure until the first error is
  found:

  - Run Memtest86+ for one cycle

  - Run memterster for one cycle (shut down as many other applications
  as possible)

  - Run Memtest86+ for 24h or more

  - Run memtester for 24h or more

  If all that does not produce error messages, your RAM may be sound,
  but I have had one weak bit that Memtest86+ needed around 60 hours
  to find. If you can reproduce the original problem reliably, a good
  additional test may be to remove half of the RAM (if you have more
  than one module) and try whether the problem is still there and if
  so, try with the other half. If you just have one module, get a
  different one and try with that. If you do overclocking, reduce
  the settings to the most conservative ones available and try with
  that.


5. Security Aspects 


 * Should I initialize (overwrite) a new LUKS/dm-crypt partition?

  If you just create a filesystem on it, most of the old data will
  still be there. If the old data is sensitive, you should overwrite
  it before encrypting. In any case, not initializing will leave the
  old data there until the specific sector gets written. That may
  enable an attacker to determine how much and where on the
  partition data was written. If you think this is a risk, you can
  prevent this by overwriting the encrypted device (here assumed to
  be named "e1") with zeros like this:

      dd_rescue -w /dev/zero /dev/mapper/e1
 
  or alternatively with one of the following more standard commands:

      cat /dev/zero > /dev/mapper/e1
      dd if=/dev/zero of=/dev/mapper/e1
       

 * 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 master key salt
  and makes access impossible. However 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.

  To do this right, overwrite the whole LUKS partition with a single
  pass of zeros. This is enough for current HDDs. For SSDs you may
  want to erase 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 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
  dd_rescue:

      dd_rescue -w /dev/zero /dev/sda10   
 

 * 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 is all my data 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 one ("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. Ask on the mailing-list on how to do that and
  make sure nobody switches off the machine.


 * 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.


 * Is LUKS secure with a low-entropy (bad) passphrase?

  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. 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.

  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.


 * 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 give use 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 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 often 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 some users will
  still chose passphrases with entropy low enough that they are
  vulnerable. Lowering it even further increases this danger
  significantly.


 * 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-startved 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.


 * Why was the default aes-cbc-plain replaced with aes-cbc-essiv?

  The problem is that cbc-plain has a fingerprint vulnerability, where
  a specially crafted file placed into the crypto-container can be
  recognized from the outside. The issue here is that for cbc-plain
  the initialization vector (IV) is the sector number. The IV gets
  XORed to the first data chunk of the sector to be encrypted. If you
  make sure that the first data block to be stored in a sector
  contains the sector number as well, the first data block to be
  encrypted is all zeros and always encrypted to the same ciphertext.
  This also works if the first data chunk just has a constant XOR
  with the sector number. By having several shifted patterns you can
  take care of the case of a non-power-of-two start sector number of
  the file.

  This mechanism allows you to create a pattern of sectors that have
  the same first ciphertext block and signal one bit per sector to the
  outside, allowing you to e.g. mark media files that way for
  recognition without decryption. For large files this is a
  practical attack. For small ones, you do not have enough blocks to
  signal and take care of different file starting offsets.

  In order to prevent this attack, the default was changed to
  cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
  encryption key as key. This makes the IV unpredictable without
  knowing the encryption key and the watermarking attack fails.


 * 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.

  However there are modes, like XTS, that are secure with "plain" IV.
  The next limit is that "plain" is 64 bit, with the upper 32 bit set
  to zero. This means that on volumes larger than 2TiB, the IV
  repeats, creating a vulnerability that potentially leaks some
  data. To avoid this, use "plain64", which uses the full sector
  number up to 64 bit. Note that "plain64" requires a kernel >=
  2.6.33. Also note that "plain64" is backwards compatible for
  volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
  does not cause any performance penalty compared to "plain".


 * What about XTS mode?

  XTS mode is potentially even more secure than cbc-essiv (but only if
  cbc-essiv is insecure in your scenario). It is a NIST standard and
  used, e.g. in Truecrypt. At the moment, if you want to use it, you
  have to specify it manually as "aes-xts-plain", i.e.

      cryptsetup -c aes-xts-plain luksFormat <device>
 
  For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ
  item on "plain" and "plain64"):

      cryptsetup -c aes-xts-plain64 luksFormat <device>
 
  There is a potential security issue with XTS mode and large blocks.
  LUKS and dm-crypt always use 512B blocks and the issue does not
  apply.


6. Backup and Data Recovery 


 * Does a backup compromise security?

  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.

  However 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.

  If you use dm-crypt, backup is simpler: As there is no key
  management, the main risk is that you cannot wipe the backup when
  wiping the original. However wiping the original for dm-crypt
  should consist of forgetting the passphrase and that you can do
  without actual access to the backup.

  In both cases, there is an additional (usually small) risk: 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.

  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...)


 * 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.


 * 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.


 * What does the on-disk structure of dm-crypt look like?

  There is none. dm-crypt takes a block device and gives encrypted
  access to each of its blocks with a key derived from the passphrase
  given. If you use a cipher different than the default, you have to
  specify that as a parameter to cryptsetup too. If you want to
  change the password, you basically have to create a second
  encrypted device with the new passphrase and copy your data over.
  On the plus side, if you accidentally overwrite any part of a
  dm-crypt device, the damage will be limited to the are you
  overwrote.


 * What does the on-disk structure of LUKS look like?

  A LUKS partition consists of a header, followed by 8 key-slot
  descriptors, followed by 8 key slots, followed by the encrypted
  data area.

  Header and key-slot descriptors fill the first 592 bytes. The
  key-slot size depends on the creation parameters, namely on the
  number of anti-forensic stripes and on key block alignment.

  With 4000 stripes (the default), each key-slot is a bit less than
  128kiB in size. Due to sector alignment of the key-slot start,
  that means the key block 0 is at offset 0x1000-0x20400, key block
  1 at offset 0x21000-0x40400, and key block 7 at offset
  0xc1000-0xe0400. The space to the next full sector address is
  padded with zeros. Never used key-slots are filled with what the
  disk originally contained there, a key-slot removed with
  "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. 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
  them.

  The exact specification of the format is here:
  http://code.google.com/p/cryptsetup/wiki/Specification


 * 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 h /dev/mapper/c1
 

 * 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:

      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 sense to use it.


 * 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.

  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.


 * Are there security risks from a backup of the LUKS header or a
   whole LUKS partition?

  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.


 * I think this is overly complicated. Is there an alternative?

  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).


7. 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!

  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.

  If you are using a plain device and you need a compatible mode, just
  specify cipher, key size and hash algorithm explicitly. For
  compatibility with cryptsetup 1.0.x defaults, simple use the
  following:

    cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
 
  LUKS stores cipher and mode in the metadata on disk, avoiding this
  problem.


 * 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.

 A. Contributors In no particular order:

  - Arno Wagner

  - Milan Broz



-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier 


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