1 Frequently Asked Questions Cryptsetup/LUKS
9 6. Backup and Data Recovery
10 7. Interoperability with other Disk Encryption Tools
11 8. Issues with Specific Versions of cryptsetup
12 9. The Initrd question
14 11. References and Further Reading
22 This is the FAQ (Frequently Asked Questions) for cryptsetup. It covers
23 Linux disk encryption with plain dm-crypt (one passphrase, no
24 management, no metadata on disk) and LUKS (multiple user keys with one
25 master key, anti-forensic features, metadata block at start of device,
26 ...). The latest version of this FAQ should usually be available at
27 https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions
32 LUKS2 COMPATIBILITY: This FAQ was originally written for LUKS1, not
33 LUKS2. Hence regarding LUKS2, some of the answers found here may not
34 apply. Updates for LUKS2 have been done and anything not applying to
35 LUKS2 should clearly say LUKS1. However, this is a Frequently Asked
36 Questions, and questions for LUKS2 are limited at this time or at least
37 those that have reached me are. In the following, "LUKS" refers to both
40 The LUKS1 on-disk format specification is at
41 https://www.kernel.org/pub/linux/utils/cryptsetup/LUKS_docs/on-disk-format.pdf
42 The LUKS2 on-disk format specification is at
43 https://gitlab.com/cryptsetup/LUKS2-docs
45 ATTENTION: If you are going to read just one thing, make it the section
46 on Backup and Data Recovery. By far the most questions on the
47 cryptsetup mailing list are from people that managed to damage the start
48 of their LUKS partitions, i.e. the LUKS header. In most cases, there
49 is nothing that can be done to help these poor souls recover their data.
50 Make sure you understand the problem and limitations imposed by the LUKS
51 security model BEFORE you face such a disaster! In particular, make
52 sure you have a current header backup before doing any potentially
53 dangerous operations. The LUKS2 header should be a bit more resilient
54 as critical data starts later and is stored twice, but you can decidedly
55 still destroy it or a keyslot permanently by accident.
57 DEBUG COMMANDS: While the --debug and --debug-json options should not
58 leak secret data, "strace" and the like can leak your full passphrase.
59 Do not post an strace output with the correct passphrase to a
60 mailing-list or online! See Item 4.5 for more explanation.
62 SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
63 unclear how to get LUKS or plain dm-crypt to run on them with the full
64 set of security assurances intact. This may or may not be a problem,
65 depending on the attacker model. See Section 5.19.
67 BACKUP: Yes, encrypted disks die, just as normal ones do. A full backup
68 is mandatory, see Section "6. Backup and Data Recovery" on options for
69 doing encrypted backup.
71 CLONING/IMAGING: If you clone or image a LUKS container, you make a copy
72 of the LUKS header and the master key will stay the same! That means
73 that if you distribute an image to several machines, the same master key
74 will be used on all of them, regardless of whether you change the
75 passphrases. Do NOT do this! If you do, a root-user on any of the
76 machines with a mapped (decrypted) container or a passphrase on that
77 machine can decrypt all other copies, breaking security. See also Item
80 DISTRIBUTION INSTALLERS: Some distribution installers offer to create
81 LUKS containers in a way that can be mistaken as activation of an
82 existing container. Creating a new LUKS container on top of an existing
83 one leads to permanent, complete and irreversible data loss. It is
84 strongly recommended to only use distribution installers after a
85 complete backup of all LUKS containers has been made.
87 UBUNTU INSTALLER: In particular the Ubuntu installer seems to be quite
88 willing to kill LUKS containers in several different ways. Those
89 responsible at Ubuntu seem not to care very much (it is very easy to
90 recognize a LUKS container), so treat the process of installing Ubuntu
91 as a severe hazard to any LUKS container you may have.
93 NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from STDIN
94 (e.g. via GnuPG) on LUKS format, it does not give you the warning that
95 you are about to format (and e.g. will lose any pre-existing LUKS
96 container on the target), as it assumes it is used from a script. In
97 this scenario, the responsibility for warning the user and possibly
98 checking for an existing LUKS header is shifted to the script. This is
99 a more general form of the previous item.
101 LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not used
102 in deriving the master key. It is used in decrypting a master key that
103 is randomly selected on header creation. This means that if you create
104 a new LUKS header on top of an old one with exactly the same parameters
105 and exactly the same passphrase as the old one, it will still have a
106 different master key and your data will be permanently lost.
108 PASSPHRASE CHARACTER SET: Some people have had difficulties with this
109 when upgrading distributions. It is highly advisable to only use the 95
110 printable characters from the first 128 characters of the ASCII table,
111 as they will always have the same binary representation. Other
112 characters may have different encoding depending on system configuration
113 and your passphrase will not work with a different encoding. A table of
114 the standardized first 128 ASCII characters can, e.g. be found on
115 https://en.wikipedia.org/wiki/ASCII
117 KEYBOARD NUM-PAD: Apparently some pre-boot authentication environments
118 (these are done by the distro, not by cryptsetup, so complain there)
119 treat digits entered on the num-pad and ones entered regularly
120 different. This may be because the BIOS USB keyboard driver is used and
121 that one may have bugs on some computers. If you cannot open your
122 device in pre-boot, try entering the digits over the regular digit keys.
125 * 1.3 System specific warnings
127 - The Ubuntu Natty uinstaller has a "won't fix" defect that may destroy
128 LUKS containers. This is quite old an not relevant for most people.
130 https://bugs.launchpad.net/ubuntu/+source/partman-crypto/+bug/420080
133 * 1.4 My LUKS-device is broken! Help!
135 First: Do not panic! In many cases the data is still recoverable.
136 Do not do anything hasty! Steps:
138 - Take some deep breaths. Maybe add some relaxing music. This may
139 sound funny, but I am completely serious. Often, critical damage is
140 done only after the initial problem.
142 - Do not reboot. The keys may still be in the kernel if the device is
145 - Make sure others do not reboot the system.
147 - Do not write to your disk without a clear understanding why this will
148 not make matters worse. Do a sector-level backup before any writes.
149 Often you do not need to write at all to get enough access to make a
154 - Read section 6 of this FAQ.
156 - Ask on the mailing-list if you need more help.
159 * 1.5 Who wrote this?
161 Current FAQ maintainer is Arno Wagner <arno@wagner.name>. If you want
162 to send me encrypted email, my current PGP key is DSA key CB5D9718,
163 fingerprint 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718.
165 Other contributors are listed at the end. If you want to contribute,
166 send your article, including a descriptive headline, to the maintainer,
167 or the dm-crypt mailing list with something like "FAQ ..."
168 in the subject. You can also send more raw information and have
169 me write the section. Please note that by contributing to this FAQ,
170 you accept the license described below.
172 This work is under the "Attribution-Share Alike 3.0 Unported" license,
173 which means distribution is unlimited, you may create derived works, but
174 attributions to original authors and this license statement must be
175 retained and the derived work must be under the same license. See
176 https://creativecommons.org/licenses/by-sa/3.0/ for more details of the
179 Side note: I did text license research some time ago and I think this
180 license is best suited for the purpose at hand and creates the least
184 * 1.6 Where is the project website?
186 There is the project website at
187 https://gitlab.com/cryptsetup/cryptsetup/ Please do not post
188 questions there, nobody will read them. Use the mailing-list
192 * 1.7 Is there a mailing-list?
194 Instructions on how to subscribe to the mailing-list are on the
195 project website. People are generally helpful and friendly on the
198 The question of how to unsubscribe from the list does crop up sometimes.
199 For this you need your list management URL, which is sent to you
200 initially and once at the start of each month. Go to the URL mentioned
201 in the email and select "unsubscribe". This page also allows you to
202 request a password reminder.
204 Alternatively, you can send an Email to dm-crypt-request@saout.de with
205 just the word "help" in the subject or message body. Make sure to send
206 it from your list address.
208 The mailing list archive is here:
209 https://marc.info/?l=dm-crypt
212 * 1.8 Unsubscribe from the mailing-list
214 Send mail to dm-crypt-unsubscribe@saout.de from the subscribed account.
215 You will get an email with instructions.
217 Basically, you just have to respond to it unmodified to get
218 unsubscribed. The listserver admin functions are not very fast. It can
219 take 15 minutes or longer for a reply to arrive (I suspect greylisting
220 is in use), so be patient.
222 Also note that nobody on the list can unsubscribe you, sending demands
223 to be unsubscribed to the list just annoys people that are entirely
224 blameless for you being subscribed.
226 If you are subscribed, a subscription confirmation email was sent to
227 your email account and it had to be answered before the subscription
228 went active. The confirmation emails from the listserver have subjects
229 like these (with other numbers):
231 Subject: confirm 9964cf10.....
233 and are sent from dm-crypt-request@saout.de. You should check whether
234 you have anything like it in your sent email folder. If you find
235 nothing and are sure you did not confirm, then you should look into a
236 possible compromise of your email account.
241 * 2.1 LUKS Container Setup mini-HOWTO
243 This item tries to give you a very brief list of all the steps you
244 should go through when creating a new LUKS encrypted container, i.e.
245 encrypted disk, partition or loop-file.
247 01) All data will be lost, if there is data on the target, make a
250 02) Make very sure you use the right target disk, partition or
253 03) If the target was in use previously, it is a good idea to wipe it
254 before creating the LUKS container in order to remove any trace of old
255 file systems and data. For example, some users have managed to run
256 e2fsck on a partition containing a LUKS container, possibly because of
257 residual ext2 superblocks from an earlier use. This can do arbitrary
258 damage up to complete and permanent loss of all data in the LUKS
261 To just quickly wipe file systems (old data may remain), use
263 wipefs -a <target device>
265 To wipe file system and data, use something like
267 cat /dev/zero > <target device>
269 This can take a while. To get a progress indicator, you can use the
270 tool dd_rescue (->google) instead or use my stream meter "wcs" (source
271 here: https://www.tansi.org/tools/index.html) in the following fashion:
273 cat /dev/zero | wcs > <target device>
275 Plain "dd" also gives you the progress on a SIGUSR1, see its man-page.
277 Be very sure you have the right target, all data will be lost!
279 Note that automatic wiping is on the TODO list for cryptsetup, so at
280 some time in the future this will become unnecessary.
282 Alternatively, plain dm-crypt can be used for a very fast wipe with
283 crypto-grade randomness, see Item 2.19
285 04) Create the LUKS container.
289 cryptsetup luksFormat --type luks1 <target device>
293 cryptsetup luksFormat --type luks2 <target device>
296 Just follow the on-screen instructions.
298 Note: Passphrase iteration count is based on time and hence security
299 level depends on CPU power of the system the LUKS container is created
300 on. For example on a Raspberry Pi and LUKS1, I found some time ago that
301 the iteration count is 15 times lower than for a regular PC (well, for
302 my old one). Depending on security requirements, this may need
303 adjustment. For LUKS1, you can just look at the iteration count on
304 different systems and select one you like. You can also change the
305 benchmark time with the -i parameter to create a header for a slower
308 For LUKS2, the parameters are more complex. ARGON2 has iteration,
309 parallelism and memory parameter. cryptsetup actually may adjust the
310 memory parameter for time scaling. Hence to use -i is the easiest way
311 to get slower or faster opening (default: 2000 = 2sec). Just make sure
312 to not drop this too low or you may get a memory parameter that is to
313 small to be secure. The luksDump command lists the memory parameter of
314 a created LUKS2 keyslot in kB. That parameter should probably be not
315 much lower than 100000, i.e. 100MB, but don't take my word for it.
317 05) Map the container. Here it will be mapped to /dev/mapper/c1:
319 cryptsetup luksOpen <target device> c1
321 06) (Optionally) wipe the container (make sure you have the right
324 cat /dev/zero > /dev/mapper/c1
326 This will take a while. Note that this creates a small information
327 leak, as an attacker can determine whether a 512 byte block is zero if
328 the attacker has access to the encrypted container multiple times.
329 Typically a competent attacker that has access multiple times can
330 install a passphrase sniffer anyways, so this leakage is not very
331 significant. For getting a progress indicator, see step 03.
333 07) Create a file system in the mapped container, for example an
334 ext3 file system (any other file system is possible):
336 mke2fs -j /dev/mapper/c1
338 08) Mount your encrypted file system, here on /mnt:
340 mount /dev/mapper/c1 /mnt
342 09) Make a LUKS header backup and plan for a container backup.
343 See Section 6 for details.
345 Done. You can now use the encrypted file system to store data. Be sure
346 to read through the rest of the FAQ, these are just the very basics. In
347 particular, there are a number of mistakes that are easy to make, but
348 will compromise your security.
351 * 2.2 LUKS on partitions or raw disks? What about RAID?
354 This is a complicated question, and made more so by the availability of
355 RAID and LVM. I will try to give some scenarios and discuss advantages
356 and disadvantages. Note that I say LUKS for simplicity, but you can do
357 all the things described with plain dm-crypt as well. Also note that
358 your specific scenario may be so special that most or even all things I
359 say below do not apply.
361 Be aware that if you add LVM into the mix, things can get very
362 complicated. Same with RAID but less so. In particular, data recovery
363 can get exceedingly difficult. Only add LVM if you have a really good
364 reason and always remember KISS is what separates an engineer from an
365 amateur. Of course, if you really need the added complexity, KISS is
366 satisfied. But be very sure as there is a price to pay for it. In
367 engineering, complexity is always the enemy and needs to be fought
368 without mercy when encountered.
370 Also consider using RAID instead of LVM, as at least with the old
371 superblock format 0.90, the RAID superblock is in the place (end of
372 disk) where the risk of it damaging the LUKS header is smallest and you
373 can have your array assembled by the RAID controller (i.e. the kernel),
374 as it should be. Use partition type 0xfd for that. I recommend staying
375 away from superblock formats 1.0, 1.1 and 1.2 unless you really need
380 (1) Encrypted partition: Just make a partition to your liking, and put
381 LUKS on top of it and a filesystem into the LUKS container. This gives
382 you isolation of differently-tasked data areas, just as ordinary
383 partitioning does. You can have confidential data, non-confidential
384 data, data for some specific applications, user-homes, root, etc.
385 Advantages are simplicity as there is a 1:1 mapping between partitions
386 and filesystems, clear security functionality and the ability to
387 separate data into different, independent (!) containers.
389 Note that you cannot do this for encrypted root, that requires an
390 initrd. On the other hand, an initrd is about as vulnerable to a
391 competent attacker as a non-encrypted root, so there really is no
392 security advantage to doing it that way. An attacker that wants to
393 compromise your system will just compromise the initrd or the kernel
394 itself. The better way to deal with this is to make sure the root
395 partition does not store any critical data and to move that to
396 additional encrypted partitions. If you really are concerned your root
397 partition may be sabotaged by somebody with physical access (who would
398 however strangely not, say, sabotage your BIOS, keyboard, etc.), protect
399 it in some other way. The PC is just not set-up for a really secure
400 boot-chain (whatever some people may claim).
402 (2) Fully encrypted raw block device: For this, put LUKS on the raw
403 device (e.g. /dev/sdb) and put a filesystem into the LUKS container, no
404 partitioning whatsoever involved. This is very suitable for things like
405 external USB disks used for backups or offline data-storage.
407 (3) Encrypted RAID: Create your RAID from partitions and/or full
408 devices. Put LUKS on top of the RAID device, just if it were an
409 ordinary block device. Applications are just the same as above, but you
410 get redundancy. (Side note as many people seem to be unaware of it: You
411 can do RAID1 with an arbitrary number of components in Linux.) See also
414 (4) Now, some people advocate doing the encryption below the RAID layer.
415 That has several serious problems. One is that suddenly debugging RAID
416 issues becomes much harder. You cannot do automatic RAID assembly
417 anymore. You need to keep the encryption keys for the different RAID
418 components in sync or manage them somehow. The only possible advantage
419 is that things may run a little faster as more CPUs do the encryption,
420 but if speed is a priority over security and simplicity, you are doing
421 this wrong anyways. A good way to mitigate a speed issue is to get a
422 CPU that does hardware AES as most do today.
425 * 2.3 How do I set up encrypted swap?
427 As things that are confidential can end up in swap (keys, passphrases,
428 etc. are usually protected against being swapped to disk, but other
429 things may not be), it may be advisable to do something about the issue.
430 One option is to run without swap, which generally works well in a
431 desktop-context. It may cause problems in a server-setting or under
432 special circumstances. The solution to that is to encrypt swap with a
433 random key at boot-time.
435 NOTE: This is for Debian, and should work for Debian-derived
436 distributions. For others you may have to write your own startup script
437 or use other mechanisms.
439 01) Add the swap partition to /etc/crypttab. A line like the
440 following should do it:
442 swap /dev/<partition> /dev/urandom swap,noearly
444 Warning: While Debian refuses to overwrite partitions with a filesystem
445 or RAID signature on it, as your disk IDs may change (adding or removing
446 disks, failure of disk during boot, etc.), you may want to take
447 additional precautions. Yes, this means that your kernel device names
448 like sda, sdb, ... can change between reboots! This is not a concern
449 if you have only one disk. One possibility is to make sure the
450 partition number is not present on additional disks or also swap there.
451 Another is to encapsulate the swap partition (by making it a 1-partition
452 RAID1 or by using LVM), as that gets a persistent identifier.
453 Specifying it directly by UUID does not work, unfortunately, as the UUID
454 is part of the swap signature and that is not visible from the outside
455 due to the encryption and in addition changes on each reboot with this
458 Note: Use /dev/random if you are paranoid or in a potential low-entropy
459 situation (embedded system, etc.). This may cause the operation to take
460 a long time during boot however. If you are in a "no entropy"
461 situation, you cannot encrypt swap securely. In this situation you
462 should find some entropy, also because nothing else using crypto will be
463 secure, like ssh, ssl or GnuPG.
465 Note: The "noearly" option makes sure things like LVM, RAID, etc. are
466 running. As swap is non-critical for boot, it is fine to start it late.
468 02) Add the swap partition to /etc/fstab. A line like the following
471 /dev/mapper/swap none swap sw 0 0
473 That is it. Reboot or start it manually to activate encrypted swap.
474 Manual start would look like this:
476 /etc/init.d/cryptdisks start
477 swapon /dev/mapper/swap
480 * 2.4 What is the difference between "plain" and LUKS format?
482 First, unless you happen to understand the cryptographic background
483 well, you should use LUKS. It does protect the user from a lot of
484 common mistakes. Plain dm-crypt is for experts.
486 Plain format is just that: It has no metadata on disk, reads all
487 parameters from the commandline (or the defaults), derives a master-key
488 from the passphrase and then uses that to de-/encrypt the sectors of the
489 device, with a direct 1:1 mapping between encrypted and decrypted
492 Primary advantage is high resilience to damage, as one damaged encrypted
493 sector results in exactly one damaged decrypted sector. Also, it is not
494 readily apparent that there even is encrypted data on the device, as an
495 overwrite with crypto-grade randomness (e.g. from
496 /dev/urandom) looks exactly the same on disk.
498 Side-note: That has limited value against the authorities. In civilized
499 countries, they cannot force you to give up a crypto-key anyways. In
500 quite a few countries around the world, they can force you to give up
501 the keys (using imprisonment or worse to pressure you, sometimes without
502 due process), and in the worst case, they only need a nebulous
503 "suspicion" about the presence of encrypted data. Sometimes this
504 applies to everybody, sometimes only when you are suspected of having
505 "illicit data" (definition subject to change) and sometimes specifically
506 when crossing a border. Note that this is going on in countries like
507 the US and the UK to different degrees and sometimes with courts
508 restricting what the authorities can actually demand.
510 My advice is to either be ready to give up the keys or to not have
511 encrypted data when traveling to those countries, especially when
512 crossing the borders. The latter also means not having any high-entropy
513 (random) data areas on your disk, unless you can explain them and
514 demonstrate that explanation. Hence doing a zero-wipe of all free
515 space, including unused space, may be a good idea.
517 Disadvantages are that you do not have all the nice features that the
518 LUKS metadata offers, like multiple passphrases that can be changed, the
519 cipher being stored in the metadata, anti-forensic properties like
520 key-slot diffusion and salts, etc..
522 LUKS format uses a metadata header and 8 key-slot areas that are being
523 placed at the beginning of the disk, see below under "What does the LUKS
524 on-disk format looks like?". The passphrases are used to decrypt a
525 single master key that is stored in the anti-forensic stripes. LUKS2
526 adds some more flexibility.
528 Advantages are a higher usability, automatic configuration of
529 non-default crypto parameters, defenses against low-entropy passphrases
530 like salting and iterated PBKDF2 or ARGON 2 passphrase hashing, the
531 ability to change passphrases, and others.
533 Disadvantages are that it is readily obvious there is encrypted data on
534 disk (but see side note above) and that damage to the header or
535 key-slots usually results in permanent data-loss. See below under "6.
536 Backup and Data Recovery" on how to reduce that risk. Also the sector
537 numbers get shifted by the length of the header and key-slots and there
538 is a loss of that size in capacity. Unless you have a specific need,
542 * 2.5 Can I encrypt an existing, non-empty partition to use LUKS?
544 There is no converter, and it is not really needed. The way to do this
545 is to make a backup of the device in question, securely wipe the device
546 (as LUKS device initialization does not clear away old data), do a
547 luksFormat, optionally overwrite the encrypted device, create a new
548 filesystem and restore your backup on the now encrypted device. Also
549 refer to sections "Security Aspects" and "Backup and Data Recovery".
551 For backup, plain GNU tar works well and backs up anything likely to be
555 * 2.6 How do I use LUKS with a loop-device?
557 This can be very handy for experiments. Setup is just the same as with
558 any block device. If you want, for example, to use a 100MiB file as
559 LUKS container, do something like this:
561 head -c 100M /dev/zero > luksfile # create empty file
562 losetup /dev/loop0 luksfile # map file to /dev/loop0
563 cryptsetup luksFormat --type luks2 /dev/loop0 # create LUKS2 container
565 Afterwards just use /dev/loop0 as a you would use a LUKS partition.
566 To unmap the file when done, use "losetup -d /dev/loop0".
569 * 2.7 When I add a new key-slot to LUKS, it asks for a passphrase
570 but then complains about there not being a key-slot with that
573 That is as intended. You are asked a passphrase of an existing key-slot
574 first, before you can enter the passphrase for the new key-slot.
575 Otherwise you could break the encryption by just adding a new key-slot.
576 This way, you have to know the passphrase of one of the already
577 configured key-slots in order to be able to configure a new key-slot.
580 * 2.8 Encryption on top of RAID or the other way round?
583 Unless you have special needs, place encryption between RAID and
584 filesystem, i.e. encryption on top of RAID. You can do it the other
585 way round, but you have to be aware that you then need to give the
586 passphrase for each individual disk and RAID auto-detection will not
587 work anymore. Therefore it is better to encrypt the RAID device, e.g.
590 This means that the typical layering looks like this:
598 Raw partitions (optional)
602 The big advantage of this is that you can manage the RAID container just
603 like any other regular RAID container, it does not care that its content
604 is encrypted. This strongly cuts down on complexity, something very
605 valuable with storage encryption.
608 * 2.9 How do I read a dm-crypt key from file?
610 Use the --key-file option, like this:
612 cryptsetup create --key-file keyfile e1 /dev/loop0
614 This will read the binary key from file, i.e. no hashing or
615 transformation will be applied to the keyfile before its bits are used
616 as key. Extra bits (beyond the length of the key) at the end are
617 ignored. Note that if you read from STDIN, the data will be hashed,
618 just as a key read interactively from the terminal. See the man-page
619 sections "NOTES ON PASSPHRASE PROCESSING..." for more detail.
622 * 2.10 How do I read a LUKS slot key from file?
624 What you really do here is to read a passphrase from file, just as you
625 would with manual entry of a passphrase for a key-slot. You can add a
626 new passphrase to a free key-slot, set the passphrase of an specific
627 key-slot or put an already configured passphrase into a file. Make sure
628 no trailing newline (0x0a) is contained in the input key file, or the
629 passphrase will not work because the whole file is used as input.
631 To add a new passphrase to a free key slot from file, use something
634 cryptsetup luksAddKey /dev/loop0 keyfile
636 To add a new passphrase to a specific key-slot, use something
639 cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
641 To supply a key from file to any LUKS command, use the --key-file
642 option, e.g. like this:
644 cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
648 * 2.11 How do I read the LUKS master key from file?
650 The question you should ask yourself first is why you would want to do
651 this. The only legitimate reason I can think of is if you want to have
652 two LUKS devices with the same master key. Even then, I think it would
653 be preferable to just use key-slots with the same passphrase, or to use
654 plain dm-crypt instead. If you really have a good reason, please tell
655 me. If I am convinced, I will add how to do this here.
658 * 2.12 What are the security requirements for a key read from file?
660 A file-stored key or passphrase has the same security requirements as
661 one entered interactively, however you can use random bytes and thereby
662 use bytes you cannot type on the keyboard. You can use any file you
663 like as key file, for example a plain text file with a human readable
664 passphrase. To generate a file with random bytes, use something like
667 head -c 256 /dev/random > keyfile
671 * 2.13 If I map a journaled file system using dm-crypt/LUKS, does
672 it still provide its usual transactional guarantees?
674 Yes, it does, unless a very old kernel is used. The required flags come
675 from the filesystem layer and are processed and passed onward by
676 dm-crypt (regardless of direct key management or LUKS key management).
677 A bit more information on the process by which transactional guarantees
678 are implemented can be found here:
680 https://lwn.net/Articles/400541/
682 Please note that these "guarantees" are weaker than they appear to be.
683 One problem is that quite a few disks lie to the OS about having flushed
684 their buffers. This is likely still true with SSDs. Some other things
685 can go wrong as well. The filesystem developers are aware of these
686 problems and typically can make it work anyways. That said,
687 dm-crypt/LUKS will not make things worse.
689 One specific problem you can run into is that you can get short freezes
690 and other slowdowns due to the encryption layer. Encryption takes time
691 and forced flushes will block for that time. For example, I did run
692 into frequent small freezes (1-2 sec) when putting a vmware image on
693 ext3 over dm-crypt. When I went back to ext2, the problem went away.
694 This seems to have gotten better with kernel 2.6.36 and the reworking of
695 filesystem flush locking mechanism (less blocking of CPU activity during
696 flushes). This should improve further and eventually the problem should
700 * 2.14 Can I use LUKS or cryptsetup with a more secure (external)
701 medium for key storage, e.g. TPM or a smartcard?
703 Yes, see the answers on using a file-supplied key. You do have to write
704 the glue-logic yourself though. Basically you can have cryptsetup read
705 the key from STDIN and write it there with your own tool that in turn
706 gets the key from the more secure key storage.
709 * 2.15 Can I resize a dm-crypt or LUKS container?
711 Yes, you can, as neither dm-crypt nor LUKS1 stores partition size and
712 LUKS2 uses a generic "whole device" size as default. Note that LUKS2
713 can use specified data-area sizes as a non-standard case and that these
714 may cause issues when resizing a LUKS2 container if set to a specific
717 Whether you should do this is a different question. Personally I
718 recommend backup, recreation of the dm-crypt or LUKS container with new
719 size, recreation of the filesystem and restore. This gets around the
720 tricky business of resizing the filesystem. Resizing a dm-crypt or LUKS
721 container does not resize the filesystem in it. A backup is really
722 non-optional here, as a lot can go wrong, resulting in partial or
723 complete data loss. But if you have that backup, you can also just
726 You also need to be aware of size-based limitations. The one currently
727 relevant is that aes-xts-plain should not be used for encrypted
728 container sizes larger than 2TiB. Use aes-xts-plain64 for that.
731 * 2.16 How do I Benchmark the Ciphers, Hashes and Modes?
733 Since version 1.60 cryptsetup supports the "benchmark" command.
738 You can get more than the default benchmarks, see the man-page for the
739 relevant parameters. Note that XTS mode takes two keys, hence the
740 listed key sizes are double that for other modes and half of it is the
741 cipher key, the other half is the XTS key.
744 * 2.17 How do I Verify I have an Authentic cryptsetup Source Package?
746 Current maintainer is Milan Broz and he signs the release packages with
747 his PGP key. The key he currently uses is the "RSA key ID D93E98FC",
748 fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B D93E 98FC. While I
749 have every confidence this really is his key and that he is who he
750 claims to be, don't depend on it if your life is at stake. For that
751 matter, if your life is at stake, don't depend on me being who I claim
754 That said, as cryptsetup is under good version control and a malicious
755 change should be noticed sooner or later, but it may take a while.
756 Also, the attacker model makes compromising the sources in a non-obvious
757 way pretty hard. Sure, you could put the master-key somewhere on disk,
758 but that is rather obvious as soon as somebody looks as there would be
759 data in an empty LUKS container in a place it should not be. Doing this
760 in a more nefarious way, for example hiding the master-key in the salts,
761 would need a look at the sources to be discovered, but I think that
762 somebody would find that sooner or later as well.
764 That said, this discussion is really a lot more complicated and longer
765 as an FAQ can sustain. If in doubt, ask on the mailing list.
768 * 2.18 Is there a concern with 4k Sectors?
770 Not from dm-crypt itself. Encryption will be done in 512B blocks, but
771 if the partition and filesystem are aligned correctly and the filesystem
772 uses multiples of 4kiB as block size, the dm-crypt layer will just
773 process 8 x 512B = 4096B at a time with negligible overhead. LUKS does
774 place data at an offset, which is 2MiB per default and will not break
775 alignment. See also Item 6.12 of this FAQ for more details. Note that
776 if your partition or filesystem is misaligned, dm-crypt can make the
777 effect worse though. Also note that SSDs typically have much larger
778 blocks internally (e.g. 128kB or even larger).
781 * 2.19 How can I wipe a device with crypto-grade randomness?
783 The conventional recommendation if you want to do more than just a
784 zero-wipe is to use something like
786 cat /dev/urandom > <target-device>
788 That used to very slow and painful at 10-20MB/s on a fast computer, but
789 newer kernels can give you > 200MB/s (depending on hardware). An
790 alternative is using cryptsetup and a plain dm-crypt device with a
791 random key, which is fast and on the same level of security. The
792 defaults are quite enough.
794 For device set-up, do the following:
796 cryptsetup open --type plain -d /dev/urandom /dev/<device> target
798 This maps the container as plain under /dev/mapper/target with a random
799 password. For the actual wipe you have several options. Basically, you
800 pipe zeroes into the opened container that then get encrypted. Simple
801 wipe without progress-indicator:
803 cat /dev/zero > /dev/mapper/to_be_wiped
805 Progress-indicator by dd_rescue:
807 dd_rescue -w /dev/zero /dev/mapper/to_be_wiped
809 Progress-indicator by my "wcs" stream meter (available from
810 https://www.tansi.org/tools/index.html ):
812 cat /dev/zero | wcs > /dev/mapper/to_be_wiped
814 Or use plain "dd", which gives you the progress when sent a SIGUSR1, see
817 Remove the mapping at the end and you are done.
820 * 2.20 How do I wipe only the LUKS header?
822 This does _not_ describe an emergency wipe procedure, see Item 5.4 for
823 that. This procedure here is intended to be used when the data should
824 stay intact, e.g. when you change your LUKS container to use a detached
825 header and want to remove the old one. Please only do this if you have
829 01) Determine header size in 512 Byte sectors with luksDump:
831 cryptsetup luksDump <device with LUKS container>
834 Payload offset: <number>
837 02) Take the result number, multiply by 512 zeros and write to
838 the start of the device, e.g. like this:
840 dd bs=512 count=<number> if=/dev/zero of=<device>
843 LUKS2: (warning, untested! Remember that backup?) This assumes the
844 LUKS2 container uses the defaults, in particular there is only one data
845 segment. 01) Determine the data-segment offset using luksDump, same
851 offset: <number> [bytes]
854 02) Overwrite the stated number of bytes from the start of the device.
855 Just to give yet another way to get a defined number of zeros:
857 head -c /dev/zero > /dev/<device>
863 * 3.1 My dm-crypt/LUKS mapping does not work! What general steps
864 are there to investigate the problem?
866 If you get a specific error message, investigate what it claims first.
867 If not, you may want to check the following things.
869 - Check that "/dev", including "/dev/mapper/control" is there. If it is
870 missing, you may have a problem with the "/dev" tree itself or you may
871 have broken udev rules.
873 - Check that you have the device mapper and the crypt target in your
874 kernel. The output of "dmsetup targets" should list a "crypt" target.
875 If it is not there or the command fails, add device mapper and
876 crypt-target to the kernel.
878 - Check that the hash-functions and ciphers you want to use are in the
879 kernel. The output of "cat /proc/crypto" needs to list them.
882 * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
884 The default cipher, hash or mode may have changed (the mode changed from
885 1.0.x to 1.1.x). See under "Issues With Specific Versions of
889 * 3.3 When I call cryptsetup from cron/CGI, I get errors about
892 If you get errors about unknown parameters or the like that are not
893 present when cryptsetup is called from the shell, make sure you have no
894 older version of cryptsetup on your system that then gets called by
895 cron/CGI. For example some distributions install cryptsetup into
896 /usr/sbin, while a manual install could go to /usr/local/sbin. As a
897 debugging aid, call "cryptsetup --version" from cron/CGI or the
898 non-shell mechanism to be sure the right version gets called.
901 * 3.4 Unlocking a LUKS device takes very long. Why?
903 The unlock time for a key-slot (see Section 5 for an explanation what
904 iteration does) is calculated when setting a passphrase. By default it
905 is 1 second (2 seconds for LUKS2). If you set a passphrase on a fast
906 machine and then unlock it on a slow machine, the unlocking time can be
907 much longer. Also take into account that up to 8 key-slots (LUKS2: up
908 to 32 key-slots) have to be tried in order to find the right one.
910 If this is the problem, you can add another key-slot using the slow
911 machine with the same passphrase and then remove the old key-slot. The
912 new key-slot will have the unlock time adjusted to the slow machine.
913 Use luksKeyAdd and then luksKillSlot or luksRemoveKey. You can also use
914 the -i option to reduce iteration time (and security level) when setting
915 a passphrase. Default is 1000 (1 sec) for LUKS1 and 2000 (2sec) for
918 However, this operation will not change volume key iteration count ("MK
919 iterations" for LUKS1, "Iterations" under "Digests" for LUKS2). In
920 order to change that, you will have to backup the data in the LUKS
921 container (i.e. your encrypted data), luksFormat on the slow machine
922 and restore the data. Note that MK iterations are not very security
926 * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
927 device. What is wrong?
929 Some old versions of cryptsetup have a bug where the header does not get
930 completely wiped during LUKS format and an older ext2/swap signature
931 remains on the device. This confuses blkid.
933 Fix: Wipe the unused header areas by doing a backup and restore of
934 the header with cryptsetup 1.1.x or later:
936 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
937 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
944 * 4.1 I get the error "LUKS keyslot x is invalid." What does that mean?
946 For LUKS1, this means that the given keyslot has an offset that points
947 outside the valid keyslot area. Typically, the reason is a corrupted
948 LUKS1 header because something was written to the start of the device
949 the LUKS1 container is on. For LUKS2, I do not know when this error can
950 happen, but I expect it will be something similar. Refer to Section
951 "Backup and Data Recovery" and ask on the mailing list if you have
952 trouble diagnosing and (if still possible) repairing this.
955 * 4.2 I cannot unlock my LUKS container! What could be the problem?
957 First, make sure you have a correct passphrase. Then make sure you have
958 the correct key-map and correct keyboard. And then make sure you have
959 the correct character set and encoding, see also "PASSPHRASE CHARACTER
960 SET" under Section 1.2.
962 If you are sure you are entering the passphrase right, there is the
963 possibility that the respective key-slot has been damaged. There is no
964 way to recover a damaged key-slot, except from a header backup (see
965 Section 6). For security reasons, there is also no checksum in the
966 key-slots that could tell you whether a key-slot has been damaged. The
967 only checksum present allows recognition of a correct passphrase, but
968 that only works with that correct passphrase and a respective key-slot
971 In order to find out whether a key-slot is damaged one has to look for
972 "non-random looking" data in it. There is a tool that automates this
973 for LUKS1 in the cryptsetup distribution from version 1.6.0 onwards. It
974 is located in misc/keyslot_checker/. Instructions how to use and how to
975 interpret results are in the README file. Note that this tool requires
976 a libcryptsetup from cryptsetup 1.6.0 or later (which means
977 libcryptsetup.so.4.5.0 or later). If the tool complains about missing
978 functions in libcryptsetup, you likely have an earlier version from your
979 distribution still installed. You can either point the symbolic link(s)
980 from libcryptsetup.so.4 to the new version manually, or you can
981 uninstall the distribution version of cryptsetup and re-install that
982 from cryptsetup >= 1.6.0 again to fix this.
985 * 4.3 Can a bad RAM module cause problems?
987 LUKS and dm-crypt can give the RAM quite a workout, especially when
988 combined with software RAID. In particular the combination RAID5 +
989 LUKS1 + XFS seems to uncover RAM problems that do not cause obvious
990 problems otherwise. Symptoms vary, but often the problem manifests
991 itself when copying large amounts of data, typically several times
992 larger than your main memory.
994 Note: One thing you should always do on large data copying or movements
995 is to run a verify, for example with the "-d" option of "tar" or by
996 doing a set of MD5 checksums on the source or target with
998 find . -type f -exec md5sum \{\} \; > checksum-file
1000 and then a "md5sum -c checksum-file" on the other side. If you get
1001 mismatches here, RAM is the primary suspect. A lesser suspect is an
1002 overclocked CPU. I have found countless hardware problems in verify
1003 runs after copying data or making backups. Bit errors are much more
1004 common than most people think.
1006 Some RAM issues are even worse and corrupt structures in one of the
1007 layers. This typically results in lockups, CPU state dumps in the
1008 system logs, kernel panic or other things. It is quite possible to have
1009 a problem with an encrypted device, but not with an otherwise the same
1010 unencrypted device. The reason for that is that encryption has an error
1011 amplification property: If you flip one bit in an encrypted data block,
1012 the decrypted version has half of its bits flipped. This is actually an
1013 important security property for modern ciphers. With the usual modes in
1014 cryptsetup (CBC, ESSIV, XTS), you can get a completely changed 512 byte
1015 block for a bit error. A corrupt block causes a lot more havoc than the
1016 occasionally flipped single bit and can result in various obscure
1019 Note that a verify run on copying between encrypted or unencrypted
1020 devices will reliably detect corruption, even when the copying itself
1021 did not report any problems. If you find defect RAM, assume all backups
1022 and copied data to be suspect, unless you did a verify.
1025 * 4.4 How do I test RAM?
1027 First you should know that overclocking often makes memory problems
1028 worse. So if you overclock (which I strongly recommend against in a
1029 system holding data that has any worth), run the tests with the
1030 overclocking active.
1032 There are two good options. One is Memtest86+ and the other is
1033 "memtester" by Charles Cazabon. Memtest86+ requires a reboot and then
1034 takes over the machine, while memtester runs from a root-shell. Both
1035 use different testing methods and I have found problems fast with either
1036 one that the other needed long to find. I recommend running the
1037 following procedure until the first error is found:
1039 - Run Memtest86+ for one cycle
1041 - Run memtester for one cycle (shut down as many other applications
1042 as possible and use the largest memory area you can get)
1044 - Run Memtest86+ for 24h or more
1046 - Run memtester for 24h or more
1048 If all that does not produce error messages, your RAM may be sound,
1049 but I have had one weak bit in the past that Memtest86+ needed around
1050 60 hours to find. If you can reproduce the original problem reliably,
1051 a good additional test may be to remove half of the RAM (if you have
1052 more than one module) and try whether the problem is still there and if
1053 so, try with the other half. If you just have one module, get a
1054 different one and try with that. If you do overclocking, reduce the
1055 settings to the most conservative ones available and try with that.
1058 * 4.5 Is there a risk using debugging tools like strace?
1060 There most definitely is. A dump from strace and friends can contain
1061 all data entered, including the full passphrase. Example with strace
1062 and passphrase "test":
1064 > strace cryptsetup luksOpen /dev/sda10 c1
1066 read(6, "test\n", 512) = 5
1069 Depending on different factors and the tool used, the passphrase may
1070 also be encoded and not plainly visible. Hence it is never a good idea
1071 to give such a trace from a live container to anybody. Recreate the
1072 problem with a test container or set a temporary passphrase like "test"
1073 and use that for the trace generation. Item 2.6 explains how to create
1074 a loop-file backed LUKS container that may come in handy for this
1077 See also Item 6.10 for another set of data you should not give to
1084 * 5.1 How long is a secure passphrase?
1086 This is just the short answer. For more info and explanation of some of
1087 the terms used in this item, read the rest of Section 5. The actual
1088 recommendation is at the end of this item.
1090 First, passphrase length is not really the right measure, passphrase
1091 entropy is. If your passphrase is 200 times the letter "a", it is long
1092 but has very low entropy and is pretty insecure.
1094 For example, a random lowercase letter (a-z) gives you 4.7 bit of
1095 entropy, one element of a-z0-9 gives you 5.2 bits of entropy, an element
1096 of a-zA-Z0-9 gives you 5.9 bits and a-zA-Z0-9!@#$%\^&:-+ gives you 6.2
1097 bits. On the other hand, a random English word only gives you 0.6...1.3
1098 bits of entropy per character. Using sentences that make sense gives
1099 lower entropy, series of random words gives higher entropy. Do not use
1100 sentences that can be tied to you or found on your computer. This type
1101 of attack is done routinely today.
1103 That said, it does not matter too much what scheme you use, but it does
1104 matter how much entropy your passphrase contains, because an attacker
1105 has to try on average
1107 1/2 * 2^(bits of entropy in passphrase)
1109 different passphrases to guess correctly.
1111 Historically, estimations tended to use computing time estimates, but
1112 more modern approaches try to estimate cost of guessing a passphrase.
1114 As an example, I will try to get an estimate from the numbers in
1115 https://gist.github.com/epixoip/a83d38f412b4737e99bbef804a270c40 This
1116 thing costs 23kUSD and does 68Ghashes/sec for SHA1. This is in 2017.
1118 Incidentally, my older calculation for a machine around 1000 times
1119 slower was off by a factor of about 1000, but in the right direction,
1120 i.e. I estimated the attack to be too easy. Nobody noticed ;-) On the
1121 plus side, the tables are now (2017) pretty much accurate.
1123 More references can be found at the end of this document. Note that
1124 these are estimates from the defender side, so assuming something is
1125 easier than it actually is is fine. An attacker may still have
1126 significantly higher cost than estimated here.
1128 LUKS1 used SHA1 (since version 1.7.0 it uses SHA256) for hashing per
1129 default. We will leave aside the check whether a try actually decrypts
1130 a key-slot. I will assume a useful lifetime of the hardware of 2 years.
1131 (This is on the low side.) Disregarding downtime, the machine can then
1134 N = 68*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
1136 passphrases for EUR/USD 23k. That is one 62 bit passphrase hashed once
1137 with SHA1 for EUR/USD 23k. This can be parallelized, it can be done
1138 faster than 2 years with several of these machines.
1140 For LUKS2, things look a bit better, as the advantage of using graphics
1141 cards is massively reduced. Using the recommendations below should
1142 hence be fine for LUKS2 as well and give a better security margin.
1144 For plain dm-crypt (no hash iteration) this is it. This gives (with
1145 SHA1, plain dm-crypt default is ripemd160 which seems to be slightly
1148 Passphrase entropy Cost to break
1158 For LUKS1, you have to take into account hash iteration in PBKDF2.
1159 For a current CPU, there are about 100k iterations (as can be queried
1160 with ''cryptsetup luksDump''.
1162 The table above then becomes:
1164 Passphrase entropy Cost to break
1176 To get reasonable security for the next 10 years, it is a good idea
1177 to overestimate by a factor of at least 1000.
1179 Then there is the question of how much the attacker is willing to spend.
1180 That is up to your own security evaluation. For general use, I will
1181 assume the attacker is willing to spend up to 1 million EUR/USD. Then
1182 we get the following recommendations:
1184 Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
1185 or a random English sentence of > 135 characters length.
1187 LUKS1 and LUKS2: Use > 65 bit. That is e.g. 14 random chars from a-z
1188 or a random English sentence of > 108 characters length.
1190 If paranoid, add at least 20 bit. That is roughly four additional
1191 characters for random passphrases and roughly 32 characters for a
1192 random English sentence.
1195 * 5.2 Is LUKS insecure? Everybody can see I have encrypted data!
1197 In practice it does not really matter. In most civilized countries you
1198 can just refuse to hand over the keys, no harm done. In some countries
1199 they can force you to hand over the keys if they suspect encryption.
1200 The suspicion is enough, they do not have to prove anything. This is
1201 for practical reasons, as even the presence of a header (like the LUKS
1202 header) is not enough to prove that you have any keys. It might have
1203 been an experiment, for example. Or it was used as encrypted swap with
1204 a key from /dev/random. So they make you prove you do not have
1205 encrypted data. Of course, if true, that is impossible and hence the
1206 whole idea is not compatible with fair laws. Note that in this context,
1207 countries like the US or the UK are not civilized and do not have fair
1210 This means that if you have a large set of random-looking data, they can
1211 already lock you up. Hidden containers (encryption hidden within
1212 encryption), as possible with Truecrypt, do not help either. They will
1213 just assume the hidden container is there and unless you hand over the
1214 key, you will stay locked up. Don't have a hidden container? Tough
1215 luck. Anybody could claim that.
1217 Still, if you are concerned about the LUKS header, use plain dm-crypt
1218 with a good passphrase. See also Section 2, "What is the difference
1219 between "plain" and LUKS format?"
1222 * 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
1224 If you just create a filesystem on it, most of the old data will still
1225 be there. If the old data is sensitive, you should overwrite it before
1226 encrypting. In any case, not initializing will leave the old data there
1227 until the specific sector gets written. That may enable an attacker to
1228 determine how much and where on the partition data was written. If you
1229 think this is a risk, you can prevent this by overwriting the encrypted
1230 device (here assumed to be named "e1") with zeros like this:
1232 dd_rescue -w /dev/zero /dev/mapper/e1
1234 or alternatively with one of the following more standard commands:
1236 cat /dev/zero > /dev/mapper/e1
1237 dd if=/dev/zero of=/dev/mapper/e1
1241 * 5.4 How do I securely erase a LUKS container?
1243 For LUKS, if you are in a desperate hurry, overwrite the LUKS header and
1244 key-slot area. For LUKS1 and LUKS2, just be generous and overwrite the
1245 first 100MB. A single overwrite with zeros should be enough. If you
1246 anticipate being in a desperate hurry, prepare the command beforehand.
1247 Example with /dev/sde1 as the LUKS partition and default parameters:
1249 head -c 100000000 /dev/zero > /dev/sde1; sync
1251 A LUKS header backup or full backup will still grant access to most or
1252 all data, so make sure that an attacker does not have access to backups
1253 or destroy them as well.
1255 Also note that SSDs and also some HDDs (SMR and hybrid HDDs, for
1256 example) may not actually overwrite the header and only do that an
1257 unspecified and possibly very long time later. The only way to be sure
1258 there is physical destruction. If the situation permits, do both
1259 overwrite and physical destruction.
1261 If you have time, overwrite the whole drive with a single pass of random
1262 data. This is enough for most HDDs. For SSDs or FLASH (USB sticks) or
1263 SMR or hybrid drives, you may want to overwrite the whole drive several
1264 times to be sure data is not retained. This is possibly still insecure
1265 as the respective technologies are not fully understood in this regard.
1266 Still, due to the anti-forensic properties of the LUKS key-slots, a
1267 single overwrite could be enough. If in doubt, use physical destruction
1268 in addition. Here is a link to some current research results on erasing
1269 SSDs and FLASH drives:
1270 https://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
1272 Keep in mind to also erase all backups.
1274 Example for a random-overwrite erase of partition sde1 done with
1277 dd_rescue -w /dev/urandom /dev/sde1
1281 * 5.5 How do I securely erase a backup of a LUKS partition or header?
1283 That depends on the medium it is stored on. For HDD and SSD, use
1284 overwrite with random data. For an SSD, FLASH drive (USB stick) hybrid
1285 HDD or SMR HDD, you may want to overwrite the complete drive several
1286 times and use physical destruction in addition, see last item. For
1287 re-writable CD/DVD, a single overwrite should be enough, due to the
1288 anti-forensic properties of the LUKS keyslots. For write-once media,
1289 use physical destruction. For low security requirements, just cut the
1290 CD/DVD into several parts. For high security needs, shred or burn the
1293 If your backup is on magnetic tape, I advise physical destruction by
1294 shredding or burning, after (!) overwriting. The problem with magnetic
1295 tape is that it has a higher dynamic range than HDDs and older data may
1296 well be recoverable after overwrites. Also write-head alignment issues
1297 can lead to data not actually being deleted during overwrites.
1299 The best option is to actually encrypt the backup, for example with
1300 PGP/GnuPG and then just destroy all copies of the encryption key if
1301 needed. Best keep them on paper, as that has excellent durability and
1302 secure destruction is easy, for example by burning and then crushing the
1303 ashes to a fine powder. A blender and water also works nicely.
1306 * 5.6 What about backup? Does it compromise security?
1308 That depends. See item 6.7.
1311 * 5.7 Why is all my data permanently gone if I overwrite the LUKS header?
1313 Overwriting the LUKS header in part or in full is the most common reason
1314 why access to LUKS containers is lost permanently. Overwriting can be
1315 done in a number of fashions, like creating a new filesystem on the raw
1316 LUKS partition, making the raw partition part of a RAID array and just
1317 writing to the raw partition.
1319 The LUKS1 header contains a 256 bit "salt" per key-slot and without that
1320 no decryption is possible. While the salts are not secret, they are
1321 key-grade material and cannot be reconstructed. This is a
1322 cryptographically strong "cannot". From observations on the cryptsetup
1323 mailing-list, people typically go though the usual stages of grief
1324 (Denial, Anger, Bargaining, Depression, Acceptance) when this happens to
1325 them. Observed times vary between 1 day and 2 weeks to complete the
1326 cycle. Seeking help on the mailing-list is fine. Even if we usually
1327 cannot help with getting back your data, most people found the feedback
1330 If your header does not contain an intact key-slot salt, best go
1331 directly to the last stage ("Acceptance") and think about what to do
1332 now. There is one exception that I know of: If your LUKS1 container is
1333 still open, then it may be possible to extract the master key from the
1334 running system. See Item "How do I recover the master key from a mapped
1335 LUKS1 container?" in Section "Backup and Data Recovery".
1337 For LUKS2, things are both better and worse. First, the salts are in a
1338 less vulnerable position now. But, on the other hand, the keys of a
1339 mapped (open) container are now stored in the kernel key-store, and
1340 while there probably is some way to get them out of there, I am not sure
1341 how much effort that needs.
1344 * 5.8 What is a "salt"?
1346 A salt is a random key-grade value added to the passphrase before it is
1347 processed. It is not kept secret. The reason for using salts is as
1348 follows: If an attacker wants to crack the password for a single LUKS
1349 container, then every possible passphrase has to be tried. Typically an
1350 attacker will not try every binary value, but will try words and
1351 sentences from a dictionary.
1353 If an attacker wants to attack several LUKS containers with the same
1354 dictionary, then a different approach makes sense: Compute the resulting
1355 slot-key for each dictionary element and store it on disk. Then the
1356 test for each entry is just the slow unlocking with the slot key (say
1357 0.00001 sec) instead of calculating the slot-key first (1 sec). For a
1358 single attack, this does not help. But if you have more than one
1359 container to attack, this helps tremendously, also because you can
1360 prepare your table before you even have the container to attack! The
1361 calculation is also very simple to parallelize. You could, for example,
1362 use the night-time unused CPU power of your desktop PCs for this.
1364 This is where the salt comes in. If the salt is combined with the
1365 passphrase (in the simplest form, just appended to it), you suddenly
1366 need a separate table for each salt value. With a reasonably-sized salt
1367 value (256 bit, e.g.) this is quite infeasible.
1370 * 5.9 Is LUKS secure with a low-entropy (bad) passphrase?
1372 Short answer: yes. Do not use a low-entropy passphrase.
1374 Note: For LUKS2, protection for bad passphrases is a bit better
1375 due to the use of Argon2, but that is only a gradual improvement.
1378 This needs a bit of theory. The quality of your passphrase is directly
1379 related to its entropy (information theoretic, not thermodynamic). The
1380 entropy says how many bits of "uncertainty" or "randomness" are in you
1381 passphrase. In other words, that is how difficult guessing the
1384 Example: A random English sentence has about 1 bit of entropy per
1385 character. A random lowercase (or uppercase) character has about 4.7
1388 Now, if n is the number of bits of entropy in your passphrase and t
1389 is the time it takes to process a passphrase in order to open the
1390 LUKS container, then an attacker has to spend at maximum
1392 attack_time_max = 2^n * t
1394 time for a successful attack and on average half that. There is no way
1395 getting around that relationship. However, there is one thing that does
1396 help, namely increasing t, the time it takes to use a passphrase, see
1399 Still, if you want good security, a high-entropy passphrase is the only
1400 option. For example, a low-entropy passphrase can never be considered
1401 secure against a TLA-level (Three Letter Agency level, i.e.
1402 government-level) attacker, no matter what tricks are used in the
1403 key-derivation function. Use at least 64 bits for secret stuff. That
1404 is 64 characters of English text (but only if randomly chosen) or a
1405 combination of 12 truly random letters and digits.
1407 For passphrase generation, do not use lines from very well-known texts
1408 (religious texts, Harry Potter, etc.) as they are too easy to guess.
1409 For example, the total Harry Potter has about 1'500'000 words (my
1410 estimation). Trying every 64 character sequence starting and ending at
1411 a word boundary would take only something like 20 days on a single CPU
1412 and is entirely feasible. To put that into perspective, using a number
1413 of Amazon EC2 High-CPU Extra Large instances (each gives about 8 real
1414 cores), this test costs currently about 50USD/EUR, but can be made to
1415 run arbitrarily fast.
1417 On the other hand, choosing 1.5 lines from, say, the Wheel of Time, is
1418 in itself not more secure, but the book selection adds quite a bit of
1419 entropy. (Now that I have mentioned it here, don't use tWoT either!) If
1420 you add 2 or 3 typos and switch some words around, then this is good
1421 passphrase material.
1424 * 5.10 What is "iteration count" and why is decreasing it a bad idea?
1427 Iteration count is the number of PBKDF2 iterations a passphrase is put
1428 through before it is used to unlock a key-slot. Iterations are done
1429 with the explicit purpose to increase the time that it takes to unlock a
1430 key-slot. This provides some protection against use of low-entropy
1433 The idea is that an attacker has to try all possible passphrases. Even
1434 if the attacker knows the passphrase is low-entropy (see last item), it
1435 is possible to make each individual try take longer. The way to do this
1436 is to repeatedly hash the passphrase for a certain time. The attacker
1437 then has to spend the same time (given the same computing power) as the
1438 user per try. With LUKS1, the default is 1 second of PBKDF2 hashing.
1440 Example 1: Lets assume we have a really bad passphrase (e.g. a
1441 girlfriends name) with 10 bits of entropy. With the same CPU, an
1442 attacker would need to spend around 500 seconds on average to break that
1443 passphrase. Without iteration, it would be more like 0.0001 seconds on
1446 Example 2: The user did a bit better and has 32 chars of English text.
1447 That would be about 32 bits of entropy. With 1 second iteration, that
1448 means an attacker on the same CPU needs around 136 years. That is
1449 pretty impressive for such a weak passphrase. Without the iterations,
1450 it would be more like 50 days on a modern CPU, and possibly far less.
1452 In addition, the attacker can both parallelize and use special hardware
1453 like GPUs or FPGAs to speed up the attack. The attack can also happen
1454 quite some time after the luksFormat operation and CPUs can have become
1455 faster and cheaper. For that reason you want a bit of extra security.
1456 Anyways, in Example 1 your are screwed. In example 2, not necessarily.
1457 Even if the attack is faster, it still has a certain cost associated
1458 with it, say 10000 EUR/USD with iteration and 1 EUR/USD without
1459 iteration. The first can be prohibitively expensive, while the second
1460 is something you try even without solid proof that the decryption will
1461 yield something useful.
1463 The numbers above are mostly made up, but show the idea. Of course the
1464 best thing is to have a high-entropy passphrase.
1466 Would a 100 sec iteration time be even better? Yes and no.
1467 Cryptographically it would be a lot better, namely 100 times better.
1468 However, usability is a very important factor for security technology
1469 and one that gets overlooked surprisingly often. For LUKS, if you have
1470 to wait 2 minutes to unlock the LUKS container, most people will not
1471 bother and use less secure storage instead. It is better to have less
1472 protection against low-entropy passphrases and people actually use LUKS,
1473 than having them do without encryption altogether.
1475 Now, what about decreasing the iteration time? This is generally a very
1476 bad idea, unless you know and can enforce that the users only use
1477 high-entropy passphrases. If you decrease the iteration time without
1478 ensuring that, then you put your users at increased risk, and
1479 considering how rarely LUKS containers are unlocked in a typical
1480 work-flow, you do so without a good reason. Don't do it. The iteration
1481 time is already low enough that users with low entropy passphrases are
1482 vulnerable. Lowering it even further increases this danger
1485 LUKS2: Pretty much the same reasoning applies. The advantages of using
1486 GPUs or FPGAs in an attack have been significantly reduced, but that
1487 is the only main difference.
1490 * 5.11 Some people say PBKDF2 is insecure?
1492 There is some discussion that a hash-function should have a "large
1493 memory" property, i.e. that it should require a lot of memory to be
1494 computed. This serves to prevent attacks using special programmable
1495 circuits, like FPGAs, and attacks using graphics cards. PBKDF2 does not
1496 need a lot of memory and is vulnerable to these attacks. However, the
1497 publication usually referred in these discussions is not very convincing
1498 in proving that the presented hash really is "large memory" (that may
1499 change, email the FAQ maintainer when it does) and it is of limited
1500 usefulness anyways. Attackers that use clusters of normal PCs will not
1501 be affected at all by a "large memory" property. For example the US
1502 Secret Service is known to use the off-hour time of all the office PCs
1503 of the Treasury for password breaking. The Treasury has about 110'000
1504 employees. Assuming every one has an office PC, that is significant
1505 computing power, all of it with plenty of memory for computing "large
1506 memory" hashes. Bot-net operators also have all the memory they want.
1507 The only protection against a resourceful attacker is a high-entropy
1508 passphrase, see items 5.9 and 5.10.
1510 That said, LUKS2 defaults to Argon2, which has a large-memory property
1511 and massively reduces the advantages of GPUs and FPGAs.
1514 * 5.12 What about iteration count with plain dm-crypt?
1516 Simple: There is none. There is also no salting. If you use plain
1517 dm-crypt, the only way to be secure is to use a high entropy passphrase.
1518 If in doubt, use LUKS instead.
1521 * 5.13 Is LUKS with default parameters less secure on a slow CPU?
1523 Unfortunately, yes. However the only aspect affected is the protection
1524 for low-entropy passphrase or master-key. All other security aspects
1525 are independent of CPU speed.
1527 The master key is less critical, as you really have to work at it to
1528 give it low entropy. One possibility to mess this up is to supply the
1529 master key yourself. If that key is low-entropy, then you get what you
1530 deserve. The other known possibility to create a LUKS container with a
1531 bad master key is to use /dev/urandom for key generation in an
1532 entropy-starved situation (e.g. automatic installation on an embedded
1533 device without network and other entropy sources or installation in a VM
1534 under certain circumstances).
1536 For the passphrase, don't use a low-entropy passphrase. If your
1537 passphrase is good, then a slow CPU will not matter. If you insist on a
1538 low-entropy passphrase on a slow CPU, use something like
1539 "--iter-time=10000" or higher and wait a long time on each LUKS unlock
1540 and pray that the attacker does not find out in which way exactly your
1541 passphrase is low entropy. This also applies to low-entropy passphrases
1542 on fast CPUs. Technology can do only so much to compensate for problems
1543 in front of the keyboard.
1545 Also note that power-saving modes will make your CPU slower. This will
1546 reduce iteration count on LUKS container creation. It will keep unlock
1547 times at the expected values though at this CPU speed.
1550 * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
1552 Note: This item applies both to plain dm-crypt and to LUKS
1554 The problem is that cbc-plain has a fingerprint vulnerability, where a
1555 specially crafted file placed into the crypto-container can be
1556 recognized from the outside. The issue here is that for cbc-plain the
1557 initialization vector (IV) is the sector number. The IV gets XORed to
1558 the first data chunk of the sector to be encrypted. If you make sure
1559 that the first data block to be stored in a sector contains the sector
1560 number as well, the first data block to be encrypted is all zeros and
1561 always encrypted to the same ciphertext. This also works if the first
1562 data chunk just has a constant XOR with the sector number. By having
1563 several shifted patterns you can take care of the case of a
1564 non-power-of-two start sector number of the file.
1566 This mechanism allows you to create a pattern of sectors that have the
1567 same first ciphertext block and signal one bit per sector to the
1568 outside, allowing you to e.g. mark media files that way for recognition
1569 without decryption. For large files this is a practical attack. For
1570 small ones, you do not have enough blocks to signal and take care of
1571 different file starting offsets.
1573 In order to prevent this attack, the default was changed to cbc-essiv.
1574 ESSIV uses a keyed hash of the sector number, with the encryption key as
1575 key. This makes the IV unpredictable without knowing the encryption key
1576 and the watermarking attack fails.
1579 * 5.15 Are there any problems with "plain" IV? What is "plain64"?
1581 First, "plain" and "plain64" are both not secure to use with CBC, see
1584 However there are modes, like XTS, that are secure with "plain" IV. The
1585 next limit is that "plain" is 64 bit, with the upper 32 bit set to zero.
1586 This means that on volumes larger than 2TiB, the IV repeats, creating a
1587 vulnerability that potentially leaks some data. To avoid this, use
1588 "plain64", which uses the full sector number up to 64 bit. Note that
1589 "plain64" requires a kernel 2.6.33 or more recent. Also note that
1590 "plain64" is backwards compatible for volume sizes of maximum size 2TiB,
1591 but not for those > 2TiB. Finally, "plain64" does not cause any
1592 performance penalty compared to "plain".
1595 * 5.16 What about XTS mode?
1597 XTS mode is potentially even more secure than cbc-essiv (but only if
1598 cbc-essiv is insecure in your scenario). It is a NIST standard and
1599 used, e.g. in Truecrypt. From version 1.6.0 of cryptsetup onwards,
1600 aes-xts-plain64 is the default for LUKS. If you want to use it with a
1601 cryptsetup before version 1.6.0 or with plain dm-crypt, you have to
1602 specify it manually as "aes-xts-plain", i.e.
1604 cryptsetup -c aes-xts-plain luksFormat <device>
1606 For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ item
1607 on "plain" and "plain64"):
1609 cryptsetup -c aes-xts-plain64 luksFormat <device>
1611 There is a potential security issue with XTS mode and blocks larger
1612 than 2^20 bytes or so. LUKS and dm-crypt always use smaller blocks
1613 and the issue does not apply.
1616 * 5.17 Is LUKS FIPS-140-2 certified?
1618 No. But that is more a problem of FIPS-140-2 than of LUKS. From a
1619 technical point-of-view, LUKS with the right parameters would be
1620 FIPS-140-2 compliant, but in order to make it certified, somebody has to
1621 pay real money for that. And then, whenever cryptsetup is changed or
1622 extended, the certification lapses and has to be obtained again.
1624 From the aspect of actual security, LUKS with default parameters should
1625 be as good as most things that are FIPS-140-2 certified, although you
1626 may want to make sure to use /dev/random (by specifying --use-random on
1627 luksFormat) as randomness source for the master key to avoid being
1628 potentially insecure in an entropy-starved situation.
1631 * 5.18 What about Plausible Deniability?
1633 First let me attempt a definition for the case of encrypted filesystems:
1634 Plausible deniability is when you store data inside an encrypted
1635 container and it is not possible to prove it is there without having a
1636 special passphrase. And at the same time it must be "plausible" that
1637 there actually is no hidden data there.
1639 As a simple entropy-analysis will show that here may be data there, the
1640 second part is what makes it tricky.
1642 There seem to be a lot of misunderstandings about this idea, so let me
1643 make it clear that this refers to the situation where the attackers can
1644 prove that there is data that either may be random or may be part of a
1645 plausible-deniability scheme, they just cannot prove which one it is.
1646 Hence a plausible-deniability scheme must hold up when the attackers
1647 know there is something potentially fishy. If you just hide data and
1648 rely on it not being found, that is just simple deniability, not
1649 "plausible" deniability and I am not talking about that in the
1650 following. Simple deniability against a low-competence attacker may be
1651 as simple as renaming a file or putting data into an unused part of a
1652 disk. Simple deniability against a high-skill attacker with time to
1653 invest is usually pointless unless you go for advanced steganographic
1654 techniques, which have their own drawbacks, such as low data capacity.
1656 Now, the idea of plausible deniability is compelling and on a first
1657 glance it seems possible to do it. And from a cryptographic point of
1658 view, it actually is possible.
1660 So, does the idea work in practice? No, unfortunately. The reasoning
1661 used by its proponents is fundamentally flawed in several ways and the
1662 cryptographic properties fail fatally when colliding with the real
1665 First, why should "I do not have a hidden partition" be any more
1666 plausible than "I forgot my crypto key" or "I wiped that partition with
1667 random data, nothing in there"? I do not see any reason.
1669 Second, there are two types of situations: Either they cannot force you
1670 to give them the key (then you simply do not) or they can. In the
1671 second case, they can always do bad things to you, because they cannot
1672 prove that you have the key in the first place! This means they do not
1673 have to prove you have the key, or that this random looking data on your
1674 disk is actually encrypted data. So the situation will allow them to
1675 waterboard/lock-up/deport you anyways, regardless of how "plausible"
1676 your deniability is. Do not have a hidden partition you could show to
1677 them, but there are indications you may? Too bad for you.
1678 Unfortunately "plausible deniability" also means you cannot prove there
1681 Third, hidden partitions are not that hidden. There are basically just
1682 two possibilities: a) Make a large crypto container, but put a smaller
1683 filesystem in there and put the hidden partition into the free space.
1684 Unfortunately this is glaringly obvious and can be detected in an
1685 automated fashion. This means that the initial suspicion to put you
1686 under duress in order to make you reveal your hidden data is given. b)
1687 Make a filesystem that spans the whole encrypted partition, and put the
1688 hidden partition into space not currently used by that filesystem.
1689 Unfortunately that is also glaringly obvious, as you then cannot write
1690 to the filesystem without a high risk of destroying data in the hidden
1691 container. Have not written anything to the encrypted filesystem in a
1692 while? Too bad, they have the suspicion they need to do unpleasant
1695 To be fair, if you prepare option b) carefully and directly before going
1696 into danger, it may work. But then, the mere presence of encrypted data
1697 may already be enough to get you into trouble in those places were they
1698 can demand encryption keys.
1700 Here is an additional reference for some problems with plausible
1702 https://www.schneier.com/academic/paperfiles/paper-truecrypt-dfs.pdf
1703 I strongly suggest you read it.
1705 So, no, I will not provide any instructions on how to do it with plain
1706 dm-crypt or LUKS. If you insist on shooting yourself in the foot, you
1707 can figure out how to do it yourself.
1710 * 5.19 What about SSDs, Flash, Hybrid and SMR Drives?
1712 The problem is that you cannot reliably erase parts of these devices,
1713 mainly due to wear-leveling and possibly defect management and delayed
1714 writes to the main data area.
1716 For example for SSDs, when overwriting a sector, what the device does is
1717 to move an internal sector (may be 128kB or even larger) to some pool of
1718 discarded, not-yet erased unused sectors, take a fresh empty sector from
1719 the empty-sector pool and copy the old sector over with the changes to
1720 the small part you wrote. This is done in some fashion so that larger
1721 writes do not cause a lot of small internal updates.
1723 The thing is that the mappings between outside-addressable sectors and
1724 inside sectors is arbitrary (and the vendors are not talking). Also the
1725 discarded sectors are not necessarily erased immediately. They may
1728 For plain dm-crypt, the consequences are that older encrypted data may
1729 be lying around in some internal pools of the device. Thus may or may
1730 not be a problem and depends on the application. Remember the same can
1731 happen with a filesystem if consecutive writes to the same area of a
1732 file can go to different sectors.
1734 However, for LUKS, the worst case is that key-slots and LUKS header may
1735 end up in these internal pools. This means that password management
1736 functionality is compromised (the old passwords may still be around,
1737 potentially for a very long time) and that fast erase by overwriting the
1738 header and key-slot area is insecure.
1740 Also keep in mind that the discarded/used pool may be large. For
1741 example, a 240GB SSD has about 16GB of spare area in the chips that it
1742 is free to do with as it likes. You would need to make each individual
1743 key-slot larger than that to allow reliable overwriting. And that
1744 assumes the disk thinks all other space is in use. Reading the internal
1745 pools using forensic tools is not that hard, but may involve some
1750 If you trust the device vendor (you probably should not...) you can try
1751 an ATA "secure erase" command. That is not present in USB keys though
1752 and may or may not be secure for a hybrid drive.
1754 If you can do without password management and are fine with doing
1755 physical destruction for permanently deleting data (always after one or
1756 several full overwrites!), you can use plain dm-crypt.
1758 If you want or need all the original LUKS security features to work, you
1759 can use a detached LUKS header and put that on a conventional, magnetic
1760 disk. That leaves potentially old encrypted data in the pools on the
1761 main disk, but otherwise you get LUKS with the same security as on a
1762 traditional magnetic disk. Note however that storage vendors are prone
1763 to lying to their customers. For example, it recently came out that
1764 HDDs sold without any warning or mentioning in the data-sheets were
1765 actually using SMR and that will write data first to a faster area and
1766 only overwrite the original data area some time later when things are
1769 If you are concerned about your laptop being stolen, you are likely fine
1770 using LUKS on an SSD or hybrid drive. An attacker would need to have
1771 access to an old passphrase (and the key-slot for this old passphrase
1772 would actually need to still be somewhere in the SSD) for your data to
1773 be at risk. So unless you pasted your old passphrase all over the
1774 Internet or the attacker has knowledge of it from some other source and
1775 does a targeted laptop theft to get at your data, you should be fine.
1778 * 5.20 LUKS1 is broken! It uses SHA-1!
1780 No, it is not. SHA-1 is (academically) broken for finding collisions,
1781 but not for using it in a key-derivation function. And that collision
1782 vulnerability is for non-iterated use only. And you need the hash-value
1785 This basically means that if you already have a slot-key, and you have
1786 set the PBKDF2 iteration count to 1 (it is > 10'000 normally), you could
1787 (maybe) derive a different passphrase that gives you the the same
1788 slot-key. But if you have the slot-key, you can already unlock the
1789 key-slot and get the master key, breaking everything. So basically,
1790 this SHA-1 vulnerability allows you to open a LUKS1 container with high
1791 effort when you already have it open.
1793 The real problem here is people that do not understand crypto and claim
1794 things are broken just because some mechanism is used that has been
1795 broken for a specific different use. The way the mechanism is used
1796 matters very much. A hash that is broken for one use can be completely
1797 secure for other uses and here it is.
1799 Since version 1.7.0, cryptsetup uses SHA-256 as default to ensure that
1800 it will be compatible in the future. There are already some systems
1801 where SHA-1 is completely phased out or disabled by a security policy.
1804 * 5.21 Why is there no "Nuke-Option"?
1806 A "Nuke-Option" or "Kill-switch" is a password that when entered upon
1807 unlocking instead wipes the header and all passwords. So when somebody
1808 forces you to enter your password, you can destroy the data instead.
1810 While this sounds attractive at first glance, it does not make sense
1811 once a real security analysis is done. One problem is that you have to
1812 have some kind of HSM (Hardware Security Module) in order to implement
1813 it securely. In the movies, a HSM starts to smoke and melt once the
1814 Nuke-Option has been activated. In actual reality, it just wipes some
1815 battery-backed RAM cells. A proper HSM costs something like
1816 20'000...100'000 EUR/USD and there a Nuke-Option may make some sense.
1817 BTW, a chipcard or a TPM is not a HSM, although some vendors are
1818 promoting that myth.
1820 Now, a proper HSMs will have a wipe option but not a Nuke-Option, i.e.
1821 you can explicitly wipe the HSM, but by a different process than
1822 unlocking it takes. Why is that? Simple: If somebody can force you to
1823 reveal passwords, then they can also do bad things to you if you do not
1824 or if you enter a nuke password instead. Think locking you up for a few
1825 years for "destroying evidence" or for far longer and without trial for
1826 being a "terrorist suspect". No HSM maker will want to expose its
1827 customers to that risk.
1829 Now think of the typical LUKS application scenario, i.e. disk
1830 encryption. Usually the ones forcing you to hand over your password
1831 will have access to the disk as well, and, if they have any real
1832 suspicion, they will mirror your disk before entering anything supplied
1833 by you. This neatly negates any Nuke-Option. If they have no suspicion
1834 (just harassing people that cross some border for example), the
1835 Nuke-Option would work, but see above about likely negative consequences
1836 and remember that a Nuke-Option may not work reliably on SSD and hybrid
1839 Hence my advice is to never take data that you do not want to reveal
1840 into any such situation in the first place. There is no need to
1841 transfer data on physical carriers today. The Internet makes it quite
1842 possible to transfer data between arbitrary places and modern encryption
1843 makes it secure. If you do it right, nobody will even be able to
1844 identify source or destination. (How to do that is out of scope of this
1845 document. It does require advanced skills in this age of pervasive
1848 Hence, LUKS has no kill option because it would do much more harm than
1851 Still, if you have a good use-case (i.e. non-abstract real-world
1852 situation) where a Nuke-Option would actually be beneficial, please let
1856 * 5.22 Does cryptsetup open network connections to websites, etc. ?
1858 This question seems not to make much sense at first glance, but here is
1859 an example form the real world: The TrueCrypt GUI has a "Donation"
1860 button. Press it, and a web-connection to the TrueCrypt website is
1861 opened via the default browser, telling everybody that listens that you
1862 use TrueCrypt. In the worst case, things like this can get people
1865 So: Cryptsetup will never open any network connections except the
1866 local netlink socket it needs to talk to the kernel crypto API.
1868 In addition, the installation package should contain all documentation,
1869 including this FAQ, so that you do not have to go to a web-site to read
1870 it. (If your distro cuts the docu, please complain to them.) In
1871 security software, any connection initiated to anywhere outside your
1872 machine should always be the result of an explicit request for such a
1873 connection by the user and cryptsetup will stay true to that principle.
1876 6. Backup and Data Recovery
1879 * 6.1 Why do I need Backup?
1881 First, disks die. The rate for well-treated (!) disk is about 5% per
1882 year, which is high enough to worry about. There is some indication
1883 that this may be even worse for some SSDs. This applies both to LUKS
1884 and plain dm-crypt partitions.
1886 Second, for LUKS, if anything damages the LUKS header or the key-stripe
1887 area then decrypting the LUKS device can become impossible. This is a
1888 frequent occurrence. For example an accidental format as FAT or some
1889 software overwriting the first sector where it suspects a partition boot
1890 sector typically makes a LUKS1 partition permanently inaccessible. See
1891 more below on LUKS header damage.
1893 So, data-backup in some form is non-optional. For LUKS, you may also
1894 want to store a header backup in some secure location. This only needs
1895 an update if you change passphrases.
1898 * 6.2 How do I backup a LUKS header?
1900 While you could just copy the appropriate number of bytes from the start
1901 of the LUKS partition, the best way is to use command option
1902 "luksHeaderBackup" of cryptsetup. This protects also against errors
1903 when non-standard parameters have been used in LUKS partition creation.
1906 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
1908 To restore, use the inverse command, i.e.
1910 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
1912 If you are unsure about a header to be restored, make a backup of the
1913 current one first! You can also test the header-file without restoring
1914 it by using the --header option for a detached header like this:
1916 cryptsetup --header <file> luksOpen <device> </dev/mapper/name>
1918 If that unlocks your key-slot, you are good. Do not forget to close
1921 Under some circumstances (damaged header), this fails. Then use the
1922 following steps in case it is LUKS1:
1924 First determine the master-key size:
1926 cryptsetup luksDump <device>
1928 gives a line of the form
1932 with bits equal to 256 for the old defaults and 512 for the new
1933 defaults. 256 bits equals a total header size of 1'052'672 Bytes and
1934 512 bits one of 2MiB. (See also Item 6.12) If luksDump fails, assume
1935 2MiB, but be aware that if you restore that, you may also restore the
1936 first 1M or so of the filesystem. Do not change the filesystem if you
1937 were unable to determine the header size! With that, restoring a
1938 too-large header backup is still safe.
1940 Second, dump the header to file. There are many ways to do it, I
1941 prefer the following:
1943 head -c 1052672 <device> > header_backup.dmp
1947 head -c 2M <device> > header_backup.dmp
1949 for a 2MiB header. Verify the size of the dump-file to be sure.
1951 To restore such a backup, you can try luksHeaderRestore or do a more
1954 cat header_backup.dmp > <device>
1958 * 6.3 How do I test for a LUKS header?
1962 cryptsetup -v isLuks <device>
1964 on the device. Without the "-v" it just signals its result via
1965 exit-status. You can also use the more general test
1969 which will also detect other types and give some more info. Omit
1970 "-p" for old versions of blkid that do not support it.
1973 * 6.4 How do I backup a LUKS or dm-crypt partition?
1975 There are two options, a sector-image and a plain file or filesystem
1976 backup of the contents of the partition. The sector image is already
1977 encrypted, but cannot be compressed and contains all empty space. The
1978 filesystem backup can be compressed, can contain only part of the
1979 encrypted device, but needs to be encrypted separately if so desired.
1981 A sector-image will contain the whole partition in encrypted form, for
1982 LUKS the LUKS header, the keys-slots and the data area. It can be done
1983 under Linux e.g. with dd_rescue (for a direct image copy) and with
1984 "cat" or "dd". Examples:
1986 cat /dev/sda10 > sda10.img
1987 dd_rescue /dev/sda10 sda10.img
1989 You can also use any other backup software that is capable of making a
1990 sector image of a partition. Note that compression is ineffective for
1991 encrypted data, hence it does not make sense to use it.
1993 For a filesystem backup, you decrypt and mount the encrypted partition
1994 and back it up as you would a normal filesystem. In this case the
1995 backup is not encrypted, unless your encryption method does that. For
1996 example you can encrypt a backup with "tar" as follows with GnuPG:
1998 tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
2000 And verify the backup like this if you are at "path":
2002 cat backup.tbz2.gpg | gpg - | tar djf -
2004 Note: Always verify backups, especially encrypted ones!
2006 There is one problem with verifying like this: The kernel may still have
2007 some files cached and in fact verify them against RAM or may even verify
2008 RAM against RAM, which defeats the purpose of the exercise. The
2009 following command empties the kernel caches:
2011 echo 3 > /proc/sys/vm/drop_caches
2013 Run it after backup and before verify.
2015 In both cases GnuPG will ask you interactively for your symmetric key.
2016 The verify will only output errors. Use "tar dvjf -" to get all
2017 comparison results. To make sure no data is written to disk
2018 unencrypted, turn off swap if it is not encrypted before doing the
2021 Restore works like certification with the 'd' ('difference') replaced
2022 by 'x' ('eXtract'). Refer to the man-page of tar for more explanations
2023 and instructions. Note that with default options tar will overwrite
2024 already existing files without warning. If you are unsure about how
2025 to use tar, experiment with it in a location where you cannot do damage.
2027 You can of course use different or no compression and you can use an
2028 asymmetric key if you have one and have a backup of the secret key that
2031 A second option for a filesystem-level backup that can be used when the
2032 backup is also on local disk (e.g. an external USB drive) is to use a
2033 LUKS container there and copy the files to be backed up between both
2034 mounted containers. Also see next item.
2037 * 6.5 Do I need a backup of the full partition? Would the header
2038 and key-slots not be enough?
2040 Backup protects you against two things: Disk loss or corruption and user
2041 error. By far the most questions on the dm-crypt mailing list about how
2042 to recover a damaged LUKS partition are related to user error. For
2043 example, if you create a new filesystem on a non-mapped LUKS container,
2044 chances are good that all data is lost permanently.
2046 For this case, a header+key-slot backup would often be enough. But keep
2047 in mind that a well-treated (!) HDD has roughly a failure risk of 5% per
2048 year. It is highly advisable to have a complete backup to protect
2052 * 6.6 What do I need to backup if I use "decrypt_derived"?
2054 This is a script in Debian, intended for mounting /tmp or swap with a
2055 key derived from the master key of an already decrypted device. If you
2056 use this for an device with data that should be persistent, you need to
2057 make sure you either do not lose access to that master key or have a
2058 backup of the data. If you derive from a LUKS device, a header backup
2059 of that device would cover backing up the master key. Keep in mind that
2060 this does not protect against disk loss.
2062 Note: If you recreate the LUKS header of the device you derive from
2063 (using luksFormat), the master key changes even if you use the same
2064 passphrase(s) and you will not be able to decrypt the derived device
2065 with the new LUKS header.
2068 * 6.7 Does a backup compromise security?
2070 Depends on how you do it. However if you do not have one, you are going
2071 to eventually lose your encrypted data.
2073 There are risks introduced by backups. For example if you
2074 change/disable a key-slot in LUKS, a binary backup of the partition will
2075 still have the old key-slot. To deal with this, you have to be able to
2076 change the key-slot on the backup as well, securely erase the backup or
2077 do a filesystem-level backup instead of a binary one.
2079 If you use dm-crypt, backup is simpler: As there is no key management,
2080 the main risk is that you cannot wipe the backup when wiping the
2081 original. However wiping the original for dm-crypt should consist of
2082 forgetting the passphrase and that you can do without actual access to
2085 In both cases, there is an additional (usually small) risk with binary
2086 backups: An attacker can see how many sectors and which ones have been
2087 changed since the backup. To prevent this, use a filesystem level
2088 backup method that encrypts the whole backup in one go, e.g. as
2089 described above with tar and GnuPG.
2091 My personal advice is to use one USB disk (low value data) or three
2092 disks (high value data) in rotating order for backups, and either use
2093 independent LUKS partitions on them, or use encrypted backup with tar
2096 If you do network-backup or tape-backup, I strongly recommend to go
2097 the filesystem backup path with independent encryption, as you
2098 typically cannot reliably delete data in these scenarios, especially
2099 in a cloud setting. (Well, you can burn the tape if it is under your
2103 * 6.8 What happens if I overwrite the start of a LUKS partition or
2104 damage the LUKS header or key-slots?
2106 There are two critical components for decryption: The salt values in the
2107 key-slot descriptors of the header and the key-slots. For LUKS2 they
2108 are a bit better protected. but for LUKS1, these are right in the first
2109 sector. If the salt values are overwritten or changed, nothing (in the
2110 cryptographically strong sense) can be done to access the data, unless
2111 there is a backup of the LUKS header. If a key-slot is damaged, the
2112 data can still be read with a different key-slot, if there is a
2113 remaining undamaged and used key-slot. Note that in order to make a
2114 key-slot completely unrecoverable, changing about 4-6 bits in random
2115 locations of its 128kiB size is quite enough.
2118 * 6.9 What happens if I (quick) format a LUKS partition?
2120 I have not tried the different ways to do this, but very likely you will
2121 have written a new boot-sector, which in turn overwrites the LUKS
2122 header, including the salts, making your data permanently irretrievable,
2123 unless you have a LUKS header backup. For LUKS2 this may still be
2124 recoverable without that header backup, for LUKS1 it is not. You may
2125 also damage the key-slots in part or in full. See also last item.
2128 * 6.10 How do I recover the master key from a mapped LUKS1 container?
2130 Note: LUKS2 uses the kernel keyring to store keys and hence this
2131 procedure does not work unless you have explicitly disabled the use of
2132 the keyring with "--disable-keyring" on opening.
2134 This is typically only needed if you managed to damage your LUKS1
2135 header, but the container is still mapped, i.e. "luksOpen"ed. It also
2136 helps if you have a mapped container that you forgot or do not know a
2137 passphrase for (e.g. on a long running server.)
2139 WARNING: Things go wrong, do a full backup before trying this!
2141 WARNING: This exposes the master key of the LUKS1 container. Note that
2142 both ways to recreate a LUKS header with the old master key described
2143 below will write the master key to disk. Unless you are sure you have
2144 securely erased it afterwards, e.g. by writing it to an encrypted
2145 partition, RAM disk or by erasing the filesystem you wrote it to by a
2146 complete overwrite, you should change the master key afterwards.
2147 Changing the master key requires a full data backup, luksFormat and then
2148 restore of the backup. Alternatively the tool cryptsetup-reencrypt from
2149 the cryptsetup package can be used to change the master key (see its
2150 man-page), but a full backup is still highly recommended.
2152 First, there is a script by Milan that automates the whole process,
2153 except generating a new LUKS1 header with the old master key (it prints
2154 the command for that though):
2156 https://gitlab.com/cryptsetup/cryptsetup/blob/master/misc/luks-header-from-active
2158 You can also do this manually. Here is how:
2160 - Get the master key from the device mapper. This is done by the
2161 following command. Substitute c5 for whatever you mapped to:
2163 # dmsetup table --target crypt --showkey /dev/mapper/c5
2166 0 200704 crypt aes-cbc-essiv:sha256
2167 a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
2170 The result is actually one line, wrapped here for clarity. The long
2171 hex string is the master key.
2173 - Convert the master key to a binary file representation. You can do
2174 this manually, e.g. with hexedit. You can also use the tool "xxd"
2177 echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
2180 - Do a luksFormat to create a new LUKS1 header.
2182 NOTE: If your header is intact and you just forgot the passphrase,
2183 you can just set a new passphrase, see next sub-item.
2185 Unmap the device before you do that (luksClose). Then do
2187 cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
2189 Note that if the container was created with other than the default
2190 settings of the cryptsetup version you are using, you need to give
2191 additional parameters specifying the deviations. If in doubt, try the
2192 script by Milan. It does recover the other parameters as well.
2194 Side note: This is the way the decrypt_derived script gets at the master
2195 key. It just omits the conversion and hashes the master key string.
2197 - If the header is intact and you just forgot the passphrase, just
2198 set a new passphrase like this:
2200 cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
2202 You may want to disable the old one afterwards.
2205 * 6.11 What does the on-disk structure of dm-crypt look like?
2207 There is none. dm-crypt takes a block device and gives encrypted access
2208 to each of its blocks with a key derived from the passphrase given. If
2209 you use a cipher different than the default, you have to specify that as
2210 a parameter to cryptsetup too. If you want to change the password, you
2211 basically have to create a second encrypted device with the new
2212 passphrase and copy your data over. On the plus side, if you
2213 accidentally overwrite any part of a dm-crypt device, the damage will be
2214 limited to the area you overwrote.
2217 * 6.12 What does the on-disk structure of LUKS1 look like?
2219 Note: For LUKS2, refer to the LUKS2 document referenced in Item 1.2
2221 A LUKS1 partition consists of a header, followed by 8 key-slot
2222 descriptors, followed by 8 key slots, followed by the encrypted data
2225 Header and key-slot descriptors fill the first 592 bytes. The key-slot
2226 size depends on the creation parameters, namely on the number of
2227 anti-forensic stripes, key material offset and master key size.
2229 With the default parameters, each key-slot is a bit less than 128kiB in
2230 size. Due to sector alignment of the key-slot start, that means the key
2231 block 0 is at offset 0x1000-0x20400, key block 1 at offset
2232 0x21000-0x40400, and key block 7 at offset 0xc1000-0xe0400. The space
2233 to the next full sector address is padded with zeros. Never used
2234 key-slots are filled with what the disk originally contained there, a
2235 key-slot removed with "luksRemoveKey" or "luksKillSlot" gets filled with
2236 0xff. Due to 2MiB default alignment, start of the data area for
2237 cryptsetup 1.3 and later is at 2MiB, i.e. at 0x200000. For older
2238 versions, it is at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB +
2239 4096 bytes from the start of the partition. Incidentally,
2240 "luksHeaderBackup" for a LUKS container created with default parameters
2241 dumps exactly the first 2MiB (or 1'052'672 bytes for headers created
2242 with cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
2245 For non-default parameters, you have to figure out placement yourself.
2246 "luksDump" helps. See also next item. For the most common non-default
2247 settings, namely aes-xts-plain with 512 bit key, the offsets are: 1st
2248 keyslot 0x1000-0x3f800, 2nd keyslot 0x40000-0x7e000, 3rd keyslot
2249 0x7e000-0xbd800, ..., and start of bulk data at 0x200000.
2251 The exact specification of the format is here:
2252 https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
2254 For your convenience, here is the LUKS1 header with hex offsets.
2256 The spec counts key-slots from 1 to 8, but the cryptsetup tool counts
2257 from 0 to 7. The numbers here refer to the cryptsetup numbers.
2260 Refers to LUKS1 On-Disk Format Specification Version 1.2.3
2264 offset length name data type description
2265 -----------------------------------------------------------------------
2266 0x0000 0x06 magic byte[] 'L','U','K','S', 0xba, 0xbe
2268 0x0006 0x02 version uint16_t LUKS version
2270 0x0008 0x20 cipher-name char[] cipher name spec.
2272 0x0028 0x20 cipher-mode char[] cipher mode spec.
2274 0x0048 0x20 hash-spec char[] hash spec.
2276 0x0068 0x04 payload-offset uint32_t bulk data offset in sectors
2277 104 4 (512 bytes per sector)
2278 0x006c 0x04 key-bytes uint32_t number of bytes in key
2280 0x0070 0x14 mk-digest byte[] master key checksum
2281 112 20 calculated with PBKDF2
2282 0x0084 0x20 mk-digest-salt byte[] salt for PBKDF2 when
2283 132 32 calculating mk-digest
2284 0x00a4 0x04 mk-digest-iter uint32_t iteration count for PBKDF2
2285 164 4 when calculating mk-digest
2286 0x00a8 0x28 uuid char[] partition UUID
2288 0x00d0 0x30 key-slot-0 key slot key slot 0
2290 0x0100 0x30 key-slot-1 key slot key slot 1
2292 0x0130 0x30 key-slot-2 key slot key slot 2
2294 0x0160 0x30 key-slot-3 key slot key slot 3
2296 0x0190 0x30 key-slot-4 key slot key slot 4
2298 0x01c0 0x30 key-slot-5 key slot key slot 5
2300 0x01f0 0x30 key-slot-6 key slot key slot 6
2302 0x0220 0x30 key-slot-7 key slot key slot 7
2308 offset length name data type description
2309 -------------------------------------------------------------------------
2310 0x0000 0x04 active uint32_t key slot enabled/disabled
2312 0x0004 0x04 iterations uint32_t PBKDF2 iteration count
2314 0x0008 0x20 salt byte[] PBKDF2 salt
2316 0x0028 0x04 key-material-offset uint32_t key start sector
2317 40 4 (512 bytes/sector)
2318 0x002c 0x04 stripes uint32_t number of anti-forensic
2323 * 6.13 What is the smallest possible LUKS1 container?
2325 Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With modern
2326 Linux partitioning tools that also align to 1MB, this will result in
2327 alignment to 2k sectors and typical Flash/SSD sectors, which is highly
2328 desirable for a number of reasons. Changing the alignment is not
2331 That said, with default parameters, the data area starts at exactly 2MB
2332 offset (at 0x101000 for cryptsetup versions before 1.3). The smallest
2333 data area you can have is one sector of 512 bytes. Data areas of 0
2334 bytes can be created, but fail on mapping.
2336 While you cannot put a filesystem into something this small, it may
2337 still be used to contain, for example, key. Note that with current
2338 formatting tools, a partition for a container this size will be 3MiB
2339 anyways. If you put the LUKS container into a file (via losetup and a
2340 loopback device), the file needs to be 2097664 bytes in size, i.e. 2MiB
2343 The two ways to influence the start of the data area are key-size and
2346 For alignment, you can go down to 1 on the parameter. This will still
2347 leave you with a data-area starting at 0x101000, i.e. 1MiB+4096B
2348 (default parameters) as alignment will be rounded up to the next
2349 multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run on a larger
2350 file and dump the LUKS header to get actual information.
2352 For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
2353 (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode). You
2354 can do 64 bit (e.g. blowfish-64 with CBC), but anything below 128 bit
2355 has to be considered insecure today.
2357 Example 1 - AES 128 bit with CBC:
2359 cryptsetup luksFormat -s 128 --align-payload=8 <device>
2361 This results in a data offset of 0x81000, i.e. 516KiB or 528384
2362 bytes. Add one 512 byte sector and the smallest LUKS container size
2363 with these parameters is 516KiB + 512B or 528896 bytes.
2365 Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
2367 cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
2369 This results in a data offset of 0x41000, i.e. 260kiB or 266240
2370 bytes, with a minimal LUKS1 container size of 260kiB + 512B or 266752
2374 * 6.14 I think this is overly complicated. Is there an alternative?
2376 Not really. Encryption comes at a price. You can use plain dm-crypt to
2377 simplify things a bit. It does not allow multiple passphrases, but on
2378 the plus side, it has zero on disk description and if you overwrite some
2379 part of a plain dm-crypt partition, exactly the overwritten parts are
2380 lost (rounded up to full sectors).
2382 * 6.15 Can I clone a LUKS container?
2384 You can, but it breaks security, because the cloned container has the
2385 same header and hence the same master key. Even if you change the
2386 passphrase(s), the master key stays the same. That means whoever has
2387 access to one of the clones can decrypt them all, completely bypassing
2390 While you can use cryptsetup-reencrypt to change the master key,
2391 this is probably more effort than to create separate LUKS containers
2394 The right way to do this is to first luksFormat the target container,
2395 then to clone the contents of the source container, with both containers
2396 mapped, i.e. decrypted. You can clone the decrypted contents of a LUKS
2397 container in binary mode, although you may run into secondary issues
2398 with GUIDs in filesystems, partition tables, RAID-components and the
2399 like. These are just the normal problems binary cloning causes.
2401 Note that if you need to ship (e.g.) cloned LUKS containers with a
2402 default passphrase, that is fine as long as each container was
2403 individually created (and hence has its own master key). In this case,
2404 changing the default passphrase will make it secure again.
2407 7. Interoperability with other Disk Encryption Tools
2410 * 7.1 What is this section about?
2412 Cryptsetup for plain dm-crypt can be used to access a number of on-disk
2413 formats created by tools like loop-aes patched into losetup. This
2414 sometimes works and sometimes does not. This section collects insights
2415 into what works, what does not and where more information is required.
2417 Additional information may be found in the mailing-list archives,
2418 mentioned at the start of this FAQ document. If you have a solution
2419 working that is not yet documented here and think a wider audience may
2420 be interested, please email the FAQ maintainer.
2423 * 7.2 loop-aes: General observations.
2425 One problem is that there are different versions of losetup around.
2426 loop-aes is a patch for losetup. Possible problems and deviations
2427 from cryptsetup option syntax include:
2429 - Offsets specified in bytes (cryptsetup: 512 byte sectors)
2431 - The need to specify an IV offset
2433 - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
2435 - Key size needs specifying (e.g. "-s 128" for 128 bit keys)
2437 - Passphrase hash algorithm needs specifying
2439 Also note that because plain dm-crypt and loop-aes format does not have
2440 metadata, and while the loopAES extension for cryptsetup tries
2441 autodetection (see command loopaesOpen), it may not always work. If you
2442 still have the old set-up, using a verbosity option (-v) on mapping with
2443 the old tool or having a look into the system logs after setup could
2444 give you the information you need. Below, there are also some things
2445 that worked for somebody.
2448 * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32
2450 In this case, the main problem seems to be that this variant of
2451 losetup takes the offset (-o option) in bytes, while cryptsetup takes
2452 it in sectors of 512 bytes each.
2454 Example: The losetup command
2456 losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
2457 mount /dev/loop0 mount-point
2461 cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
2462 mount /dev/mapper/e1 mount-point
2466 * 7.4 loop-aes with 160 bit key
2468 This seems to be sometimes used with twofish and blowfish and represents
2469 a 160 bit ripemed160 hash output padded to 196 bit key length. It seems
2470 the corresponding options for cryptsetup are
2472 --cipher twofish-cbc-null -s 192 -h ripemd160:20
2476 * 7.5 loop-aes v1 format OpenSUSE
2478 Apparently this is done by older OpenSUSE distros and stopped working
2479 from OpenSUSE 12.1 to 12.2. One user had success with the following:
2481 cryptsetup create <target> <device> -c aes -s 128 -h sha256
2485 * 7.6 Kernel encrypted loop device (cryptoloop)
2487 There are a number of different losetup implementations for using
2488 encrypted loop devices so getting this to work may need a bit of
2491 NOTE: Do NOT use this for new containers! Some of the existing
2492 implementations are insecure and future support is uncertain.
2494 Example for a compatible mapping:
2496 losetup -e twofish -N /dev/loop0 /image.img
2500 cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain
2502 with the mapping being done to /dev/mapper/image_plain instead of
2507 Cipher, mode and password hash (or no hash):
2509 -e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256]
2510 -e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256]
2513 Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512 bytes):
2516 -o 2560 => -o 5 -p 5 # 2560/512 = 5
2519 There is no replacement for --pass-fd, it has to be emulated using
2520 keyfiles, see the cryptsetup man-page.
2523 8. Issues with Specific Versions of cryptsetup
2526 * 8.1 When using the create command for plain dm-crypt with
2527 cryptsetup 1.1.x, the mapping is incompatible and my data is not
2530 With cryptsetup 1.1.x, the distro maintainer can define different
2531 default encryption modes. You can check the compiled-in defaults using
2532 "cryptsetup --help". Moreover, the plain device default changed because
2533 the old IV mode was vulnerable to a watermarking attack.
2535 If you are using a plain device and you need a compatible mode, just
2536 specify cipher, key size and hash algorithm explicitly. For
2537 compatibility with cryptsetup 1.0.x defaults, simple use the following:
2539 cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
2541 LUKS stores cipher and mode in the metadata on disk, avoiding this
2545 * 8.2 cryptsetup on SLED 10 has problems...
2547 SLED 10 is missing an essential kernel patch for dm-crypt, which is
2548 broken in its kernel as a result. There may be a very old version of
2549 cryptsetup (1.0.x) provided by SLED, which should also not be used
2550 anymore as well. My advice would be to drop SLED 10.
2553 * 8.3 Gcrypt 1.6.x and later break Whirlpool
2555 It is the other way round: In gcrypt 1.5.x, Whirlpool is broken and it
2556 was fixed in 1.6.0 and later. If you selected whirlpool as hash on
2557 creation of a LUKS container, it does not work anymore with the fixed
2558 library. This shows one serious risk of using rarely used settings.
2560 Note that at the time this FAQ item was written, 1.5.4 was the latest
2561 1.5.x version and it has the flaw, i.e. works with the old Whirlpool
2562 version. Possibly later 1.5.x versions will work as well. If not,
2565 The only two ways to access older LUKS containers created with Whirlpool
2566 are to either decrypt with an old gcrypt version that has the flaw or to
2567 use a compatibility feature introduced in cryptsetup 1.6.4 and gcrypt
2568 1.6.1 or later. Version 1.6.0 cannot be used.
2572 - Make at least a header backup or better, refresh your full backup.
2573 (You have a full backup, right? See Item 6.1 and following.)
2575 - Make sure you have cryptsetup 1.6.4 or later and check the gcrypt
2578 cryptsetup luksDump <your luks device> --debug | grep backend
2580 If gcrypt is at version 1.5.x or before:
2582 - Reencrypt the LUKS header with a different hash. (Requires entering
2583 all keyslot passphrases. If you do not have all, remove the ones you
2584 do not have before.):
2586 cryptsetup-reencrypt --keep-key --hash sha256 <your luks device>
2588 If gcrypt is at version 1.6.1 or later:
2590 - Patch the hash name in the LUKS header from "whirlpool" to
2591 "whirlpool_gcryptbug". This activates the broken implementation.
2592 The detailed header layout is in Item 6.12 of this FAQ and in the
2593 LUKS on-disk format specification. One way to change the hash is
2594 with the following command:
2596 echo -n -e 'whirlpool_gcryptbug\0' | dd of=<luks device> bs=1 seek=72 conv=notrunc
2598 - You can now open the device again. It is highly advisable to change
2599 the hash now with cryptsetup-reencrypt as described above. While you
2600 can reencrypt to use the fixed whirlpool, that may not be a good idea
2601 as almost nobody seems to use it and hence the long time until the
2605 9. The Initrd question
2608 * 9.1 My initrd is broken with cryptsetup
2610 That is not nice! However the initrd is supplied by your distribution,
2611 not by the cryptsetup project and hence you should complain to them. We
2612 cannot really do anything about it.
2615 * 9.2 CVE-2016-4484 says cryptsetup is broken!
2617 Not really. It says the initrd in some Debian versions have a behavior
2618 that under some very special and unusual conditions may be considered
2621 What happens is that you can trick the initrd to go to a rescue-shell if
2622 you enter the LUKS password wrongly in a specific way. But falling back
2623 to a rescue shell on initrd errors is a sensible default behavior in the
2624 first place. It gives you about as much access as booting a rescue
2625 system from CD or USB-Stick or as removing the disk would give you. So
2626 this only applies when an attacker has physical access, but cannot boot
2627 anything else or remove the disk. These will be rare circumstances
2628 indeed, and if you rely on the default distribution initrd to keep you
2629 safe under these circumstances, then you have bigger problems than this
2630 somewhat expected behavior.
2632 The CVE was exaggerated and should not be assigned to upstream
2633 cryptsetup in the first place (it is a distro specific initrd issue).
2634 It was driven more by a try to make a splash for self-aggrandizement,
2635 than by any actual security concerns. Ignore it.
2638 * 9.3 How do I do my own initrd with cryptsetup?
2640 Note: The instructions here apply to an initrd in initramfs format, not
2641 to an initrd in initrd format. The latter is a filesystem image, not a
2642 cpio-archive, and seems to not be widely used anymore.
2644 It depends on the distribution. Below, I give a very simple example and
2645 step-by-step instructions for Debian. With a bit of work, it should be
2646 possible to adapt this to other distributions. Note that the
2647 description is pretty general, so if you want to do other things with an
2648 initrd it provides a useful starting point for that too.
2650 01) Unpacking an existing initrd to use as template
2652 A Linux initrd is in gzip'ed cpio format. To unpack it, use something
2655 md tmp; cd tmp; cat ../initrd | gunzip | cpio -id
2657 After this, you have the full initrd content in tmp/
2659 02) Inspecting the init-script
2661 The init-script is the only thing the kernel cares about. All activity
2662 starts there. Its traditional location is /sbin/init on disk, but /init
2663 in an initrd. In an initrd unpacked as above it is tmp/init.
2665 While init can be a binary despite usually being called "init script",
2666 in Debian the main init on the root partition is a binary, but the init
2667 in the initrd (and only that one is called by the kernel) is a script
2668 and starts like this:
2673 The "sh" used here is in tmp/bin/sh as just unpacked, and in Debian it
2674 currently is a busybox.
2676 03) Creating your own initrd
2678 The two examples below should give you most of what is needed. This is
2679 tested with LUKS1 and should work with LUKS2 as well. If not, please
2682 Here is a really minimal example. It does nothing but set up some
2683 things and then drop to an interactive shell. It is perfect to try out
2684 things that you want to go into the init-script.
2687 export PATH=/sbin:/bin
2688 [ -d /sys ] || mkdir /sys
2689 [ -d /proc ] || mkdir /proc
2690 [ -d /tmp ] || mkdir /tmp
2691 mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
2692 mount -t proc -o nodev,noexec,nosuid proc /proc
2693 echo "initrd is running, starting BusyBox..."
2694 exec /bin/sh --login
2697 Here is an example that opens the first LUKS-partition it finds with the
2698 hard-coded password "test2" and then mounts it as root-filesystem. This
2699 is intended to be used on an USB-stick that after boot goes into a safe,
2700 as it contains the LUKS-passphrase in plain text and is not secure to be
2701 left in the system. The script contains debug-output that should make it
2702 easier to see what is going on. Note that the final hand-over to the init
2703 on the encrypted root-partition is done by "exec switch_root /mnt/root
2704 /sbin/init", after mounting the decrypted LUKS container with "mount
2705 /dev/mapper/c1 /mnt/root". The second argument of switch_root is relative
2706 to the first argument, i.e. the init started with this command is really
2707 /mnt/sbin/init before switch_root runs.
2710 export PATH=/sbin:/bin
2711 [ -d /sys ] || mkdir /sys
2712 [ -d /proc ] || mkdir /proc
2713 [ -d /tmp ] || mkdir /tmp
2714 mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
2715 mount -t proc -o nodev,noexec,nosuid proc /proc
2716 echo "detecting LUKS containers in sda1-10, sdb1-10"; sleep 1
2719 for j in 1 2 3 4 5 6 7 8 9 10
2724 cryptsetup isLuks $d >/dev/null 2>&1
2726 echo -n " result: "$r""
2727 # 0 = is LUKS, 1 = is not LUKS, 4 = other error
2728 if expr $r = 0 > /dev/null
2730 echo " is LUKS, attempting unlock"
2731 echo -n "test2" | cryptsetup luksOpen --key-file=- $d c1
2733 echo " result of unlock attempt: "$r""
2735 if expr $r = 0 > /dev/null
2737 echo "*** LUKS partition unlocked, switching root ***
2738 echo " (waiting 30 seconds before doing that)"
2739 mount /dev/mapper/c1 /mnt/root
2741 exec switch_root /mnt/root /sbin/init
2748 echo "FAIL finding root on LUKS, loading BusyBox..."; sleep 5
2749 exec /bin/sh --login
2752 04) What if I want a binary in the initrd, but libraries are missing?
2754 That is a bit tricky. One option is to compile statically, but that
2755 does not work for everything. Debian puts some libraries into lib/ and
2756 lib64/ which are usually enough. If you need more, you can add the
2757 libraries you need there. That may or may not need a configuration
2758 change for the dynamic linker "ld" as well. Refer to standard Linux
2759 documentation on how to add a library to a Linux system. A running
2760 initrd is just a running Linux system after all, it is not special in
2763 05) How do I repack the initrd?
2765 Simply repack the changed directory. While in tmp/, do
2768 find . | cpio --create --format='newc' | gzip > ../new_initrd
2770 Rename "new_initrd" to however you want it called (the name of
2771 the initrd is a kernel-parameter) and move to /boot. That is it.
2777 * 10.1 Is the cryptography of LUKS2 different?
2779 Mostly not. The header has changed in its structure, but the
2780 crytpgraphy is the same. The one exception is that PBKDF2 has been
2781 replaced by Argon2 to give better resilience against attacks by
2782 graphics cards and other hardware with lots of computing power but
2783 limited local memory per computing element.
2786 * 10.2 What new features does LUKS2 have?
2788 There are quite a few. I recommend reading the man-page and the on-disk
2789 format specification, see Item 1.2.
2792 - A lot of the metadata is JSON, allowing for easier extension
2793 - Max 32 key-slots per default
2794 - Better protection for bad passphrases now available with Argon2
2795 - Authenticated encryption
2796 - The LUKS2 header is less vulnerable to corruption and has a 2nd copy
2799 * 10.3 Why does LUKS2 need so much memory?
2801 LUKS2 uses Argon2 instead of PBKDF2. That causes the increase in memory.
2805 * 10.4 Why use Argon2 in LUKS 2 instead of PBKDF2?
2807 LUKS tries to be secure with not-so-good passwords. Bad passwords need to
2808 be protected in some way against an attacker that just tries all possible
2809 combinations. (For good passwords, you can just wait for the attacker to
2810 die of old age...) The situation with LUKS is not quite the same as with a
2811 password stored in a database, but there are similarities.
2813 LUKS does not store passwords on disk. Instead, the passwords are used to
2814 decrypt the master-key with it and that one is stored on disk in encrypted
2815 form. If you have a good password, with, say, more than 80 bits of
2816 entropy, you could just put the password through a single crypto-hash (to
2817 turn it into something that can be used as a key) and that would be secure.
2818 This is what plain dm-crypt does.
2820 If the password has lower entropy, you want to make this process cost some
2821 effort, so that each try takes time and resources and slows the attacker
2822 down. LUKS1 uses PBKDF2 for that, adding an iteration count and a salt.
2823 The iteration count is per default set to that it takes 1 second per try on
2824 the CPU of the device where the respective passphrase was set. The salt is
2825 there to prevent precomputation.
2827 The problem with that is that if you use a graphics card, you can massively
2828 speed up these computations as PBKDF2 needs very little memory to compute
2829 it. A graphics card is (grossly simplified) a mass of small CPUs with some
2830 small very fast local memory per CPU and a large slow memory (the 4/6/8 GB
2831 a current card may have). If you can keep a computation in the small,
2832 CPU-local memory, you can gain a speed factor of 1000 or more when trying
2833 passwords with PBKDF2.
2835 Argon2 was created to address this problem. It adds a "large memory
2836 property" where computing the result with less memory than the memory
2837 parameter requires is massively (exponentially) slowed down. That means,
2838 if you set, for example, 4GB of memory, computing Argon2 on a graphics card
2839 with around 100kB of memory per "CPU" makes no sense at all because it is
2840 far too slow. An attacker has hence to use real CPUs and furthermore is
2841 limited by main memory bandwidth.
2843 Hence the large amount of memory used is a security feature and should not
2844 be turned off or reduced. If you really (!) understand what you are doing
2845 and can assure good passwords, you can either go back to PBKDF2 or set a
2846 low amount of memory used for Argon2 when creating the header.
2849 * 10.5 LUKS2 is insecure! It uses less memory than the Argon2 RFC say!
2851 Well, not really. The RFC recommends 6GiB of memory for use with disk
2852 encryption. That is a bit insane and something clearly went wrong in the
2853 standardization process here. First, that makes Argon2 unusable on any 32
2854 bit Linux and that is clearly a bad thing. Second, there are many small
2855 Linux devices around that do not have 6GiB of RAM in the first place. For
2856 example, the current Raspberry Pi has 1GB, 2GB or 4GB of RAM, and with the
2857 RFC recommendations, none of these could compute Argon2 hashes.
2859 Hence LUKS2 uses a more real-world approach. Iteration is set to a
2860 minimum of 4 because there are some theoretical attacks that work up to an
2861 iteration count of 3. The thread parameter is set to 4. To achieve 2
2862 second/slot unlock time, LUKS2 adjusts the memory parameter down if
2863 needed. In the other direction, it will respect available memory and not
2864 exceed it. On a current PC, the memory parameter will be somewhere around
2865 1GB, which should be quite generous. The minimum I was able to set in an
2866 experiment with "-i 1" was 400kB of memory and that is too low to be
2867 secure. A Raspberry Pi would probably end up somewhere around 50MB (have
2868 not tried it) and that should still be plenty.
2870 That said, if you have a good, high-entropy passphrase, LUKS2 is secure
2871 with any memory parameter.
2874 * 10.6 How does re-encryption store data while it is running?
2876 All metadata necessary to perform a recovery of said segment (in case of
2877 crash) is stored in the LUKS2 metadata area. No matter if the LUKS2
2878 reencryption was run in online or offline mode.
2881 * 10.7 What do I do if re-encryption crashes?
2883 In case of a reencryption application crash, try to close the original
2884 device via following command first:
2886 cryptsetup close <my_crypt_device>.
2888 Cryptsetup assesses if it's safe to teardown the reencryption device stack
2889 or not. It will also cut off I/O (via dm-error mapping) to current
2890 hotzone segment (to make later recovery possible). If it can't be torn
2891 down, i.e. due to a mounted fs, you must unmount the filesystem first.
2892 Never try to tear down reencryption dm devices manually using e.g.
2893 dmsetup tool, at least not unless cryptsetup says it's safe to do so. It
2894 could damage the data beyond repair.
2897 * 10.8 Do I need to enter two passphrases to recover a crashed
2900 Cryptsetup (command line utility) expects the passphrases to be identical
2901 for the keyslot containing old volume key and for the keyslot containing
2902 new one. So the recovery happens during normal the "cryptsetup open"
2903 operation or the equivalent during boot.
2905 Re-encryption recovery can be also performed in offline mode by
2906 the "cryptsetup repair" command.
2909 * 10.9 What is an unbound keyslot and what is it used for?
2911 Quite simply, an 'unbound key' is an independent 'key' stored in a luks2
2912 keyslot that cannot be used to unlock a LUKS2 data device. More specifically,
2913 an 'unbound key' or 'unbound luks2 keyslot' contains a secret that is not
2914 currently associated with any data/crypt segment (encrypted area) in the
2915 LUKS2 'Segments' section (displayed by luksDump).
2917 This is a bit of a more general idea. It basically allows to use a keyslot
2918 as a container for a key to be used in other things than decrypting a
2921 As of April 2020, the following uses are defined:
2923 1) LUKS2 re-encryption. The new volume key is stored in an unbound keyslot
2924 which becomes a regular LUKS2 keyslot later when re-encryption is
2927 2) Somewhat similar is the use with a wrapped key scheme (e.g. with the
2928 paes cipher). In this case, the VK (Volume Key) stored in a keyslot
2929 is an encrypted binary binary blob. The KEK (Key Encryption Key) for
2930 that blob may be refreshed (Note that this KEK is not managed by
2931 cryptsetup!) and the binary blob gets changed. The KEK refresh process
2932 uses an 'unbound keyslot'. First the future effective VK is placed
2933 in the unbound keyslot and later it gets turned into the new real VK
2934 (and bound to the respective crypt segment).
2937 * 10.10 What about the size of the LUKS2 header?
2939 While the LUKS1 header has a fixed size that is determined by the cipher
2940 spec (see Item 6.12), LUKS2 is more variable. The default size is 16MB,
2941 but it can be adjusted on creation by using the --luks2-metadata-size
2942 and --luks2-keyslots-size options. Refer to the man-page for details.
2943 While adjusting the size in an existing LUKS2 container is possible,
2944 it is somewhat complicated and risky. My advice is to do a backup,
2945 recreate the container with changed parameters and restore that backup.
2948 * 10.11 Does LUKS2 store metadata anywhere except in the header?
2950 It does not. But note that if you use the experimental integrity support,
2951 there will be an integrity header as well at the start of the data area
2952 and things get a bit more complicated. All metadata will still be at the
2953 start of the device, nothing gets stored somewhere in the middle or at
2957 11. References and Further Reading
2959 * Purpose of this Section
2961 The purpose of this section is to collect references to all materials
2962 that do not fit the FAQ but are relevant in some fashion. This can be
2963 core topics like the LUKS spec or disk encryption, but it can also be
2964 more tangential, like secure storage management or cryptography used in
2965 LUKS. It should still have relevance to cryptsetup and its
2968 If you want to see something added here, send email to the maintainer
2969 (or the cryptsetup mailing list) giving an URL, a description (1-3 lines
2970 preferred) and a section to put it in. You can also propose new
2973 At this time I would like to limit the references to things that are
2974 available on the web.
2978 - LUKS on-disk format spec: See Item 1.2
2980 * Other Documentation
2982 - Arch Linux on LUKS, LVM and full-disk encryption:
2983 https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system
2987 - Some code examples are in the source package under docs/examples
2989 - LUKS AF Splitter in Ruby by John Lane: https://rubygems.org/gems/afsplitter
2991 * Brute-forcing passphrases
2993 - http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
2995 - https://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
2999 * SSD and Flash Disk Related
3003 * Attacks Against Disk Encryption
3005 * Risk Management as Relevant for Disk Encryption
3013 In no particular order: