8 6. Backup and Data Recovery
9 7. Interoperability with other Disk Encryption Tools
10 8. Issues with Specific Versions of cryptsetup
11 9. References and Further Reading
20 This is the FAQ (Frequently Asked Questions) for cryptsetup. It
21 covers Linux disk encryption with plain dm-crypt (one passphrase,
22 no management, no metadata on disk) and LUKS (multiple user keys
23 with one master key, anti-forensic features, metadata block at
24 start of device, ...). The latest version of this FAQ should
25 usually be available at
26 http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
31 ATTENTION: If you are going to read just one thing, make it the
32 section on Backup and Data Recovery. By far the most questions on
33 the cryptsetup mailing list are from people that managed to damage
34 the start of their LUKS partitions, i.e. the LUKS header. In
35 most cases, there is nothing that can be done to help these poor
36 souls recover their data. Make sure you understand the problem and
37 limitations imposed by the LUKS security model BEFORE you face
38 such a disaster! In particular, make sure you have a current header
39 backup before doing any potentially dangerous operations.
41 SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
42 unclear how to get LUKS or plain dm-crypt to run on them with the
43 full set of security features intact. This may or may not be a
44 problem, depending on the attacher model. See Section 5.19.
46 BACKUP: Yes, encrypted disks die, just as normal ones do. A full
47 backup is mandatory, see Section "6. Backup and Data Recovery" on
48 options for doing encrypted backup.
50 CLONING/IMAGING: If you clone or image a LUKS container, you make a
51 copy of the LUKS header and the master key will stay the same!
52 That means that if you distribute an image to several machines, the
53 same master key will be used on all of them, regardless of whether
54 you change the passphrases. Do NOT do this! If you do, a root-user
55 on any of the machines with a mapped (decrypted) container or a
56 passphrase on that machine can decrypt all other copies, breaking
57 security. See also Item 6.15.
59 DISTRIBUTION INSTALLERS: Some distribution installers offer to
60 create LUKS containers in a way that can be mistaken as activation
61 of an existing container. Creating a new LUKS container on top of
62 an existing one leads to permanent, complete and irreversible data
63 loss. It is strongly recommended to only use distribution
64 installers after a complete backup of all LUKS containers has been
67 NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from
68 STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
69 warning that you are about to format (and e.g. will lose any
70 pre-existing LUKS container on the target), as it assumes it is
71 used from a script. In this scenario, the responsibility for
72 warning the user and possibly checking for an existing LUKS header
73 is shifted to the script. This is a more general form of the
76 LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
77 used in deriving the master key. It is used in decrypting a master
78 key that is randomly selected on header creation. This means that
79 if you create a new LUKS header on top of an old one with
80 exactly the same parameters and exactly the same passphrase as the
81 old one, it will still have a different master key and your data
82 will be permanently lost.
84 PASSPHRASE CHARACTER SET: Some people have had difficulties with
85 this when upgrading distributions. It is highly advisable to only
86 use the 95 printable characters from the first 128 characters of
87 the ASCII table, as they will always have the same binary
88 representation. Other characters may have different encoding
89 depending on system configuration and your passphrase will not
90 work with a different encoding. A table of the standardized first
91 128 ASCII characters can, e.g. be found on
92 http://en.wikipedia.org/wiki/ASCII
95 * 1.3 System specific warnings
97 - Ubuntu as of 4/2011: It seems the installer offers to create
98 LUKS partitions in a way that several people mistook for an offer
99 to activate their existing LUKS partition. The installer gives no
100 or an inadequate warning and will destroy your old LUKS header,
101 causing permanent data loss. See also the section on Backup and
104 This issue has been acknowledged by the Ubuntu dev team, see here:
105 http://launchpad.net/bugs/420080
107 Update 7/2012: I am unsure whether this has been fixed by now, best
111 * 1.4 My LUKS-device is broken! Help!
113 First: Do not panic! In many cases the data is still recoverable.
114 Do not do anything hasty! Steps:
116 - Take some deep breaths. Maybe add some relaxing music. This may
117 sound funny, but I am completely serious. Often, critical damage is
118 done only after the initial problem.
120 - Do not reboot. The keys mays still be in the kernel if the device
123 - Make sure others do not reboot the system.
125 - Do not write to your disk without a clear understanding why this
126 will not make matters worse. Do a sector-level backup before any
127 writes. Often you do not need to write at all to get enough access
128 to make a backup of the data.
132 - Read section 6 of this FAQ.
134 - Ask on the mailing-list if you need more help.
137 * 1.5 Who wrote this?
139 Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
140 contributors are listed at the end. If you want to contribute, send
141 your article, including a descriptive headline, to the maintainer,
142 or the dm-crypt mailing list with something like "FAQ ..." in the
143 subject. You can also send more raw information and have me write
144 the section. Please note that by contributing to this FAQ, you
145 accept the license described below.
147 This work is under the "Attribution-Share Alike 3.0 Unported"
148 license, which means distribution is unlimited, you may create
149 derived works, but attributions to original authors and this
150 license statement must be retained and the derived work must be
151 under the same license. See
152 http://creativecommons.org/licenses/by-sa/3.0/ for more details of
155 Side note: I did text license research some time ago and I think
156 this license is best suited for the purpose at hand and creates the
160 * 1.5 Where is the project website?
162 There is the project website at http://code.google.com/p/cryptsetup/
163 Please do not post questions there, nobody will read them. Use
164 the mailing-list instead.
167 * 1.6 Is there a mailing-list?
169 Instructions on how to subscribe to the mailing-list are at on the
170 project website. People are generally helpful and friendly on the
173 The question of how to unsubscribe from the list does crop up
174 sometimes. For this you need your list management URL, which is
175 sent to you initially and once at the start of each month. Go to
176 the URL mentioned in the email and select "unsubscribe". This page
177 also allows you to request a password reminder.
179 Alternatively, you can send an Email to dm-crypt-request@saout.de
180 with just the word "help" in the subject or message body. Make sure
181 to send it from your list address.
183 The mailing list archive is here:
184 http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
187 * 1.7 Unsubscribe from the mailing-list
189 Send mail to dm-crypt-unsubscribe@saout.de from the subscribed
190 account. You will get an email with instructions.
192 Basically, you just have to respond to it unmodified to get
193 unsubscribed. The listserver admin functions are not very fast. It
194 can take 15 minutes or longer for a reply to arrive (I suspect
195 greylisting is in use), so be patient.
197 Also note that nobody on the list can unsubscribe you, sending
198 demands to be unsubscribed to the list just annoys people that are
199 entirely blameless for you being subscribed.
201 If you are subscribed, a subscription confirmation email was sent
202 to your email account and it had to be answered before the
203 subscription went active. The confirmation emails from the
204 listserver have subjects like these (with other numbers):
206 Subject: confirm 9964cf10.....
208 and are sent from dm-crypt-request@saout.de. You should check
209 whether you have anything like it in your sent email folder. If
210 you find nothing and are sure you did not confirm, then you should
211 look into a possible compromise of your email account.
217 * 2.1 LUKS Container Setup mini-HOWTO
219 This item tries to give you a very brief list of all the steps you
220 should go though when creating a new LUKS encrypted container, i.e.
221 encrypted disk, partition or loop-file.
223 01) All data will be lost, if there is data on the target, make a
226 02) Make very sure you have the right target disk, partition or
229 03) If the target was in use previously, it is a good idea to
230 wipe it before creating the LUKS container in order to remove any
231 trace of old file systems and data. For example, some users have
232 managed to run e2fsck on a partition containing a LUKS container,
233 possibly because of residual ext2 superblocks from an earlier use.
234 This can do arbitrary damage up to complete and permanent loss of
235 all data in the LUKS container.
237 To just quickly wipe file systems (old data may remain), use
239 wipefs -a <target device>
241 To wipe file system and data, use something like
243 cat /dev/zero > <target device>
245 This can take a while. To get a progress indicator, you can use
246 the tool dd_rescue (->google) instead or use my stream meter "wcs"
247 (source here: http://www.tansi.org/tools/index.html) in the
250 cat /dev/zero | wcs > <target device>
252 Be very sure you have the right target, all data will be lost!
254 Note that automatic wiping is on the TODO list for cryptsetup, so
255 at some time in the future this will become unnecessary.
257 04) Create the LUKS container:
258 cryptsetup luksFormat <target device>
260 Just follow the on-screen instructions.
262 05) Map the container. Here it will be mapped to /dev/mapper/c1:
263 cryptsetup luksOpen <target device> c1
265 06) (Optionally) wipe the container (make sure you have the right target!):
266 cat /dev/zero > /dev/mapper/c1
268 Note that this creates a small information leak, as an attacker can
269 determine whether a 512 byte block is zero if the attacker has
270 access to the encrypted container multiple times. Typically a
271 competent attacker that has access multiple times can install a
272 passphrase sniffer anyways, so this leakage is not very
273 significant. For getting a progress indicator, see step 03.
275 Note that at some time in the future, cryptsetup will do this for
276 you, but currently it is a TODO list item.
278 07) Create a file system in the mapped container, for example an
279 ext3 file system (any other file system is possible):
281 mke2fs -j /dev/mapper/c1
283 08) Mount your encrypted file system, here on /mnt:
284 mount /dev/mapper/c1 /mnt
286 Done. You can now use the encrypted file system to store data. Be
287 sure to read though the rest of the FAQ, these are just the very
288 basics. In particular, there are a number of mistakes that are
289 easy to make, but will compromise your security.
292 * 2.2 What is the difference between "plain" and LUKS format?
294 First, unless you happen to understand the cryptographic background
295 well, you should use LUKS. It does protect the user from a lot of
296 common mistakes. Plain dm-crypt is for experts.
298 Plain format is just that: It has no metadata on disk, reads all
299 parameters from the commandline (or the defaults), derives a
300 master-key from the passphrase and then uses that to de-/encrypt
301 the sectors of the device, with a direct 1:1 mapping between
302 encrypted and decrypted sectors.
304 Primary advantage is high resilience to damage, as one damaged
305 encrypted sector results in exactly one damaged decrypted sector.
306 Also, it is not readily apparent that there even is encrypted data
307 on the device, as an overwrite with crypto-grade randomness (e.g.
308 from /dev/urandom) looks exactly the same on disk.
310 Side-note: That has limited value against the authorities. In
311 civilized countries, they cannot force you to give up a crypto-key
312 anyways. In the US, the UK and dictatorships around the world,
313 they can force you to give up the keys (using imprisonment or worse
314 to pressure you), and in the worst case, they only need a
315 nebulous "suspicion" about the presence of encrypted data. My
316 advice is to either be ready to give up the keys or to not have
317 encrypted data when traveling to those countries, especially when
318 crossing the borders.
320 Disadvantages are that you do not have all the nice features that
321 the LUKS metadata offers, like multiple passphrases that can be
322 changed, the cipher being stored in the metadata, anti-forensic
323 properties like key-slot diffusion and salts, etc..
325 LUKS format uses a metadata header and 8 key-slot areas that are
326 being placed at the beginning of the disk, see below under "What
327 does the LUKS on-disk format looks like?". The passphrases are used
328 to decrypt a single master key that is stored in the anti-forensic
331 Advantages are a higher usability, automatic configuration of
332 non-default crypto parameters, defenses against low-entropy
333 passphrases like salting and iterated PBKDF2 passphrase hashing,
334 the ability to change passphrases, and others.
336 Disadvantages are that it is readily obvious there is encrypted
337 data on disk (but see side note above) and that damage to the
338 header or key-slots usually results in permanent data-loss. See
339 below under "6. Backup and Data Recovery" on how to reduce that
340 risk. Also the sector numbers get shifted by the length of the
341 header and key-slots and there is a loss of that size in capacity
342 (1MB+4096B for defaults and 2MB for the most commonly used
343 non-default XTS mode).
346 * 2.3 Can I encrypt an already existing, non-empty partition to use
349 There is no converter, and it is not really needed. The way to do
350 this is to make a backup of the device in question, securely wipe
351 the device (as LUKS device initialization does not clear away old
352 data), do a luksFormat, optionally overwrite the encrypted device,
353 create a new filesystem and restore your backup on the now
354 encrypted device. Also refer to sections "Security Aspects" and
355 "Backup and Data Recovery".
357 For backup, plain GNU tar works well and backs up anything likely
358 to be in a filesystem.
361 * 2.4 How do I use LUKS with a loop-device?
363 This can be very handy for experiments. Setup is just the same as
364 with any block device. If you want, for example, to use a 100MiB
365 file as LUKS container, do something like this:
367 head -c 100M /dev/zero > luksfile # create empty file
368 losetup /dev/loop0 luksfile # map luksfile to /dev/loop0
369 cryptsetup luksFormat /dev/loop0 # create LUKS on loop device
371 Afterwards just use /dev/loop0 as a you would use a LUKS partition.
372 To unmap the file when done, use "losetup -d /dev/loop0".
375 * 2.5 When I add a new key-slot to LUKS, it asks for a passphrase but
376 then complains about there not being a key-slot with that
379 That is as intended. You are asked a passphrase of an existing
380 key-slot first, before you can enter the passphrase for the new
381 key-slot. Otherwise you could break the encryption by just adding a
382 new key-slot. This way, you have to know the passphrase of one of
383 the already configured key-slots in order to be able to configure a
387 * 2.6 Encryption on top of RAID or the other way round?
389 Unless you have special needs, place encryption between RAID and
390 filesystem, i.e. encryption on top of RAID. You can do it the other
391 way round, but you have to be aware that you then need to give the
392 passphrase for each individual disk and RAID autodetection will
393 not work anymore. Therefore it is better to encrypt the RAID
394 device, e.g. /dev/dm0 .
396 This means that the typical layering looks like this:
408 The big advantage is that you can manage the RAID container just
409 like any RAID container, it does not care that what is in it is
413 * 2.7 How do I read a dm-crypt key from file?
415 Note that the file will still be hashed first, just like keyboard
416 input. Use the --key-file option, like this:
418 cryptsetup create --key-file keyfile e1 /dev/loop0
421 * 2.8 How do I read a LUKS slot key from file?
423 What you really do here is to read a passphrase from file, just as
424 you would with manual entry of a passphrase for a key-slot. You can
425 add a new passphrase to a free key-slot, set the passphrase of an
426 specific key-slot or put an already configured passphrase into a
427 file. In the last case make sure no trailing newline (0x0a) is
428 contained in the key file, or the passphrase will not work because
429 the whole file is used as input.
431 To add a new passphrase to a free key slot from file, use something
434 cryptsetup luksAddKey /dev/loop0 keyfile
436 To add a new passphrase to a specific key-slot, use something like
439 cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
441 To supply a key from file to any LUKS command, use the --key-file
442 option, e.g. like this:
444 cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
447 * 2.9 How do I read the LUKS master key from file?
449 The question you should ask yourself first is why you would want to
450 do this. The only legitimate reason I can think of is if you want
451 to have two LUKS devices with the same master key. Even then, I
452 think it would be preferable to just use key-slots with the same
453 passphrase, or to use plain dm-crypt instead. If you really have a
454 good reason, please tell me. If I am convinced, I will add how to
458 * 2.10 What are the security requirements for a key read from file?
460 A file-stored key or passphrase has the same security requirements
461 as one entered interactively, however you can use random bytes and
462 thereby use bytes you cannot type on the keyboard. You can use any
463 file you like as key file, for example a plain text file with a
464 human readable passphrase. To generate a file with random bytes,
465 use something like this:
467 head -c 256 /dev/random > keyfile
470 * 2.11 If I map a journaled file system using dm-crypt/LUKS, does it
471 still provide its usual transactional guarantees?
473 Yes, it does, unless a very old kernel is used. The required flags
474 come from the filesystem layer and are processed and passed onwards
475 by dm-crypt. A bit more information on the process by which
476 transactional guarantees are implemented can be found here:
478 http://lwn.net/Articles/400541/
480 Please note that these "guarantees" are weaker than they appear to
481 be. One problem is that quite a few disks lie to the OS about
482 having flushed their buffers. Some other things can go wrong as
483 well. The filesystem developers are aware of these problems and
484 typically can make it work anyways. That said, dm-crypt/LUKS will
485 not make things worse.
487 One specific problem you can run into though is that you can get
488 short freezes and other slowdowns due to the encryption layer.
489 Encryption takes time and forced flushes will block for that time.
490 For example, I did run into frequent small freezes (1-2 sec) when
491 putting a vmware image on ext3 over dm-crypt. When I went back to
492 ext2, the problem went away. This seems to have gotten better with
493 kernel 2.6.36 and the reworking of filesystem flush locking
494 mechanism (less blocking of CPU activity during flushes). It
495 should improve further and eventually the problem should go away.
498 * 2.12 Can I use LUKS or cryptsetup with a more secure (external)
499 medium for key storage, e.g. TPM or a smartcard?
501 Yes, see the answers on using a file-supplied key. You do have to
502 write the glue-logic yourself though. Basically you can have
503 cryptsetup read the key from STDIN and write it there with your
504 own tool that in turn gets the key from the more secure key
507 For TPM support, you may want to have a look at tpm-luks at
508 https://github.com/shpedoikal/tpm-luks. Note that tpm-luks is not
509 related to the cryptsetup project.
512 * 2.13 Can I resize a dm-crypt or LUKS partition?
514 Yes, you can, as neither dm-crypt nor LUKS stores partition size.
515 Whether you should is a different question. Personally I recommend
516 backup, recreation of the encrypted partition with new size,
517 recreation of the filesystem and restore. This gets around the
518 tricky business of resizing the filesystem. Resizing a dm-crypt or
519 LUKS container does not resize the filesystem in it. The backup is
520 really non-optional here, as a lot can go wrong, resulting in
521 partial or complete data loss. Using something like gparted to
522 resize an encrypted partition is slow, but typically works. This
523 will not change the size of the filesystem hidden under the
526 You also need to be aware of size-based limitations. The one
527 currently relevant is that aes-xts-plain should not be used for
528 encrypted container sizes larger than 2TiB. Use aes-xts-plain64
532 * 2.14 How do I Benchmark the Ciphers, Hashes and Modes?
534 Since version 1.60 cryptsetup supports the "benchmark" command.
539 It will output first iterations/second for the key-derivation
540 function PBKDF2 parameterized with different hash-functions, and
541 then the raw encryption speed of ciphers with different modes and
542 key-sizes. You can get more than the default benchmarks, see the
543 man-page for the relevant parameters. Note that XTS mode takes two
544 keys, hence the listed key sizes are double that for other modes
545 and half of it is the cipher key, the other half is the XTS key.
551 * 3.1 My dm-crypt/LUKS mapping does not work! What general steps are
552 there to investigate the problem?
554 If you get a specific error message, investigate what it claims
555 first. If not, you may want to check the following things.
557 - Check that "/dev", including "/dev/mapper/control" is there. If it
558 is missing, you may have a problem with the "/dev" tree itself or
559 you may have broken udev rules.
561 - Check that you have the device mapper and the crypt target in your
562 kernel. The output of "dmsetup targets" should list a "crypt"
563 target. If it is not there or the command fails, add device mapper
564 and crypt-target to the kernel.
566 - Check that the hash-functions and ciphers you want to use are in
567 the kernel. The output of "cat /proc/crypto" needs to list them.
570 * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
572 The default cipher, hash or mode may have changed (the mode changed
573 from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
577 * 3.3 When I call cryptsetup from cron/CGI, I get errors about
580 If you get errors about unknown parameters or the like that are not
581 present when cryptsetup is called from the shell, make sure you
582 have no older version of cryptsetup on your system that then gets
583 called by cron/CGI. For example some distributions install
584 cryptsetup into /usr/sbin, while a manual install could go to
585 /usr/local/sbin. As a debugging aid, call "cryptsetup --version"
586 from cron/CGI or the non-shell mechanism to be sure the right
590 * 3.4 Unlocking a LUKS device takes very long. Why?
592 The iteration time for a key-slot (see Section 5 for an explanation
593 what iteration does) is calculated when setting a passphrase. By
594 default it is 1 second on the machine where the passphrase is set.
595 If you set a passphrase on a fast machine and then unlock it on a
596 slow machine, the unlocking time can be much longer. Also take into
597 account that up to 8 key-slots have to be tried in order to find the
600 If this is problem, you can add another key-slot using the slow
601 machine with the same passphrase and then remove the old key-slot.
602 The new key-slot will have an iteration count adjusted to 1 second
603 on the slow machine. Use luksKeyAdd and then luksKillSlot or
606 However, this operation will not change volume key iteration count
607 (MK iterations in output of "cryptsetup luksDump"). In order to
608 change that, you will have to backup the data in the LUKS
609 container (i.e. your encrypted data), luksFormat on the slow
610 machine and restore the data. Note that in the original LUKS
611 specification this value was fixed to 10, but it is now derived
612 from the PBKDF2 benchmark as well and set to iterations in 0.125
613 sec or 1000, whichever is larger. Also note that MK iterations
614 are not very security relevant. But as each key-slot already takes
615 1 second, spending the additional 0.125 seconds really does not
619 * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
620 device. What is wrong?
622 Some old versions of cryptsetup have a bug where the header does
623 not get completely wiped during LUKS format and an older ext2/swap
624 signature remains on the device. This confuses blkid.
626 Fix: Wipe the unused header areas by doing a backup and restore of
627 the header with cryptsetup 1.1.x:
629 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
630 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
633 * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
635 There seems to be some interference between the hardening and and
636 the way cryptsetup benchmarks PBKDF2. The solution to this is
637 currently not quite clear for an encrypted root filesystem. For
638 other uses, you can apparently specify USE="dynamic" as compile
639 flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470
645 * 4.1 I get the error "LUKS keyslot x is invalid." What does that
648 This means that the given keyslot has an offset that points
649 outside the valid keyslot area. Typically, the reason is a
650 corrupted LUKS header because something was written to the start of
651 the device the LUKS container is on. Refer to Section "Backup and
652 Data Recovery" and ask on the mailing list if you have trouble
653 diagnosing and (if still possible) repairing this.
656 * 4.2 I cannot unlock my LUKS container! What could be the problem?
658 First, make sure you have a correct passphrase. Then make sure you
659 have the correct key-map and correct keyboard. And then make sure
660 you have the correct character set and encoding, see also
661 "PASSPHRASE CHARACTER SET" under Section 1.2.
663 If you are sure you are entering the passphrase right, there is the
664 possibility that the respective key-slot has been damaged. There
665 is no way to recover a damaged key-slot, except from a header
666 backup (see Section 6). For security reasons, there is also no
667 checksum in the key-slots that could tell you whether a key-slot has
668 been damaged. The only checksum present allows recognition of a
669 correct passphrase, but that only works if the passphrase is
670 correct and the respective key-slot is intact.
672 In order to find out whether a key-slot is damaged one has to look
673 for "non-random looking" data in it. There is a tool that
674 automatizes this in the cryptsetup distribution from version 1.6.0
675 onwards. It is located in misc/keyslot_checker/. Instructions how
676 to use and how to interpret results are in the README file. Note
677 that this tool requires a libcryptsetup from cryptsetup 1.6.0 or
678 later (which means libcryptsetup.so.4.5.0 or later). If the tool
679 complains about missing functions in libcryptsetup, you likely
680 have an earlier version from your distribution still installed. You
681 can either point the symbolic link(s) from libcryptsetup.so.4 to
682 the new version manually, or you can uninstall the distribution
683 version of cryptsetup and re-install that from cryptsetup >= 1.6.0
687 * 4.3 Can a bad RAM module cause problems?
689 LUKS and dm-crypt can give the RAM quite a workout, especially when
690 combined with software RAID. In particular the combination RAID5 +
691 LUKS + XFS seems to uncover RAM problems that never caused obvious
692 problems before. Symptoms vary, but often the problem manifest
693 itself when copying large amounts of data, typically several times
694 larger than your main memory.
696 Side note: One thing you should always do on large data
697 copy/movements is to run a verify, for example with the "-d"
698 option of "tar" or by doing a set of MD5 checksums on the source
701 find . -type f -exec md5sum \{\} \; > checksum-file
703 and then a "md5sum -c checksum-file" on the other side. If you get
704 mismatches here, RAM is the primary suspect. A lesser suspect is
705 an overclocked CPU. I have found countless hardware problems in
706 verify runs after copying or making backups. Bit errors are much
707 more common than most people think.
709 Some RAM issues are even worse and corrupt structures in one of the
710 layers. This typically results in lockups, CPU state dumps in the
711 system logs, kernel panic or other things. It is quite possible to
712 have the problem with an encrypted device, but not with an
713 otherwise the same unencrypted device. The reason for that is that
714 encryption has an error amplification property: You flip one bit
715 in an encrypted data block, and the decrypted version has half of
716 its bits flipped. This is an important security property for modern
717 ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
718 get up to a completely changed 512 byte block per bit error. A
719 corrupt block causes a lot more havoc than the occasionally
720 flipped single bit and can result in various obscure errors.
722 Note, that a verify run on copying between encrypted or
723 unencrypted devices will reliably detect corruption, even when the
724 copying itself did not report any problems. If you find defect
725 RAM, assume all backups and copied data to be suspect, unless you
729 * 4.4 How do I test RAM?
731 First you should know that overclocking often makes memory
732 problems worse. So if you overclock (which I strongly recommend
733 against in a system holding data that has some worth), run the
734 tests with the overclocking active.
736 There are two good options. One is Memtest86+ and the other is
737 "memtester" by Charles Cazabon. Memtest86+ requires a reboot and
738 then takes over the machine, while memtester runs from a
739 root-shell. Both use different testing methods and I have found
740 problems fast with each one that the other needed long to find. I
741 recommend running the following procedure until the first error is
744 - Run Memtest86+ for one cycle
746 - Run memtester for one cycle (shut down as many other applications
749 - Run Memtest86+ for 24h or more
751 - Run memtester for 24h or more
753 If all that does not produce error messages, your RAM may be sound,
754 but I have had one weak bit that Memtest86+ needed around 60 hours
755 to find. If you can reproduce the original problem reliably, a good
756 additional test may be to remove half of the RAM (if you have more
757 than one module) and try whether the problem is still there and if
758 so, try with the other half. If you just have one module, get a
759 different one and try with that. If you do overclocking, reduce
760 the settings to the most conservative ones available and try with
767 * 5.1 How long is a secure passphrase ?
769 This is just the short answer. For more info and explanation of
770 some of the terms used in this item, read the rest of Section 5.
771 The actual recommendation is at the end of this item.
773 First, passphrase length is not really the right measure,
774 passphrase entropy is. For example, a random lowercase letter (a-z)
775 gives you 4.7 bit of entropy, one element of a-z0-9 gives you 5.2
776 bits of entropy, an element of a-zA-Z0-9 gives you 5.9 bits and
777 a-zA-Z0-9!@#$%^&:-+ gives you 6.2 bits. On the other hand, a random
778 English word only gives you 0.6...1.3 bits of entropy per
779 character. Using sentences that make sense gives lower entropy,
780 series of random words gives higher entropy. Do not use sentences
781 that can be tied to you or found on your computer. This type of
782 attack is done routinely today.
784 That said, it does not matter too much what scheme you use, but it
785 does matter how much entropy your passphrase contains, because an
786 attacker has to try on average
788 1/2 * 2^(bits of entropy in passphrase)
790 different passphrases to guess correctly.
792 Historically, estimations tended to use computing time estimates,
793 but more modern approaches try to estimate cost of guessing a
796 As an example, I will try to get an estimate from the numbers in
797 http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
798 More references can be found a the end of this document. Note that
799 these are estimates from the defender side, so assuming something
800 is easier than it actually is is fine. An attacker may still have
801 vastly higher cost than estimated here.
803 LUKS uses SHA1 for hasing per default. The claim in the reference is
804 63 billion tries/second for SHA1. We will leave aside the check
805 whether a try actually decrypts a key-slot. Now, the machine has 25
806 GPUs, which I will estimate at an overall lifetime cost of USD/EUR
807 1000 each, and an useful lifetime of 2 years. (This is on the low
808 side.) Disregarding downtime, the machine can then break
810 N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
812 passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed
813 once with SHA1 for EUR/USD 25k. Note that as this can be
814 parallelized, it can be done faster than 2 years with several of
817 For plain dm-crypt (no hash iteration) this is it. This gives (with
818 SHA1, plain dm-crypt default is ripemd160 which seems to be
819 slightly slower than SHA1):
821 Passphrase entropy Cost to break
830 For LUKS, you have to take into account hash iteration in PBKDF2.
831 For a current CPU, there are about 100k iterations (as can be
832 queried with ''cryptsetup luksDump''.
834 The table above then becomes:
836 Passphrase entropy Cost to break
847 To get reasonable security for the next 10 years, it is a good idea
848 to overestimate by a factor of at least 1000.
850 Then there is the question of how much the attacker is willing to
851 spend. That is up to your own security evaluation. For general use,
852 I will assume the attacker is willing to spend up to 1 million
853 EUR/USD. Then we get the following recommendations:
855 Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
856 or a random English sentence of > 135 characters length.
858 LUKS: Use > 65 bit. That is e.g. 14 random chars from a-z or a
859 random English sentence of > 108 characters length.
861 If paranoid, add at least 20 bit. That is roughly four additional
862 characters for random passphrases and roughly 32 characters for a
863 random English sentence.
866 * 5.2 Is LUKS insecure? Everybody can see I have encrypted data!
868 In practice it does not really matter. In most civilized countries
869 you can just refuse to hand over the keys, no harm done. In some
870 countries they can force you to hand over the keys, if they suspect
871 encryption. However the suspicion is enough, they do not have to
872 prove anything. This is for practical reasons, as even the presence
873 of a header (like the LUKS header) is not enough to prove that you
874 have any keys. It might have been an experiment, for example. Or it
875 was used as encrypted swap with a key from /dev/random. So they
876 make you prove you do not have encrypted data. Of course that is
877 just as impossible as the other way round.
879 This means that if you have a large set of random-looking data,
880 they can already lock you up. Hidden containers (encryption hidden
881 within encryption), as possible with Truecrypt, do not help
882 either. They will just assume the hidden container is there and
883 unless you hand over the key, you will stay locked up. Don't have
884 a hidden container? Though luck. Anybody could claim that.
886 Still, if you are concerned about the LUKS header, use plain
887 dm-crypt with a good passphrase. See also Section 2, "What is the
888 difference between "plain" and LUKS format?"
891 * 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
893 If you just create a filesystem on it, most of the old data will
894 still be there. If the old data is sensitive, you should overwrite
895 it before encrypting. In any case, not initializing will leave the
896 old data there until the specific sector gets written. That may
897 enable an attacker to determine how much and where on the
898 partition data was written. If you think this is a risk, you can
899 prevent this by overwriting the encrypted device (here assumed to
900 be named "e1") with zeros like this:
902 dd_rescue -w /dev/zero /dev/mapper/e1
904 or alternatively with one of the following more standard commands:
906 cat /dev/zero > /dev/mapper/e1
907 dd if=/dev/zero of=/dev/mapper/e1
910 * 5.4 How do I securely erase a LUKS (or other) partition?
912 For LUKS, if you are in a desperate hurry, overwrite the LUKS
913 header and key-slot area. This means overwriting the first
914 (keyslots x stripes x keysize) + offset bytes. For the default
915 parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the
916 LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
917 512 bit key) this is 2MiB. (The different offset stems from
918 differences in the sector alignment of the key-slots.) If in doubt,
919 just be generous and overwrite the first 10MB or so, it will likely
920 still be fast enough. A single overwrite with zeros should be
921 enough. If you anticipate being in a desperate hurry, prepare the
922 command beforehand. Example with /dev/sde1 as the LUKS partition
923 and default parameters:
925 head -c 1052672 /dev/zero > /dev/sde1; sync
927 A LUKS header backup or full backup will still grant access to
928 most or all data, so make sure that an attacker does not have
929 access to backups or destroy them as well.
931 If you have time, overwrite the whole LUKS partition with a single
932 pass of zeros. This is enough for current HDDs. For SSDs or FLASH
933 (USB sticks) you may want to overwrite the whole drive several
934 times to be sure data is not retained by wear leveling. This is
935 possibly still insecure as SSD technology is not fully understood
936 in this regard. Still, due to the anti-forensic properties of the
937 LUKS key-slots, a single overwrite of an SSD or FLASH drive could
938 be enough. If in doubt, use physical destruction in addition. Here
939 is a link to some current research results on erasing SSDs and
941 http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
943 Keep in mind to also erase all backups.
945 Example for a zero-overwrite erase of partition sde1 done with
948 dd_rescue -w /dev/zero /dev/sde1
951 * 5.5 How do I securely erase a backup of a LUKS partition or header?
953 That depends on the medium it is stored on. For HDD and SSD, use
954 overwrite with zeros. For an SSD or FLASH drive (USB stick), you
955 may want to overwrite the complete SSD several times and use
956 physical destruction in addition, see last item. For re-writable
957 CD/DVD, a single overwrite should also be enough, due to the
958 anti-forensic properties of the LUKS keyslots. For write-once
959 media, use physical destruction. For low security requirements,
960 just cut the CD/DVD into several parts. For high security needs,
961 shred or burn the medium. If your backup is on magnetic tape, I
962 advise physical destruction by shredding or burning, after
963 overwriting . The problem with magnetic tape is that it has a
964 higher dynamic range than HDDs and older data may well be
965 recoverable after overwrites. Also write-head alignment issues can
966 lead to data not actually being deleted at all during overwrites.
969 * 5.6 What about backup? Does it compromise security?
971 That depends. See item 6.7.
974 * 5.7 Why is all my data permanently gone if I overwrite the LUKS
977 Overwriting the LUKS header in part or in full is the most common
978 reason why access to LUKS containers is lost permanently.
979 Overwriting can be done in a number of fashions, like creating a
980 new filesystem on the raw LUKS partition, making the raw partition
981 part of a raid array and just writing to the raw partition.
983 The LUKS header contains a 256 bit "salt" value and without that no
984 decryption is possible. While the salt is not secret, it is
985 key-grade material and cannot be reconstructed. This is a
986 cryptographically strong "cannot". From observations on the
987 cryptsetup mailing-list, people typically go though the usual
988 stages of grief (Denial, Anger, Bargaining, Depression, Acceptance)
989 when this happens to them. Observed times vary between 1 day and 2
990 weeks to complete the cycle. Seeking help on the mailing-list is
991 fine. Even if we usually cannot help with getting back your data,
992 most people found the feedback comforting.
994 If your header does not contain an intact salt, best go directly
995 to the last stage ("Acceptance") and think about what to do now.
996 There is one exception that I know of: If your LUKS container is
997 still open, then it may be possible to extract the master key from
998 the running system. See Item "How do I recover the master key from
999 a mapped LUKS container?" in Section "Backup and Data Recovery".
1002 * 5.8 What is a "salt"?
1004 A salt is a random key-grade value added to the passphrase before
1005 it is processed. It is not kept secret. The reason for using salts
1006 is as follows: If an attacker wants to crack the password for a
1007 single LUKS container, then every possible passphrase has to be
1008 tried. Typically an attacker will not try every binary value, but
1009 will try words and sentences from a dictionary.
1011 If an attacker wants to attack several LUKS containers with the
1012 same dictionary, then a different approach makes sense: Compute the
1013 resulting slot-key for each dictionary element and store it on
1014 disk. Then the test for each entry is just the slow unlocking with
1015 the slot key (say 0.00001 sec) instead of calculating the slot-key
1016 first (1 sec). For a single attack, this does not help. But if you
1017 have more than one container to attack, this helps tremendously,
1018 also because you can prepare your table before you even have the
1019 container to attack! The calculation is also very simple to
1020 parallelize. You could, for example, use the night-time unused CPU
1021 power of your desktop PCs for this.
1023 This is where the salt comes in. If the salt is combined with the
1024 passphrase (in the simplest form, just appended to it), you
1025 suddenly need a separate table for each salt value. With a
1026 reasonably-sized salt value (256 bit, e.g.) this is quite
1030 * 5.9 Is LUKS secure with a low-entropy (bad) passphrase?
1032 Note: You should only use the 94 printable characters from 7 bit
1033 ASCII code to prevent your passphrase from failing when the
1034 character encoding changes, e.g. because of a system upgrade, see
1035 also the note at the very start of this FAQ under "WARNINGS".
1037 This needs a bit of theory. The quality of your passphrase is
1038 directly related to its entropy (information theoretic, not
1039 thermodynamic). The entropy says how many bits of "uncertainty" or
1040 "randomness" are in you passphrase. In other words, that is how
1041 difficult guessing the passphrase is.
1043 Example: A random English sentence has about 1 bit of entropy per
1044 character. A random lowercase (or uppercase) character has about
1047 Now, if n is the number of bits of entropy in your passphrase and t
1048 is the time it takes to process a passphrase in order to open the
1049 LUKS container, then an attacker has to spend at maximum
1051 attack_time_max = 2^n * t
1053 time for a successful attack and on average half that. There is no
1054 way getting around that relationship. However, there is one thing
1055 that does help, namely increasing t, the time it takes to use a
1056 passphrase, see next FAQ item.
1058 Still, if you want good security, a high-entropy passphrase is the
1059 only option. For example, a low-entropy passphrase can never be
1060 considered secure against a TLA-level (Three Letter Agency level,
1061 i.e. government-level) attacker, no matter what tricks are used in
1062 the key-derivation function. Use at least 64 bits for secret stuff.
1063 That is 64 characters of English text (but only if randomly chosen)
1064 or a combination of 12 truly random letters and digits.
1066 For passphrase generation, do not use lines from very well-known
1067 texts (religious texts, Harry potter, etc.) as they are to easy to
1068 guess. For example, the total Harry Potter has about 1'500'000
1069 words (my estimation). Trying every 64 character sequence starting
1070 and ending at a word boundary would take only something like 20
1071 days on a single CPU and is entirely feasible. To put that into
1072 perspective, using a number of Amazon EC2 High-CPU Extra Large
1073 instances (each gives about 8 real cores), this test costs
1074 currently about 50USD/EUR, but can be made to run arbitrarily fast.
1076 On the other hand, choosing 1.5 lines from, say, the Wheel of Time
1077 is in itself not more secure, but the book selection adds quite a
1078 bit of entropy. (Now that I have mentioned it here, don't use tWoT
1079 either!) If you add 2 or 3 typos or switch some words around, then
1080 this is good passphrase material.
1083 * 5.10 What is "iteration count" and why is decreasing it a bad idea?
1085 Iteration count is the number of PBKDF2 iterations a passphrase is
1086 put through before it is used to unlock a key-slot. Iterations are
1087 done with the explicit purpose to increase the time that it takes
1088 to unlock a key-slot. This provides some protection against use of
1089 low-entropy passphrases.
1091 The idea is that an attacker has to try all possible passphrases.
1092 Even if the attacker knows the passphrase is low-entropy (see last
1093 item), it is possible to make each individual try take longer. The
1094 way to do this is to repeatedly hash the passphrase for a certain
1095 time. The attacker then has to spend the same time (given the same
1096 computing power) as the user per try. With LUKS, the default is 1
1097 second of PBKDF2 hashing.
1099 Example 1: Lets assume we have a really bad passphrase (e.g. a
1100 girlfriends name) with 10 bits of entropy. With the same CPU, an
1101 attacker would need to spend around 500 seconds on average to
1102 break that passphrase. Without iteration, it would be more like
1103 0.0001 seconds on a modern CPU.
1105 Example 2: The user did a bit better and has 32 chars of English
1106 text. That would be about 32 bits of entropy. With 1 second
1107 iteration, that means an attacker on the same CPU needs around 136
1108 years. That is pretty impressive for such a weak passphrase.
1109 Without the iterations, it would be more like 50 days on a modern
1110 CPU, and possibly far less.
1112 In addition, the attacker can both parallelize and use special
1113 hardware like GPUs or FPGAs to speed up the attack. The attack can
1114 also happen quite some time after the luksFormat operation and CPUs
1115 can have become faster and cheaper. For that reason you want a
1116 bit of extra security. Anyways, in Example 1 your are screwed.
1117 In example 2, not necessarily. Even if the attack is faster, it
1118 still has a certain cost associated with it, say 10000 EUR/USD
1119 with iteration and 1 EUR/USD without iteration. The first can be
1120 prohibitively expensive, while the second is something you try
1121 even without solid proof that the decryption will yield something
1124 The numbers above are mostly made up, but show the idea. Of course
1125 the best thing is to have a high-entropy passphrase.
1127 Would a 100 sec iteration time be even better? Yes and no.
1128 Cryptographically it would be a lot better, namely 100 times better.
1129 However, usability is a very important factor for security
1130 technology and one that gets overlooked surprisingly often. For
1131 LUKS, if you have to wait 2 minutes to unlock the LUKS container,
1132 most people will not bother and use less secure storage instead. It
1133 is better to have less protection against low-entropy passphrases
1134 and people actually use LUKS, than having them do without
1135 encryption altogether.
1137 Now, what about decreasing the iteration time? This is generally a
1138 very bad idea, unless you know and can enforce that the users only
1139 use high-entropy passphrases. If you decrease the iteration time
1140 without ensuring that, then you put your users at increased risk,
1141 and considering how rarely LUKS containers are unlocked in a
1142 typical work-flow, you do so without a good reason. Don't do it.
1143 The iteration time is already low enough that users with entropy
1144 low passphrases are vulnerable. Lowering it even further increases
1145 this danger significantly.
1148 * 5.11 Some people say PBKDF2 is insecure?
1150 There is some discussion that a hash-function should have a "large
1151 memory" property, i.e. that it should require a lot of memory to be
1152 computed. This serves to prevent attacks using special programmable
1153 circuits, like FPGAs, and attacks using graphics cards. PBKDF2
1154 does not need a lot of memory and is vulnerable to these attacks.
1155 However, the publication usually referred in these discussions is
1156 not very convincing in proving that the presented hash really is
1157 "large memory" (that may change, email the FAQ maintainer when it
1158 does) and it is of limited usefulness anyways. Attackers that use
1159 clusters of normal PCs will not be affected at all by a "large
1160 memory" property. For example the US Secret Service is known to
1161 use the off-hour time of all the office PCs of the Treasury for
1162 password breaking. The Treasury has about 110'000 employees.
1163 Assuming every one has an office PC, that is significant computing
1164 power, all of it with plenty of memory for computing "large
1165 memory" hashes. Bot-net operators also have all the memory they
1166 want. The only protection against a resourceful attacker is a
1167 high-entropy passphrase, see items 5.9 and 5.10.
1170 * 5.12 What about iteration count with plain dm-crypt?
1172 Simple: There is none. There is also no salting. If you use plain
1173 dm-crypt, the only way to be secure is to use a high entropy
1174 passphrase. If in doubt, use LUKS instead.
1177 * 5.13 Is LUKS with default parameters less secure on a slow CPU?
1179 Unfortunately, yes. However the only aspect affected is the
1180 protection for low-entropy passphrase or master-key. All other
1181 security aspects are independent of CPU speed.
1183 The master key is less critical, as you really have to work at it
1184 to give it low entropy. One possibility is to supply the master key
1185 yourself. If that key is low-entropy, then you get what you
1186 deserve. The other known possibility is to use /dev/urandom for
1187 key generation in an entropy-starved situation (e.g. automatic
1188 installation on an embedded device without network and other entropy
1191 For the passphrase, don't use a low-entropy passphrase. If your
1192 passphrase is good, then a slow CPU will not matter. If you insist
1193 on a low-entropy passphrase on a slow CPU, use something like
1194 "--iter-time=10" or higher and wait a long time on each LUKS unlock
1195 and pray that the attacker does not find out in which way exactly
1196 your passphrase is low entropy. This also applies to low-entropy
1197 passphrases on fast CPUs. Technology can do only so much to
1198 compensate for problems in front of the keyboard.
1201 * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
1203 Note: This item applies both to plain dm-crypt and to LUKS
1205 The problem is that cbc-plain has a fingerprint vulnerability, where
1206 a specially crafted file placed into the crypto-container can be
1207 recognized from the outside. The issue here is that for cbc-plain
1208 the initialization vector (IV) is the sector number. The IV gets
1209 XORed to the first data chunk of the sector to be encrypted. If you
1210 make sure that the first data block to be stored in a sector
1211 contains the sector number as well, the first data block to be
1212 encrypted is all zeros and always encrypted to the same ciphertext.
1213 This also works if the first data chunk just has a constant XOR
1214 with the sector number. By having several shifted patterns you can
1215 take care of the case of a non-power-of-two start sector number of
1218 This mechanism allows you to create a pattern of sectors that have
1219 the same first ciphertext block and signal one bit per sector to the
1220 outside, allowing you to e.g. mark media files that way for
1221 recognition without decryption. For large files this is a
1222 practical attack. For small ones, you do not have enough blocks to
1223 signal and take care of different file starting offsets.
1225 In order to prevent this attack, the default was changed to
1226 cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
1227 encryption key as key. This makes the IV unpredictable without
1228 knowing the encryption key and the watermarking attack fails.
1231 * 5.15 Are there any problems with "plain" IV? What is "plain64"?
1233 First, "plain" and "plain64" are both not secure to use with CBC,
1234 see previous FAQ item.
1236 However there are modes, like XTS, that are secure with "plain" IV.
1237 The next limit is that "plain" is 64 bit, with the upper 32 bit set
1238 to zero. This means that on volumes larger than 2TiB, the IV
1239 repeats, creating a vulnerability that potentially leaks some
1240 data. To avoid this, use "plain64", which uses the full sector
1241 number up to 64 bit. Note that "plain64" requires a kernel >=
1242 2.6.33. Also note that "plain64" is backwards compatible for
1243 volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
1244 does not cause any performance penalty compared to "plain".
1247 * 5.16 What about XTS mode?
1249 XTS mode is potentially even more secure than cbc-essiv (but only if
1250 cbc-essiv is insecure in your scenario). It is a NIST standard and
1251 used, e.g. in Truecrypt. At the moment, if you want to use it, you
1252 have to specify it manually as "aes-xts-plain", i.e.
1254 cryptsetup -c aes-xts-plain luksFormat <device>
1256 For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ
1257 item on "plain" and "plain64"):
1259 cryptsetup -c aes-xts-plain64 luksFormat <device>
1261 There is a potential security issue with XTS mode and large blocks.
1262 LUKS and dm-crypt always use 512B blocks and the issue does not
1266 * 5.17 Is LUKS FIPS-140-2 certified?
1268 No. But that is more a problem of FIPS-140-2 than of LUKS. From a
1269 technical point-of-view, LUKS with the right parameters would be
1270 FIPS-140-2 compliant, but in order to make it certified, somebody
1271 has to pay real money for that. And then, whenever cryptsetup is
1272 changed or extended, the certification lapses and has to be
1275 From the aspect of actual security, LUKS with default parameters
1276 should be as good as most things that are FIPS-140-2 certified,
1277 although you may want to make sure to use /dev/random (by
1278 specifying --use-random on luksFormat) as randomness source for
1279 the master key to avoid being potentially insecure in an
1280 entropy-starved situation.
1283 * 5.18 What about Plausible Deniability?
1285 First let me attempt a definition for the case of encrypted
1286 filesystems: Plausible deniability is when you hide encrypted data
1287 inside an encrypted container and it is not possible to prove it is
1288 there. The idea is compelling and on first glance it seems
1289 possible to do it. And from a cryptographic point of view, it
1290 actually is possible.
1292 So, does it work in practice? No, unfortunately. The reasoning used
1293 by its proponents is fundamentally flawed in several ways and the
1294 cryptographic properties fail fatally when colliding with the real
1297 First, why should "I do not have a hidden partition" be any more
1298 plausible than "I forgot my crypto key" or "I wiped that partition
1299 with random data, nothing in there"? I do not see any reason.
1301 Second, there are two types of situations: Either they cannot force
1302 you to give them the key (then you simply do not) or the can. In
1303 the second case, they can always do bad things to you, because they
1304 cannot prove that you have the key in the first place! This means
1305 they do not have to prove you have the key, or that this random
1306 looking data on your disk is actually encrypted data. So the
1307 situation will allow them to waterboard/lock-up/deport you
1308 anyways, regardless of how "plausible" your deniability is. Do not
1309 have a hidden partition you could show to them, but there are
1310 indications you may? Too bad for you. Unfortunately "plausible
1311 deniability" also means you cannot prove there is no hidden data.
1313 Third, hidden partitions are not that hidden. There are basically
1314 just two possibilities: a) Make a large crypto container, but put a
1315 smaller filesystem in there and put the hidden partition into the
1316 free space. Unfortunately this is glaringly obvious and can be
1317 detected in an automated fashion. This means that the initial
1318 suspicion to put you under duress in order to make you reveal you
1319 hidden data is given. b) Make a filesystem that spans the whole
1320 encrypted partition, and put the hidden partition into space not
1321 currently used by that filesystem. Unfortunately that is also
1322 glaringly obvious, as you then cannot write to the filesystem
1323 without a high risk of destroying data in the hidden container.
1324 Have not written anything to the encrypted filesystem in a while?
1325 Too bad, they have the suspicion they need to do unpleasant things
1328 To be fair, if you prepare option b) carefully and directly before
1329 going into danger, it may work. But then, the mere presence of
1330 encrypted data may already be enough to get you into trouble in
1331 those places were they can demand encryption keys.
1333 Here is an additional reference for some problems with plausible
1334 deniability: http://www.schneier.com/paper-truecrypt-dfs.pdf I
1335 strongly suggest you read it.
1337 So, no, I will not provide any instructions on how to do it with
1338 plain dm-crypt or LUKS. If you insist on shooting yourself in the
1339 foot, you can figure out how to do it yourself.
1342 * 5.19 What about SSDs, Flash and Hybrid Drives?
1344 The problem is that you cannot reliably erase parts of these
1345 devices, mainly due to wear-leveling and possibly defect
1348 Basically, when overwriting a sector (of 512B), what the device
1349 does is to move an internal sector (may be 128kB or even larger) to
1350 some pool of discarded, not-yet erased unused sectors, take a
1351 fresh empty sector from the empty-sector pool and copy the old
1352 sector over with the changes to the small part you wrote. This is
1353 done in some fashion so that larger writes do not cause a lot of
1354 small internal updates.
1356 The thing is that the mappings between outside-addressable sectors
1357 and inside sectors is arbitrary (and the vendors are not talking).
1358 Also the discarded sectors are not necessarily erased immediately.
1359 They may linger a long time.
1361 For plain dm-crypt, the consequences are that older encrypted data
1362 may be lying around in some internal pools of the device. Thus may
1363 or may not be a problem and depends on the application. Remember
1364 the same can happen with a filesystem if consecutive writes to the
1365 same area of a file can go to different sectors.
1367 However, for LUKS, the worst case is that key-slots and LUKS
1368 header may end up in these internal pools. This means that password
1369 management functionality is compromised (the old passwords may
1370 still be around, potentially for a very long time) and that fast
1371 erase by overwriting the header and key-slot area is insecure.
1373 Also keep in mind that the discarded/used pool may be large. For
1374 example, a 240GB SSD has about 16GB of spare area in the chips that
1375 it is free to do with as it likes. You would need to make each
1376 individual key-slot larger than that to allow reliable overwriting.
1377 And that assumes the disk thinks all other space is in use.
1378 Reading the internal pools using forensic tools is not that hard,
1379 but may involve some soldering.
1383 If you trust the device vendor (you probably should not...) you can
1384 try an ATA "secure erase" command for SSDs. That does not work for
1385 USB keys though and may or may not be secure for a hybrid drive. If
1386 it finishes on an SSD after a few seconds, it was possibly faked.
1387 UNfortunately, for hybrid drives that indicator does not work, as
1388 the drive may well take the time to dully erase the magnetic part,
1389 but only mark the SSD/Flash part as erased while data is still in
1392 If you can do without password management and are fine with doing
1393 physical destruction for permanently deleting data (always after
1394 one or several full overwrites!), you can use plain dm-crypt or
1397 If you want or need the original LUKS security features to work,
1398 you can use a detached LUKS header and put that on a conventional,
1399 magnetic disk. That leaves potentially old encrypted data in the
1400 pools on the disk, but otherwise you get LUKS with the same
1401 security as on a magnetic disk.
1403 If you are concerned about your laptop being stolen, you are likely
1404 fine using LUKS on an SSD or hybrid drive. An attacker would need
1405 to have access to an old passphrase (and the key-slot for this old
1406 passphrase would actually need to still be somewhere in the SSD)
1407 for your data to be at risk. So unless you pasted your old
1408 passphrase all over the Internet or the attacker has knowledge of
1409 it from some other source and does a targeted laptop theft to get
1410 at your data, you should be fine.
1413 6. Backup and Data Recovery
1416 * 6.1 Why do I need Backup?
1418 First, disks die. The rate for well-treated (!) disk is about 5%
1419 per year, which is high enough to worry about. There is some
1420 indication that this may be even worse for some SSDs. This applies
1421 both to LUKS and plain dm-crypt partitions.
1423 Second, for LUKS, if anything damages the LUKS header or the
1424 key-stripe area then decrypting the LUKS device can become
1425 impossible. This is a frequent occurrence. For example an
1426 accidental format as FAT or some software overwriting the first
1427 sector where it suspects a partition boot sector typically makes a
1428 LUKS partition permanently inaccessible. See more below on LUKS
1431 So, data-backup in some form is non-optional. For LUKS, you may
1432 also want to store a header backup in some secure location. This
1433 only needs an update if you change passphrases.
1436 * 6.2 How do I backup a LUKS header?
1438 While you could just copy the appropriate number of bytes from the
1439 start of the LUKS partition, the best way is to use command option
1440 "luksHeaderBackup" of cryptsetup. This protects also against
1441 errors when non-standard parameters have been used in LUKS
1442 partition creation. Example:
1445 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
1447 To restore, use the inverse command, i.e.
1449 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
1452 * 6.3 How do I test a LUKS header?
1456 cryptsetup -v isLuks <device>
1458 on the device. Without the "-v" it just signals its result via
1459 exit-status. You can also use the more general test
1463 which will also detect other types and give some more info. Omit
1464 "-p" for old versions of blkid that do not support it.
1467 * 6.4 How do I backup a LUKS or dm-crypt partition?
1469 There are two options, a sector-image and a plain file or
1470 filesystem backup of the contents of the partition. The sector
1471 image is already encrypted, but cannot be compressed and contains
1472 all empty space. The filesystem backup can be compressed, can
1473 contain only part of the encrypted device, but needs to be
1474 encrypted separately if so desired.
1476 A sector-image will contain the whole partition in encrypted form,
1477 for LUKS the LUKS header, the keys-slots and the data area. It can
1478 be done under Linux e.g. with dd_rescue (for a direct image copy)
1479 and with "cat" or "dd". Example:
1481 cat /dev/sda10 > sda10.img
1482 dd_rescue /dev/sda10 sda10.img
1484 You can also use any other backup software that is capable of making
1485 a sector image of a partition. Note that compression is
1486 ineffective for encrypted data, hence it does not make sense to
1489 For a filesystem backup, you decrypt and mount the encrypted
1490 partition and back it up as you would a normal filesystem. In this
1491 case the backup is not encrypted, unless your encryption method
1492 does that. For example you can encrypt a backup with "tar" as
1495 tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
1497 And verify the backup like this if you are at "path":
1499 cat backup.tbz2.gpg | gpg - | tar djf -
1501 Note: Always verify backups, especially encrypted ones.
1503 In both cases GnuPG will ask you interactively for your symmetric
1504 key. The verify will only output errors. Use "tar dvjf -" to get
1505 all comparison results. To make sure no data is written to disk
1506 unencrypted, turn off swap if it is not encrypted before doing the
1509 You can of course use different or no compression and you can use
1510 an asymmetric key if you have one and have a backup of the secret
1511 key that belongs to it.
1513 A second option for a filesystem-level backup that can be used when
1514 the backup is also on local disk (e.g. an external USB drive) is
1515 to use a LUKS container there and copy the files to be backed up
1516 between both mounted containers. Also see next item.
1519 * 6.5 Do I need a backup of the full partition? Would the header and
1520 key-slots not be enough?
1522 Backup protects you against two things: Disk loss or corruption
1523 and user error. By far the most questions on the dm-crypt mailing
1524 list about how to recover a damaged LUKS partition are related
1525 to user error. For example, if you create a new filesystem on a
1526 LUKS partition, chances are good that all data is lost
1529 For this case, a header+key-slot backup would often be enough. But
1530 keep in mind that a well-treated (!) HDD has roughly a failure
1531 risk of 5% per year. It is highly advisable to have a complete
1532 backup to protect against this case.
1535 * *6.6 What do I need to backup if I use "decrypt_derived"?
1537 This is a script in Debian, intended for mounting /tmp or swap with
1538 a key derived from the master key of an already decrypted device.
1539 If you use this for an device with data that should be persistent,
1540 you need to make sure you either do not lose access to that master
1541 key or have a backup of the data. If you derive from a LUKS
1542 device, a header backup of that device would cover backing up the
1543 master key. Keep in mind that this does not protect against disk
1546 Note: If you recreate the LUKS header of the device you derive from
1547 (using luksFormat), the master key changes even if you use the same
1548 passphrase(s) and you will not be able to decrypt the derived
1549 device with the new LUKS header.
1552 * 6.7 Does a backup compromise security?
1554 Depends on how you do it. However if you do not have one, you are
1555 going to eventually lose your encrypted data.
1557 There are risks introduced by backups. For example if you
1558 change/disable a key-slot in LUKS, a binary backup of the partition
1559 will still have the old key-slot. To deal with this, you have to
1560 be able to change the key-slot on the backup as well, securely
1561 erase the backup or do a filesystem-level backup instead of a binary
1564 If you use dm-crypt, backup is simpler: As there is no key
1565 management, the main risk is that you cannot wipe the backup when
1566 wiping the original. However wiping the original for dm-crypt
1567 should consist of forgetting the passphrase and that you can do
1568 without actual access to the backup.
1570 In both cases, there is an additional (usually small) risk with
1571 binary backups: An attacker can see how many sectors and which
1572 ones have been changed since the backup. To prevent this, use a
1573 filesystem level backup method that encrypts the whole backup in
1574 one go, e.g. as described above with tar and GnuPG.
1576 My personal advice is to use one USB disk (low value data) or
1577 three disks (high value data) in rotating order for backups, and
1578 either use independent LUKS partitions on them, or use encrypted
1579 backup with tar and GnuPG.
1581 If you do network-backup or tape-backup, I strongly recommend to
1582 go the filesystem backup path with independent encryption, as you
1583 typically cannot reliably delete data in these scenarios,
1584 especially in a cloud setting. (Well, you can burn the tape if it
1585 is under your control...)
1588 * 6.8 What happens if I overwrite the start of a LUKS partition or
1589 damage the LUKS header or key-slots?
1591 There are two critical components for decryption: The salt values
1592 in the header itself and the key-slots. If the salt values are
1593 overwritten or changed, nothing (in the cryptographically strong
1594 sense) can be done to access the data, unless there is a backup
1595 of the LUKS header. If a key-slot is damaged, the data can still
1596 be read with a different key-slot, if there is a remaining
1597 undamaged and used key-slot. Note that in order to make a key-slot
1598 unrecoverable in a cryptographically strong sense, changing about
1599 4-6 bits in random locations of its 128kiB size is quite enough.
1602 * 6.9 What happens if I (quick) format a LUKS partition?
1604 I have not tried the different ways to do this, but very likely you
1605 will have written a new boot-sector, which in turn overwrites the
1606 LUKS header, including the salts, making your data permanently
1607 irretrievable, unless you have a LUKS header backup. You may also
1608 damage the key-slots in part or in full. See also last item.
1611 * 6.10 How do I recover the master key from a mapped LUKS container?
1613 This is typically only needed if you managed to damage your LUKS
1614 header, but the container is still mapped, i.e. "luksOpen"ed. It
1615 also helps if you have a mapped container that you forgot or do not
1616 know a passphrase for (e.g. on a long running server.)
1618 WARNING: Things go wrong, do a full backup before trying this!
1620 WARNING: This exposes the master key of the LUKS container. Note
1621 that both ways to recreate a LUKS header with the old master key
1622 described below will write the master key to disk. Unless you are
1623 sure you have securely erased it afterwards, e.g. by writing it to
1624 an encrypted partition, RAM disk or by erasing the filesystem you
1625 wrote it to by a complete overwrite, you should change the master
1626 key afterwards. Changing the master key requires a full data
1627 backup, luksFormat and then restore of the backup.
1629 First, there is a script by Milan that automates the whole
1630 process, except generating a new LUKS header with the old master
1631 key (it prints the command for that though):
1633 http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
1635 You can also do this manually. Here is how:
1637 - Get the master key from the device mapper. This is done by the
1638 following command. Substitute c5 for whatever you mapped to:
1640 # dmsetup table --target crypt --showkey /dev/mapper/c5
1642 0 200704 crypt aes-cbc-essiv:sha256
1643 a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
1646 The result is actually one line, wrapped here for clarity. The long
1647 hex string is the master key.
1649 - Convert the master key to a binary file representation. You can
1650 do this manually, e.g. with hexedit. You can also use the tool
1651 "xxd" from vim like this:
1653 echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
1655 - Do a luksFormat to create a new LUKS header.
1657 NOTE: If your header is intact and you just forgot the
1658 passphrase, you can just set a new passphrase, see next
1661 Unmap the device before you do that (luksClose). Then do
1663 cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
1665 Note that if the container was created with other than the default
1666 settings of the cryptsetup version you are using, you need to give
1667 additional parameters specifying the deviations. If in doubt, try
1668 the script by Milan. It does recover the other parameters as well.
1670 Side note: This is the way the decrypt_derived script gets at the
1671 master key. It just omits the conversion and hashes the master key
1674 - If the header is intact and you just forgot the passphrase, just
1675 set a new passphrase like this:
1677 cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
1679 You may want to disable the old one afterwards.
1682 * 6.11 What does the on-disk structure of dm-crypt look like?
1684 There is none. dm-crypt takes a block device and gives encrypted
1685 access to each of its blocks with a key derived from the passphrase
1686 given. If you use a cipher different than the default, you have to
1687 specify that as a parameter to cryptsetup too. If you want to
1688 change the password, you basically have to create a second
1689 encrypted device with the new passphrase and copy your data over.
1690 On the plus side, if you accidentally overwrite any part of a
1691 dm-crypt device, the damage will be limited to the are you
1695 * 6.12 What does the on-disk structure of LUKS look like?
1697 A LUKS partition consists of a header, followed by 8 key-slot
1698 descriptors, followed by 8 key slots, followed by the encrypted
1701 Header and key-slot descriptors fill the first 592 bytes. The
1702 key-slot size depends on the creation parameters, namely on the
1703 number of anti-forensic stripes, key material offset and master
1706 With the default parameters, each key-slot is a bit less than
1707 128kiB in size. Due to sector alignment of the key-slot start,
1708 that means the key block 0 is at offset 0x1000-0x20400, key
1709 block 1 at offset 0x21000-0x40400, and key block 7 at offset
1710 0xc1000-0xe0400. The space to the next full sector address is
1711 padded with zeros. Never used key-slots are filled with what the
1712 disk originally contained there, a key-slot removed with
1713 "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Due to
1714 2MiB default alignment, start of the data area for cryptsetup 1.3
1715 and later is at 2MiB, i.e. at 0x200000. For older versions, it is
1716 at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes
1717 from the start of the partition. Incidentally, "luksHeaderBackup"
1718 for a LUKS container created with default parameters dumps exactly
1719 the first 2MiB (or 1'052'672 bytes for headers created with
1720 cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
1723 For non-default parameters, you have to figure out placement
1724 yourself. "luksDump" helps. See also next item. For the most common
1725 non-default settings, namely aes-xts-plain with 512 bit key, the
1726 offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
1727 0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, ..., and start of
1728 bulk data at 0x200000.
1730 The exact specification of the format is here:
1731 http://code.google.com/p/cryptsetup/wiki/Specification
1734 * 6.13 What is the smallest possible LUKS container?
1736 Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
1737 modern Linux partitioning tools that also align to 1MB, this will
1738 result in alignment to 2k sectors and typical Flash/SSD sectors,
1739 which is highly desirable for a number of reasons. Changing the
1740 alignment is not recommended.
1742 That said, with default parameters, the data area starts at
1743 exactly 2MB offset (at 0x101000 for cryptsetup versions before
1744 1.3). The smallest data area you can have is one sector of 512
1745 bytes. Data areas of 0 bytes can be created, but fail on mapping.
1747 While you cannot put a filesystem into something this small, it may
1748 still be used to contain, for example, key. Note that with current
1749 formatting tools, a partition for a container this size will be
1750 3MiB anyways. If you put the LUKS container into a file (via
1751 losetup and a loopback device), the file needs to be 2097664 bytes
1752 in size, i.e. 2MiB + 512B.
1754 There two ways to influence the start of the data area are key-size
1757 For alignment, you can go down to 1 on the parameter. This will
1758 still leave you with a data-area starting at 0x101000, i.e.
1759 1MiB+4096B (default parameters) as alignment will be rounded up to
1760 the next multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run
1761 on a larger file and dump the LUKS header to get actual
1764 For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
1765 (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
1766 You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
1767 128 bit has to be considered insecure today.
1769 Example 1 - AES 128 bit with CBC:
1771 cryptsetup luksFormat -s 128 --align-payload=8 <device>
1773 This results in a data offset of 0x81000, i.e. 516KiB or 528384
1774 bytes. Add one 512 byte sector and the smallest LUKS container size
1775 with these parameters is 516KiB + 512B or 528896 bytes.
1777 Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
1779 cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
1781 This results in a data offset of 0x41000, i.e. 260kiB or 266240
1782 bytes, with a minimal LUKS container size of 260kiB + 512B or
1786 * 6.14 I think this is overly complicated. Is there an alternative?
1788 Not really. Encryption comes at a price. You can use plain
1789 dm-crypt to simplify things a bit. It does not allow multiple
1790 passphrases, but on the plus side, it has zero on disk description
1791 and if you overwrite some part of a plain dm-crypt partition,
1792 exactly the overwritten parts are lost (rounded up to sector
1796 * 6.15 Can I clone a LUKS container?
1798 You can, but it breaks security, because the cloned container has
1799 the same header and hence the same master key. You cannot change
1800 the master key on a LUKS container, even if you change the
1801 passphrase(s), the master key stays the same. That means whoever
1802 has access to one of the clones can decrypt them all, completely
1803 bypassing the passphrases.
1805 The right way to do this is to first luksFormat the target
1806 container, then to clone the contents of the source container, with
1807 both containers mapped, i.e. decrypted. You can clone the decrypted
1808 contents of a LUKS container in binary mode, although you may run
1809 into secondary issues with GUIDs in filesystems, partition tables,
1810 RAID-components and the like. These are just the normal problems
1811 binary cloning causes.
1813 Note that if you need to ship (e.g.) cloned LUKS containers with a
1814 default passphrase, that is fine as long as each container was
1815 individually created (and hence has its own master key). In this
1816 case, changing the default passphrase will make it secure again.
1819 7. Interoperability with other Disk Encryption Tools
1822 * 7.1 What is this section about?
1824 Cryptsetup for plain dm-crypt can be used to access a number of
1825 on-disk formats created by tools like loop-aes patched into
1826 losetup. This sometimes works and sometimes does not. This
1827 section collects insights into what works, what does not and where
1828 more information is required.
1830 Additional information may be found in the mailing-list archives,
1831 mentioned at the start of this FAQ document. If you have a
1832 solution working that is not yet documented here and think a wider
1833 audience may be interested, please email the FAQ maintainer.
1836 * 7.2 loop-aes: General observations.
1838 One problem is that there are different versions of losetup around.
1839 loop-aes is a patch for losetup. Possible problems and deviations
1840 from cryptsetup option syntax include:
1842 - Offsets specified in bytes (cryptsetup: 512 byte sectors)
1844 - The need to specify an IV offset
1846 - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
1848 - Key size needs specifying (e.g. "-s 128" for 128 bit keys)
1850 - Passphrase hash algorithm needs specifying
1852 Also note that because plain dm-crypt and loop-aes format does not
1853 have metadata, and while the loopAES extension for cryptsetup tries
1854 autodetection (see command loopaesOpen), it may not always work.
1855 If you still have the old set-up, using a verbosity option (-v)
1856 on mapping with the old tool or having a look into the system logs
1857 after setup could give you the information you need. Below, there
1858 are also some things that worked for somebody.
1861 * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32
1863 In this case, the main problem seems to be that this variant of
1864 losetup takes the offset (-o option) in bytes, while cryptsetup
1865 takes it in sectors of 512 bytes each. Example: The losetup command
1867 losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
1868 mount /dev/loop0 mount-point
1872 cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
1873 mount /dev/mapper/e1 mount-point
1876 * 7.4 loop-aes with 160 bit key
1878 This seems to be sometimes used with twofish and blowfish and
1879 represents a 160 bit ripemed160 hash output padded to 196 bit key
1880 length. It seems the corresponding options for cryptsetup are
1882 --cipher twofish-cbc-null -s 192 -h ripemd160:20
1885 * 7.5 loop-aes v1 format OpenSUSE
1887 Apparently this is done by older OpenSUSE distros and stopped
1888 working from OpenSUSE 12.1 to 12.2. One user had success with the
1891 cryptsetup create <target> <device> -c aes -s 128 -h sha256
1894 * 7.6 Kernel encrypted loop device (cryptoloop)
1896 There are a number of different losetup implementations for using
1897 encrypted loop devices so getting this to work may need a bit of
1900 NOTE: Do NOT use this for new containers! Some of the existing
1901 implementations are insecure and future support is uncertain.
1903 Example for a compatible mapping:
1905 losetup -e twofish -N /dev/loop0 /image.img
1909 cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain
1911 with the mapping being done to /dev/mapper/image_plain instead of
1916 Cipher, mode and pasword hash (or no hash):
1918 -e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256]
1919 -e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256]
1921 Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512
1925 -o 2560 => -o 5 -p 5 # 2560/512 = 5
1927 There is no replacement for --pass-fd, it has to be emulated using
1928 keyfiles, see the cryptsetup man-page.
1931 8. Issues with Specific Versions of cryptsetup
1934 * 8.1 When using the create command for plain dm-crypt with
1935 cryptsetup 1.1.x, the mapping is incompatible and my data is not
1938 With cryptsetup 1.1.x, the distro maintainer can define different
1939 default encryption modes for LUKS and plain devices. You can check
1940 these compiled-in defaults using "cryptsetup --help". Moreover, the
1941 plain device default changed because the old IV mode was
1942 vulnerable to a watermarking attack.
1944 If you are using a plain device and you need a compatible mode, just
1945 specify cipher, key size and hash algorithm explicitly. For
1946 compatibility with cryptsetup 1.0.x defaults, simple use the
1949 cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
1951 LUKS stores cipher and mode in the metadata on disk, avoiding this
1955 * 8.2 cryptsetup on SLED 10 has problems...
1957 SLED 10 is missing an essential kernel patch for dm-crypt, which
1958 is broken in its kernel as a result. There may be a very old
1959 version of cryptsetup (1.0.x) provided by SLED, which should also
1960 not be used anymore as well. My advice would be to drop SLED 10.
1963 9. References and Further Reading
1966 * Purpose of this Section
1968 The purpose of this section is to collect references to all
1969 materials that do not fit the FAQ but are relevant in some fashion.
1970 This can be core topics like the LUKS spec or disk encryption, but
1971 it can also be more tangential, like secure storage management or
1972 cryptography used in LUKS. It should still have relevance to
1973 cryptsetup and its applications.
1975 If you wan to see something added here, send email to the
1976 maintainer (or the cryptsetup mailing list) giving an URL, a
1977 description (1-3 lines preferred) and a section to put it in. You
1978 can also propose new sections.
1980 At this time I would like to limit the references to things that
1981 are available on the web.
1986 - LUKS on-disk format spec:
1987 http://code.google.com/p/cryptsetup/wiki/Specification
1992 - Some code examples are in the source package under docs/examples
1995 * Brute-forcing passphrases
1998 http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
2001 http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
2007 * SSD and Flash Disk Related
2013 * Attacks Against Disk Encryption
2016 * Risk Management as Relevant for Disk Encryption
2024 A. Contributors In no particular order: