8 6. Backup and Data Recovery
9 7. Interoperability with other Disk Encryption Tools
10 8. Issues with Specific Versions of cryptsetup
11 9. References and Further Reading
20 This is the FAQ (Frequently Asked Questions) for cryptsetup. It
21 covers Linux disk encryption with plain dm-crypt (one passphrase,
22 no management, no metadata on disk) and LUKS (multiple user keys
23 with one master key, anti-forensic features, metadata block at
24 start of device, ...). The latest version of this FAQ should
25 usually be available at
26 http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
31 ATTENTION: If you are going to read just one thing, make it the
32 section on Backup and Data Recovery. By far the most questions on
33 the cryptsetup mailing list are from people that managed to damage
34 the start of their LUKS partitions, i.e. the LUKS header. In
35 most cases, there is nothing that can be done to help these poor
36 souls recover their data. Make sure you understand the problem and
37 limitations imposed by the LUKS security model BEFORE you face
38 such a disaster! In particular, make sure you have a current header
39 backup before doing any potentially dangerous operations.
41 SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
42 unclear how to get LUKS or plain dm-crypt to run on them with the
43 full set of security features intact. This may or may not be a
44 problem, depending on the attacher model. See Section 5.19.
46 BACKUP: Yes, encrypted disks die, just as normal ones do. A full
47 backup is mandatory, see Section "6. Backup and Data Recovery" on
48 options for doing encrypted backup.
50 CLONING/IMAGING: If you clone or image a LUKS container, you make a
51 copy of the LUKS header and the master key will stay the same!
52 That means that if you distribute an image to several machines, the
53 same master key will be used on all of them, regardless of whether
54 you change the passphrases. Do NOT do this! If you do, a root-user
55 on any of the machines with a mapped (decrypted) container or a
56 passphrase on that machine can decrypt all other copies, breaking
57 security. See also Item 6.15.
59 DISTRIBUTION INSTALLERS: Some distribution installers offer to
60 create LUKS containers in a way that can be mistaken as activation
61 of an existing container. Creating a new LUKS container on top of
62 an existing one leads to permanent, complete and irreversible data
63 loss. It is strongly recommended to only use distribution
64 installers after a complete backup of all LUKS containers has been
67 NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from
68 STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
69 warning that you are about to format (and e.g. will lose any
70 pre-existing LUKS container on the target), as it assumes it is
71 used from a script. In this scenario, the responsibility for
72 warning the user and possibly checking for an existing LUKS header
73 is shifted to the script. This is a more general form of the
76 LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
77 used in deriving the master key. It is used in decrypting a master
78 key that is randomly selected on header creation. This means that
79 if you create a new LUKS header on top of an old one with
80 exactly the same parameters and exactly the same passphrase as the
81 old one, it will still have a different master key and your data
82 will be permanently lost.
84 PASSPHRASE CHARACTER SET: Some people have had difficulties with
85 this when upgrading distributions. It is highly advisable to only
86 use the 95 printable characters from the first 128 characters of
87 the ASCII table, as they will always have the same binary
88 representation. Other characters may have different encoding
89 depending on system configuration and your passphrase will not
90 work with a different encoding. A table of the standardized first
91 128 ASCII characters can, e.g. be found on
92 http://en.wikipedia.org/wiki/ASCII
95 * 1.3 System specific warnings
97 - Ubuntu as of 4/2011: It seems the installer offers to create
98 LUKS partitions in a way that several people mistook for an offer
99 to activate their existing LUKS partition. The installer gives no
100 or an inadequate warning and will destroy your old LUKS header,
101 causing permanent data loss. See also the section on Backup and
104 This issue has been acknowledged by the Ubuntu dev team, see here:
105 http://launchpad.net/bugs/420080
107 Update 7/2012: I am unsure whether this has been fixed by now, best
111 * 1.4 My LUKS-device is broken! Help!
113 First: Do not panic! In many cases the data is still recoverable.
114 Do not do anything hasty! Steps:
116 - Take some deep breaths. Maybe add some relaxing music. This may
117 sound funny, but I am completely serious. Often, critical damage is
118 done only after the initial problem.
120 - Do not reboot. The keys mays still be in the kernel if the device
123 - Make sure others do not reboot the system.
125 - Do not write to your disk without a clear understanding why this
126 will not make matters worse. Do a sector-level backup before any
127 writes. Often you do not need to write at all to get enough access
128 to make a backup of the data.
132 - Read section 6 of this FAQ.
134 - Ask on the mailing-list if you need more help.
137 * 1.5 Who wrote this?
139 Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
140 contributors are listed at the end. If you want to contribute, send
141 your article, including a descriptive headline, to the maintainer,
142 or the dm-crypt mailing list with something like "FAQ ..." in the
143 subject. You can also send more raw information and have me write
144 the section. Please note that by contributing to this FAQ, you
145 accept the license described below.
147 This work is under the "Attribution-Share Alike 3.0 Unported"
148 license, which means distribution is unlimited, you may create
149 derived works, but attributions to original authors and this
150 license statement must be retained and the derived work must be
151 under the same license. See
152 http://creativecommons.org/licenses/by-sa/3.0/ for more details of
155 Side note: I did text license research some time ago and I think
156 this license is best suited for the purpose at hand and creates the
160 * 1.5 Where is the project website?
162 There is the project website at http://code.google.com/p/cryptsetup/
163 Please do not post questions there, nobody will read them. Use
164 the mailing-list instead.
167 * 1.6 Is there a mailing-list?
169 Instructions on how to subscribe to the mailing-list are at on the
170 project website. People are generally helpful and friendly on the
173 The question of how to unsubscribe from the list does crop up
174 sometimes. For this you need your list management URL, which is
175 sent to you initially and once at the start of each month. Go to
176 the URL mentioned in the email and select "unsubscribe". This page
177 also allows you to request a password reminder.
179 Alternatively, you can send an Email to dm-crypt-request@saout.de
180 with just the word "help" in the subject or message body. Make sure
181 to send it from your list address.
183 The mailing list archive is here:
184 http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
187 * 1.7 Unsubscribe from the mailing-list
189 Send mail to dm-crypt-unsubscribe@saout.de from the subscribed
190 account. You will get an email with instructions.
192 Basically, you just have to respond to it unmodified to get
193 unsubscribed. The listserver admin functions are not very fast. It
194 can take 15 minutes or longer for a reply to arrive (I suspect
195 greylisting is in use), so be patient.
197 Also note that nobody on the list can unsubscribe you, sending
198 demands to be unsubscribed to the list just annoys people that are
199 entirely blameless for you being subscribed.
201 If you are subscribed, a subscription confirmation email was sent
202 to your email account and it had to be answered before the
203 subscription went active. The confirmation emails from the
204 listserver have subjects like these (with other numbers):
206 Subject: confirm 9964cf10.....
208 and are sent from dm-crypt-request@saout.de. You should check
209 whether you have anything like it in your sent email folder. If
210 you find nothing and are sure you did not confirm, then you should
211 look into a possible compromise of your email account.
217 * 2.1 LUKS Container Setup mini-HOWTO
219 This item tries to give you a very brief list of all the steps you
220 should go though when creating a new LUKS encrypted container, i.e.
221 encrypted disk, partition or loop-file.
223 01) All data will be lost, if there is data on the target, make a
226 02) Make very sure you have the right target disk, partition or
229 03) If the target was in use previously, it is a good idea to
230 wipe it before creating the LUKS container in order to remove any
231 trace of old file systems and data. For example, some users have
232 managed to run e2fsck on a partition containing a LUKS container,
233 possibly because of residual ext2 superblocks from an earlier use.
234 This can do arbitrary damage up to complete and permanent loss of
235 all data in the LUKS container.
237 To just quickly wipe file systems (old data may remain), use
239 wipefs -a <target device>
241 To wipe file system and data, use something like
243 cat /dev/zero > <target device>
245 This can take a while. To get a progress indicator, you can use
246 the tool dd_rescue (->google) instead or use my stream meter "wcs"
247 (source here: http://www.tansi.org/tools/index.html) in the
250 cat /dev/zero | wcs > <target device>
252 Be very sure you have the right target, all data will be lost!
254 Note that automatic wiping is on the TODO list for cryptsetup, so
255 at some time in the future this will become unnecessary.
257 04) Create the LUKS container:
258 cryptsetup luksFormat <target device>
260 Just follow the on-screen instructions.
262 05) Map the container. Here it will be mapped to /dev/mapper/c1:
263 cryptsetup luksOpen <target device> c1
265 06) (Optionally) wipe the container (make sure you have the right target!):
266 cat /dev/zero > /dev/mapper/c1
268 Note that this creates a small information leak, as an attacker can
269 determine whether a 512 byte block is zero if the attacker has
270 access to the encrypted container multiple times. Typically a
271 competent attacker that has access multiple times can install a
272 passphrase sniffer anyways, so this leakage is not very
273 significant. For getting a progress indicator, see step 03.
275 Note that at some time in the future, cryptsetup will do this for
276 you, but currently it is a TODO list item.
278 07) Create a file system in the mapped container, for example an
279 ext3 file system (any other file system is possible):
281 mke2fs -j /dev/mapper/c1
283 08) Mount your encrypted file system, here on /mnt:
284 mount /dev/mapper/c1 /mnt
286 Done. You can now use the encrypted file system to store data. Be
287 sure to read though the rest of the FAQ, these are just the very
288 basics. In particular, there are a number of mistakes that are
289 easy to make, but will compromise your security.
292 * 2.2 What is the difference between "plain" and LUKS format?
294 First, unless you happen to understand the cryptographic background
295 well, you should use LUKS. It does protect the user from a lot of
296 common mistakes. Plain dm-crypt is for experts.
298 Plain format is just that: It has no metadata on disk, reads all
299 parameters from the commandline (or the defaults), derives a
300 master-key from the passphrase and then uses that to de-/encrypt
301 the sectors of the device, with a direct 1:1 mapping between
302 encrypted and decrypted sectors.
304 Primary advantage is high resilience to damage, as one damaged
305 encrypted sector results in exactly one damaged decrypted sector.
306 Also, it is not readily apparent that there even is encrypted data
307 on the device, as an overwrite with crypto-grade randomness (e.g.
308 from /dev/urandom) looks exactly the same on disk.
310 Side-note: That has limited value against the authorities. In
311 civilized countries, they cannot force you to give up a crypto-key
312 anyways. In the US, the UK and dictatorships around the world,
313 they can force you to give up the keys (using imprisonment or worse
314 to pressure you), and in the worst case, they only need a
315 nebulous "suspicion" about the presence of encrypted data. My
316 advice is to either be ready to give up the keys or to not have
317 encrypted data when traveling to those countries, especially when
318 crossing the borders.
320 Disadvantages are that you do not have all the nice features that
321 the LUKS metadata offers, like multiple passphrases that can be
322 changed, the cipher being stored in the metadata, anti-forensic
323 properties like key-slot diffusion and salts, etc..
325 LUKS format uses a metadata header and 8 key-slot areas that are
326 being placed at the beginning of the disk, see below under "What
327 does the LUKS on-disk format looks like?". The passphrases are used
328 to decrypt a single master key that is stored in the anti-forensic
331 Advantages are a higher usability, automatic configuration of
332 non-default crypto parameters, defenses against low-entropy
333 passphrases like salting and iterated PBKDF2 passphrase hashing,
334 the ability to change passphrases, and others.
336 Disadvantages are that it is readily obvious there is encrypted
337 data on disk (but see side note above) and that damage to the
338 header or key-slots usually results in permanent data-loss. See
339 below under "6. Backup and Data Recovery" on how to reduce that
340 risk. Also the sector numbers get shifted by the length of the
341 header and key-slots and there is a loss of that size in capacity
342 (1MB+4096B for defaults and 2MB for the most commonly used
343 non-default XTS mode).
346 * 2.3 Can I encrypt an already existing, non-empty partition to use
349 There is no converter, and it is not really needed. The way to do
350 this is to make a backup of the device in question, securely wipe
351 the device (as LUKS device initialization does not clear away old
352 data), do a luksFormat, optionally overwrite the encrypted device,
353 create a new filesystem and restore your backup on the now
354 encrypted device. Also refer to sections "Security Aspects" and
355 "Backup and Data Recovery".
357 For backup, plain GNU tar works well and backs up anything likely
358 to be in a filesystem.
361 * 2.4 How do I use LUKS with a loop-device?
363 This can be very handy for experiments. Setup is just the same as
364 with any block device. If you want, for example, to use a 100MiB
365 file as LUKS container, do something like this:
367 head -c 100M /dev/zero > luksfile # create empty file
368 losetup /dev/loop0 luksfile # map luksfile to /dev/loop0
369 cryptsetup luksFormat /dev/loop0 # create LUKS on loop device
371 Afterwards just use /dev/loop0 as a you would use a LUKS partition.
372 To unmap the file when done, use "losetup -d /dev/loop0".
375 * 2.5 When I add a new key-slot to LUKS, it asks for a passphrase but
376 then complains about there not being a key-slot with that
379 That is as intended. You are asked a passphrase of an existing
380 key-slot first, before you can enter the passphrase for the new
381 key-slot. Otherwise you could break the encryption by just adding a
382 new key-slot. This way, you have to know the passphrase of one of
383 the already configured key-slots in order to be able to configure a
387 * 2.6 Encryption on top of RAID or the other way round?
389 Unless you have special needs, place encryption between RAID and
390 filesystem, i.e. encryption on top of RAID. You can do it the other
391 way round, but you have to be aware that you then need to give the
392 passphrase for each individual disk and RAID autodetection will
393 not work anymore. Therefore it is better to encrypt the RAID
394 device, e.g. /dev/dm0 .
397 * 2.7 How do I read a dm-crypt key from file?
399 Note that the file will still be hashed first, just like keyboard
400 input. Use the --key-file option, like this:
402 cryptsetup create --key-file keyfile e1 /dev/loop0
405 * 2.8 How do I read a LUKS slot key from file?
407 What you really do here is to read a passphrase from file, just as
408 you would with manual entry of a passphrase for a key-slot. You can
409 add a new passphrase to a free key-slot, set the passphrase of an
410 specific key-slot or put an already configured passphrase into a
411 file. In the last case make sure no trailing newline (0x0a) is
412 contained in the key file, or the passphrase will not work because
413 the whole file is used as input.
415 To add a new passphrase to a free key slot from file, use something
418 cryptsetup luksAddKey /dev/loop0 keyfile
420 To add a new passphrase to a specific key-slot, use something like
423 cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
425 To supply a key from file to any LUKS command, use the --key-file
426 option, e.g. like this:
428 cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
431 * 2.9 How do I read the LUKS master key from file?
433 The question you should ask yourself first is why you would want to
434 do this. The only legitimate reason I can think of is if you want
435 to have two LUKS devices with the same master key. Even then, I
436 think it would be preferable to just use key-slots with the same
437 passphrase, or to use plain dm-crypt instead. If you really have a
438 good reason, please tell me. If I am convinced, I will add how to
442 * 2.10 What are the security requirements for a key read from file?
444 A file-stored key or passphrase has the same security requirements
445 as one entered interactively, however you can use random bytes and
446 thereby use bytes you cannot type on the keyboard. You can use any
447 file you like as key file, for example a plain text file with a
448 human readable passphrase. To generate a file with random bytes,
449 use something like this:
451 head -c 256 /dev/random > keyfile
454 * 2.11 If I map a journaled file system using dm-crypt/LUKS, does it
455 still provide its usual transactional guarantees?
457 Yes, it does, unless a very old kernel is used. The required flags
458 come from the filesystem layer and are processed and passed onwards
459 by dm-crypt. A bit more information on the process by which
460 transactional guarantees are implemented can be found here:
462 http://lwn.net/Articles/400541/
464 Please note that these "guarantees" are weaker than they appear to
465 be. One problem is that quite a few disks lie to the OS about
466 having flushed their buffers. Some other things can go wrong as
467 well. The filesystem developers are aware of these problems and
468 typically can make it work anyways. That said, dm-crypt/LUKS will
469 not make things worse.
471 One specific problem you can run into though is that you can get
472 short freezes and other slowdowns due to the encryption layer.
473 Encryption takes time and forced flushes will block for that time.
474 For example, I did run into frequent small freezes (1-2 sec) when
475 putting a vmware image on ext3 over dm-crypt. When I went back to
476 ext2, the problem went away. This seems to have gotten better with
477 kernel 2.6.36 and the reworking of filesystem flush locking
478 mechanism (less blocking of CPU activity during flushes). It
479 should improve further and eventually the problem should go away.
482 * 2.12 Can I use LUKS or cryptsetup with a more secure (external)
483 medium for key storage, e.g. TPM or a smartcard?
485 Yes, see the answers on using a file-supplied key. You do have to
486 write the glue-logic yourself though. Basically you can have
487 cryptsetup read the key from STDIN and write it there with your
488 own tool that in turn gets the key from the more secure key
492 * 2.13 Can I resize a dm-crypt or LUKS partition?
494 Yes, you can, as neither dm-crypt nor LUKS stores partition size.
495 Whether you should is a different question. Personally I recommend
496 backup, recreation of the encrypted partition with new size,
497 recreation of the filesystem and restore. This gets around the
498 tricky business of resizing the filesystem. Resizing a dm-crypt or
499 LUKS container does not resize the filesystem in it. The backup is
500 really non-optional here, as a lot can go wrong, resulting in
501 partial or complete data loss. Using something like gparted to
502 resize an encrypted partition is slow, but typically works. This
503 will not change the size of the filesystem hidden under the
506 You also need to be aware of size-based limitations. The one
507 currently relevant is that aes-xts-plain should not be used for
508 encrypted container sizes larger than 2TiB. Use aes-xts-plain64
512 * 2.14 How do I Benchmark the Ciphers, Hashes and Modes?
514 Since version 1.60 cryptsetup supports the "benchmark" command.
519 It will output first iterations/second for the key-derivation
520 function PBKDF2 parameterized with different hash-functions, and
521 then the raw encryption speed of ciphers with different modes and
522 key-sizes. You can get more than the default benchmarks, see the
523 man-page for the relevant parameters. Note that XTS mode takes two
524 keys, hence the listed key sizes are double that for other modes
525 and half of it is the cipher key, the other half is the XTS key.
531 * 3.1 My dm-crypt/LUKS mapping does not work! What general steps are
532 there to investigate the problem?
534 If you get a specific error message, investigate what it claims
535 first. If not, you may want to check the following things.
537 - Check that "/dev", including "/dev/mapper/control" is there. If it
538 is missing, you may have a problem with the "/dev" tree itself or
539 you may have broken udev rules.
541 - Check that you have the device mapper and the crypt target in your
542 kernel. The output of "dmsetup targets" should list a "crypt"
543 target. If it is not there or the command fails, add device mapper
544 and crypt-target to the kernel.
546 - Check that the hash-functions and ciphers you want to use are in
547 the kernel. The output of "cat /proc/crypto" needs to list them.
550 * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
552 The default cipher, hash or mode may have changed (the mode changed
553 from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
557 * 3.3 When I call cryptsetup from cron/CGI, I get errors about
560 If you get errors about unknown parameters or the like that are not
561 present when cryptsetup is called from the shell, make sure you
562 have no older version of cryptsetup on your system that then gets
563 called by cron/CGI. For example some distributions install
564 cryptsetup into /usr/sbin, while a manual install could go to
565 /usr/local/sbin. As a debugging aid, call "cryptsetup --version"
566 from cron/CGI or the non-shell mechanism to be sure the right
570 * 3.4 Unlocking a LUKS device takes very long. Why?
572 The iteration time for a key-slot (see Section 5 for an explanation
573 what iteration does) is calculated when setting a passphrase. By
574 default it is 1 second on the machine where the passphrase is set.
575 If you set a passphrase on a fast machine and then unlock it on a
576 slow machine, the unlocking time can be much longer. Also take into
577 account that up to 8 key-slots have to be tried in order to find the
580 If this is problem, you can add another key-slot using the slow
581 machine with the same passphrase and then remove the old key-slot.
582 The new key-slot will have an iteration count adjusted to 1 second
583 on the slow machine. Use luksKeyAdd and then luksKillSlot or
586 However, this operation will not change volume key iteration count
587 (MK iterations in output of "cryptsetup luksDump"). In order to
588 change that, you will have to backup the data in the LUKS
589 container (i.e. your encrypted data), luksFormat on the slow
590 machine and restore the data. Note that in the original LUKS
591 specification this value was fixed to 10, but it is now derived
592 from the PBKDF2 benchmark as well and set to iterations in 0.125
593 sec or 1000, whichever is larger. Also note that MK iterations
594 are not very security relevant. But as each key-slot already takes
595 1 second, spending the additional 0.125 seconds really does not
599 * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
600 device. What is wrong?
602 Some old versions of cryptsetup have a bug where the header does
603 not get completely wiped during LUKS format and an older ext2/swap
604 signature remains on the device. This confuses blkid.
606 Fix: Wipe the unused header areas by doing a backup and restore of
607 the header with cryptsetup 1.1.x:
609 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
610 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
613 * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
615 There seems to be some interference between the hardening and and
616 the way cryptsetup benchmarks PBKDF2. The solution to this is
617 currently not quite clear for an encrypted root filesystem. For
618 other uses, you can apparently specify USE="dynamic" as compile
619 flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470
625 * 4.1 I get the error "LUKS keyslot x is invalid." What does that
628 This means that the given keyslot has an offset that points
629 outside the valid keyslot area. Typically, the reason is a
630 corrupted LUKS header because something was written to the start of
631 the device the LUKS container is on. Refer to Section "Backup and
632 Data Recovery" and ask on the mailing list if you have trouble
633 diagnosing and (if still possible) repairing this.
636 * 4.2 Can a bad RAM module cause problems?
638 LUKS and dm-crypt can give the RAM quite a workout, especially when
639 combined with software RAID. In particular the combination RAID5 +
640 LUKS + XFS seems to uncover RAM problems that never caused obvious
641 problems before. Symptoms vary, but often the problem manifest
642 itself when copying large amounts of data, typically several times
643 larger than your main memory.
645 Side note: One thing you should always do on large data
646 copy/movements is to run a verify, for example with the "-d"
647 option of "tar" or by doing a set of MD5 checksums on the source
650 find . -type f -exec md5sum \{\} \; > checksum-file
652 and then a "md5sum -c checksum-file" on the other side. If you get
653 mismatches here, RAM is the primary suspect. A lesser suspect is
654 an overclocked CPU. I have found countless hardware problems in
655 verify runs after copying or making backups. Bit errors are much
656 more common than most people think.
658 Some RAM issues are even worse and corrupt structures in one of the
659 layers. This typically results in lockups, CPU state dumps in the
660 system logs, kernel panic or other things. It is quite possible to
661 have the problem with an encrypted device, but not with an
662 otherwise the same unencrypted device. The reason for that is that
663 encryption has an error amplification property: You flip one bit
664 in an encrypted data block, and the decrypted version has half of
665 its bits flipped. This is an important security property for modern
666 ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
667 get up to a completely changed 512 byte block per bit error. A
668 corrupt block causes a lot more havoc than the occasionally
669 flipped single bit and can result in various obscure errors.
671 Note, that a verify run on copying between encrypted or
672 unencrypted devices will reliably detect corruption, even when the
673 copying itself did not report any problems. If you find defect
674 RAM, assume all backups and copied data to be suspect, unless you
678 * 4.3 How do I test RAM?
680 First you should know that overclocking often makes memory
681 problems worse. So if you overclock (which I strongly recommend
682 against in a system holding data that has some worth), run the
683 tests with the overclocking active.
685 There are two good options. One is Memtest86+ and the other is
686 "memtester" by Charles Cazabon. Memtest86+ requires a reboot and
687 then takes over the machine, while memtester runs from a
688 root-shell. Both use different testing methods and I have found
689 problems fast with each one that the other needed long to find. I
690 recommend running the following procedure until the first error is
693 - Run Memtest86+ for one cycle
695 - Run memtester for one cycle (shut down as many other applications
698 - Run Memtest86+ for 24h or more
700 - Run memtester for 24h or more
702 If all that does not produce error messages, your RAM may be sound,
703 but I have had one weak bit that Memtest86+ needed around 60 hours
704 to find. If you can reproduce the original problem reliably, a good
705 additional test may be to remove half of the RAM (if you have more
706 than one module) and try whether the problem is still there and if
707 so, try with the other half. If you just have one module, get a
708 different one and try with that. If you do overclocking, reduce
709 the settings to the most conservative ones available and try with
716 * 5.1 How long is a secure passphrase ?
718 This is just the short answer. For more info and explanation of
719 some of the terms used in this item, read the rest of Section 5.
720 The actual recommendation is at the end of this item.
722 First, passphrase length is not really the right measure,
723 passphrase entropy is. For example, a random lowercase letter (a-z)
724 gives you 4.7 bit of entropy, one element of a-z0-9 gives you 5.2
725 bits of entropy, an element of a-zA-Z0-9 gives you 5.9 bits and
726 a-zA-Z0-9!@#$%^&:-+ gives you 6.2 bits. On the other hand, a random
727 English word only gives you 0.6...1.3 bits of entropy per
728 character. Using sentences that make sense gives lower entropy,
729 series of random words gives higher entropy. Do not use sentences
730 that can be tied to you or found on your computer. This type of
731 attack is done routinely today.
733 That said, it does not matter too much what scheme you use, but it
734 does matter how much entropy your passphrase contains, because an
735 attacker has to try on average
737 1/2 * 2^(bits of entropy in passphrase)
739 different passphrases to guess correctly.
741 Historically, estimations tended to use computing time estimates,
742 but more modern approaches try to estimate cost of guessing a
745 As an example, I will try to get an estimate from the numbers in
746 http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
747 More references can be found a the end of this document. Note that
748 these are estimates from the defender side, so assuming something
749 is easier than it actually is is fine. An attacker may still have
750 vastly higher cost than estimated here.
752 LUKS uses SHA1 for hasing per default. The claim in the reference is
753 63 billion tries/second for SHA1. We will leave aside the check
754 whether a try actually decrypts a key-slot. Now, the machine has 25
755 GPUs, which I will estimate at an overall lifetime cost of USD/EUR
756 1000 each, and an useful lifetime of 2 years. (This is on the low
757 side.) Disregarding downtime, the machine can then break
759 N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
761 passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed
762 once with SHA1 for EUR/USD 25k. Note that as this can be
763 parallelized, it can be done faster than 2 years with several of
766 For plain dm-crypt (no hash iteration) this is it. This gives (with
767 SHA1, plain dm-crypt default is ripemd160 which seems to be
768 slightly slower than SHA1):
770 Passphrase entropy Cost to break
779 For LUKS, you have to take into account hash iteration in PBKDF2.
780 For a current CPU, there are about 100k iterations (as can be
781 queried with ''cryptsetup luksDump''.
783 The table above then becomes:
785 Passphrase entropy Cost to break
796 To get reasonable security for the next 10 years, it is a good idea
797 to overestimate by a factor of at least 1000.
799 Then there is the question of how much the attacker is willing to
800 spend. That is up to your own security evaluation. For general use,
801 I will assume the attacker is willing to spend up to 1 million
802 EUR/USD. Then we get the following recommendations:
804 Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
805 or a random English sentence of > 135 characters length.
807 LUKS: Use > 65 bit. That is e.g. 14 random chars from a-z or a
808 random English sentence of > 108 characters length.
810 If paranoid, add at least 20 bit. That is roughly four additional
811 characters for random passphrases and roughly 32 characters for a
812 random English sentence.
815 * 5.2 Is LUKS insecure? Everybody can see I have encrypted data!
817 In practice it does not really matter. In most civilized countries
818 you can just refuse to hand over the keys, no harm done. In some
819 countries they can force you to hand over the keys, if they suspect
820 encryption. However the suspicion is enough, they do not have to
821 prove anything. This is for practical reasons, as even the presence
822 of a header (like the LUKS header) is not enough to prove that you
823 have any keys. It might have been an experiment, for example. Or it
824 was used as encrypted swap with a key from /dev/random. So they
825 make you prove you do not have encrypted data. Of course that is
826 just as impossible as the other way round.
828 This means that if you have a large set of random-looking data,
829 they can already lock you up. Hidden containers (encryption hidden
830 within encryption), as possible with Truecrypt, do not help
831 either. They will just assume the hidden container is there and
832 unless you hand over the key, you will stay locked up. Don't have
833 a hidden container? Though luck. Anybody could claim that.
835 Still, if you are concerned about the LUKS header, use plain
836 dm-crypt with a good passphrase. See also Section 2, "What is the
837 difference between "plain" and LUKS format?"
840 * 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
842 If you just create a filesystem on it, most of the old data will
843 still be there. If the old data is sensitive, you should overwrite
844 it before encrypting. In any case, not initializing will leave the
845 old data there until the specific sector gets written. That may
846 enable an attacker to determine how much and where on the
847 partition data was written. If you think this is a risk, you can
848 prevent this by overwriting the encrypted device (here assumed to
849 be named "e1") with zeros like this:
851 dd_rescue -w /dev/zero /dev/mapper/e1
853 or alternatively with one of the following more standard commands:
855 cat /dev/zero > /dev/mapper/e1
856 dd if=/dev/zero of=/dev/mapper/e1
859 * 5.4 How do I securely erase a LUKS (or other) partition?
861 For LUKS, if you are in a desperate hurry, overwrite the LUKS
862 header and key-slot area. This means overwriting the first
863 (keyslots x stripes x keysize) + offset bytes. For the default
864 parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the
865 LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
866 512 bit key) this is 2MiB. (The different offset stems from
867 differences in the sector alignment of the key-slots.) If in doubt,
868 just be generous and overwrite the first 10MB or so, it will likely
869 still be fast enough. A single overwrite with zeros should be
870 enough. If you anticipate being in a desperate hurry, prepare the
871 command beforehand. Example with /dev/sde1 as the LUKS partition
872 and default parameters:
874 head -c 1052672 /dev/zero > /dev/sde1; sync
876 A LUKS header backup or full backup will still grant access to
877 most or all data, so make sure that an attacker does not have
878 access to backups or destroy them as well.
880 If you have time, overwrite the whole LUKS partition with a single
881 pass of zeros. This is enough for current HDDs. For SSDs or FLASH
882 (USB sticks) you may want to overwrite the whole drive several
883 times to be sure data is not retained by wear leveling. This is
884 possibly still insecure as SSD technology is not fully understood
885 in this regard. Still, due to the anti-forensic properties of the
886 LUKS key-slots, a single overwrite of an SSD or FLASH drive could
887 be enough. If in doubt, use physical destruction in addition. Here
888 is a link to some current research results on erasing SSDs and
890 http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
892 Keep in mind to also erase all backups.
894 Example for a zero-overwrite erase of partition sde1 done with
897 dd_rescue -w /dev/zero /dev/sde1
900 * 5.5 How do I securely erase a backup of a LUKS partition or header?
902 That depends on the medium it is stored on. For HDD and SSD, use
903 overwrite with zeros. For an SSD or FLASH drive (USB stick), you
904 may want to overwrite the complete SSD several times and use
905 physical destruction in addition, see last item. For re-writable
906 CD/DVD, a single overwrite should also be enough, due to the
907 anti-forensic properties of the LUKS keyslots. For write-once
908 media, use physical destruction. For low security requirements,
909 just cut the CD/DVD into several parts. For high security needs,
910 shred or burn the medium. If your backup is on magnetic tape, I
911 advise physical destruction by shredding or burning, after
912 overwriting . The problem with magnetic tape is that it has a
913 higher dynamic range than HDDs and older data may well be
914 recoverable after overwrites. Also write-head alignment issues can
915 lead to data not actually being deleted at all during overwrites.
918 * 5.6 What about backup? Does it compromise security?
920 That depends. See item 6.7.
923 * 5.7 Why is all my data permanently gone if I overwrite the LUKS
926 Overwriting the LUKS header in part or in full is the most common
927 reason why access to LUKS containers is lost permanently.
928 Overwriting can be done in a number of fashions, like creating a
929 new filesystem on the raw LUKS partition, making the raw partition
930 part of a raid array and just writing to the raw partition.
932 The LUKS header contains a 256 bit "salt" value and without that no
933 decryption is possible. While the salt is not secret, it is
934 key-grade material and cannot be reconstructed. This is a
935 cryptographically strong "cannot". From observations on the
936 cryptsetup mailing-list, people typically go though the usual
937 stages of grief (Denial, Anger, Bargaining, Depression, Acceptance)
938 when this happens to them. Observed times vary between 1 day and 2
939 weeks to complete the cycle. Seeking help on the mailing-list is
940 fine. Even if we usually cannot help with getting back your data,
941 most people found the feedback comforting.
943 If your header does not contain an intact salt, best go directly
944 to the last stage ("Acceptance") and think about what to do now.
945 There is one exception that I know of: If your LUKS container is
946 still open, then it may be possible to extract the master key from
947 the running system. See Item "How do I recover the master key from
948 a mapped LUKS container?" in Section "Backup and Data Recovery".
951 * 5.8 What is a "salt"?
953 A salt is a random key-grade value added to the passphrase before
954 it is processed. It is not kept secret. The reason for using salts
955 is as follows: If an attacker wants to crack the password for a
956 single LUKS container, then every possible passphrase has to be
957 tried. Typically an attacker will not try every binary value, but
958 will try words and sentences from a dictionary.
960 If an attacker wants to attack several LUKS containers with the
961 same dictionary, then a different approach makes sense: Compute the
962 resulting slot-key for each dictionary element and store it on
963 disk. Then the test for each entry is just the slow unlocking with
964 the slot key (say 0.00001 sec) instead of calculating the slot-key
965 first (1 sec). For a single attack, this does not help. But if you
966 have more than one container to attack, this helps tremendously,
967 also because you can prepare your table before you even have the
968 container to attack! The calculation is also very simple to
969 parallelize. You could, for example, use the night-time unused CPU
970 power of your desktop PCs for this.
972 This is where the salt comes in. If the salt is combined with the
973 passphrase (in the simplest form, just appended to it), you
974 suddenly need a separate table for each salt value. With a
975 reasonably-sized salt value (256 bit, e.g.) this is quite
979 * 5.9 Is LUKS secure with a low-entropy (bad) passphrase?
981 Note: You should only use the 94 printable characters from 7 bit
982 ASCII code to prevent your passphrase from failing when the
983 character encoding changes, e.g. because of a system upgrade, see
984 also the note at the very start of this FAQ under "WARNINGS".
986 This needs a bit of theory. The quality of your passphrase is
987 directly related to its entropy (information theoretic, not
988 thermodynamic). The entropy says how many bits of "uncertainty" or
989 "randomness" are in you passphrase. In other words, that is how
990 difficult guessing the passphrase is.
992 Example: A random English sentence has about 1 bit of entropy per
993 character. A random lowercase (or uppercase) character has about
996 Now, if n is the number of bits of entropy in your passphrase and t
997 is the time it takes to process a passphrase in order to open the
998 LUKS container, then an attacker has to spend at maximum
1000 attack_time_max = 2^n * t
1002 time for a successful attack and on average half that. There is no
1003 way getting around that relationship. However, there is one thing
1004 that does help, namely increasing t, the time it takes to use a
1005 passphrase, see next FAQ item.
1007 Still, if you want good security, a high-entropy passphrase is the
1008 only option. For example, a low-entropy passphrase can never be
1009 considered secure against a TLA-level (Three Letter Agency level,
1010 i.e. government-level) attacker, no matter what tricks are used in
1011 the key-derivation function. Use at least 64 bits for secret stuff.
1012 That is 64 characters of English text (but only if randomly chosen)
1013 or a combination of 12 truly random letters and digits.
1015 For passphrase generation, do not use lines from very well-known
1016 texts (religious texts, Harry potter, etc.) as they are to easy to
1017 guess. For example, the total Harry Potter has about 1'500'000
1018 words (my estimation). Trying every 64 character sequence starting
1019 and ending at a word boundary would take only something like 20
1020 days on a single CPU and is entirely feasible. To put that into
1021 perspective, using a number of Amazon EC2 High-CPU Extra Large
1022 instances (each gives about 8 real cores), this test costs
1023 currently about 50USD/EUR, but can be made to run arbitrarily fast.
1025 On the other hand, choosing 1.5 lines from, say, the Wheel of Time
1026 is in itself not more secure, but the book selection adds quite a
1027 bit of entropy. (Now that I have mentioned it here, don't use tWoT
1028 either!) If you add 2 or 3 typos or switch some words around, then
1029 this is good passphrase material.
1032 * 5.10 What is "iteration count" and why is decreasing it a bad idea?
1034 Iteration count is the number of PBKDF2 iterations a passphrase is
1035 put through before it is used to unlock a key-slot. Iterations are
1036 done with the explicit purpose to increase the time that it takes
1037 to unlock a key-slot. This provides some protection against use of
1038 low-entropy passphrases.
1040 The idea is that an attacker has to try all possible passphrases.
1041 Even if the attacker knows the passphrase is low-entropy (see last
1042 item), it is possible to make each individual try take longer. The
1043 way to do this is to repeatedly hash the passphrase for a certain
1044 time. The attacker then has to spend the same time (given the same
1045 computing power) as the user per try. With LUKS, the default is 1
1046 second of PBKDF2 hashing.
1048 Example 1: Lets assume we have a really bad passphrase (e.g. a
1049 girlfriends name) with 10 bits of entropy. With the same CPU, an
1050 attacker would need to spend around 500 seconds on average to
1051 break that passphrase. Without iteration, it would be more like
1052 0.0001 seconds on a modern CPU.
1054 Example 2: The user did a bit better and has 32 chars of English
1055 text. That would be about 32 bits of entropy. With 1 second
1056 iteration, that means an attacker on the same CPU needs around 136
1057 years. That is pretty impressive for such a weak passphrase.
1058 Without the iterations, it would be more like 50 days on a modern
1059 CPU, and possibly far less.
1061 In addition, the attacker can both parallelize and use special
1062 hardware like GPUs or FPGAs to speed up the attack. The attack can
1063 also happen quite some time after the luksFormat operation and CPUs
1064 can have become faster and cheaper. For that reason you want a
1065 bit of extra security. Anyways, in Example 1 your are screwed.
1066 In example 2, not necessarily. Even if the attack is faster, it
1067 still has a certain cost associated with it, say 10000 EUR/USD
1068 with iteration and 1 EUR/USD without iteration. The first can be
1069 prohibitively expensive, while the second is something you try
1070 even without solid proof that the decryption will yield something
1073 The numbers above are mostly made up, but show the idea. Of course
1074 the best thing is to have a high-entropy passphrase.
1076 Would a 100 sec iteration time be even better? Yes and no.
1077 Cryptographically it would be a lot better, namely 100 times better.
1078 However, usability is a very important factor for security
1079 technology and one that gets overlooked surprisingly often. For
1080 LUKS, if you have to wait 2 minutes to unlock the LUKS container,
1081 most people will not bother and use less secure storage instead. It
1082 is better to have less protection against low-entropy passphrases
1083 and people actually use LUKS, than having them do without
1084 encryption altogether.
1086 Now, what about decreasing the iteration time? This is generally a
1087 very bad idea, unless you know and can enforce that the users only
1088 use high-entropy passphrases. If you decrease the iteration time
1089 without ensuring that, then you put your users at increased risk,
1090 and considering how rarely LUKS containers are unlocked in a
1091 typical work-flow, you do so without a good reason. Don't do it.
1092 The iteration time is already low enough that users with entropy
1093 low passphrases are vulnerable. Lowering it even further increases
1094 this danger significantly.
1097 * 5.11 Some people say PBKDF2 is insecure?
1099 There is some discussion that a hash-function should have a "large
1100 memory" property, i.e. that it should require a lot of memory to be
1101 computed. This serves to prevent attacks using special programmable
1102 circuits, like FPGAs, and attacks using graphics cards. PBKDF2
1103 does not need a lot of memory and is vulnerable to these attacks.
1104 However, the publication usually referred in these discussions is
1105 not very convincing in proving that the presented hash really is
1106 "large memory" (that may change, email the FAQ maintainer when it
1107 does) and it is of limited usefulness anyways. Attackers that use
1108 clusters of normal PCs will not be affected at all by a "large
1109 memory" property. For example the US Secret Service is known to
1110 use the off-hour time of all the office PCs of the Treasury for
1111 password breaking. The Treasury has about 110'000 employees.
1112 Assuming every one has an office PC, that is significant computing
1113 power, all of it with plenty of memory for computing "large
1114 memory" hashes. Bot-net operators also have all the memory they
1115 want. The only protection against a resourceful attacker is a
1116 high-entropy passphrase, see items 5.9 and 5.10.
1119 * 5.12 What about iteration count with plain dm-crypt?
1121 Simple: There is none. There is also no salting. If you use plain
1122 dm-crypt, the only way to be secure is to use a high entropy
1123 passphrase. If in doubt, use LUKS instead.
1126 * 5.13 Is LUKS with default parameters less secure on a slow CPU?
1128 Unfortunately, yes. However the only aspect affected is the
1129 protection for low-entropy passphrase or master-key. All other
1130 security aspects are independent of CPU speed.
1132 The master key is less critical, as you really have to work at it
1133 to give it low entropy. One possibility is to supply the master key
1134 yourself. If that key is low-entropy, then you get what you
1135 deserve. The other known possibility is to use /dev/urandom for
1136 key generation in an entropy-starved situation (e.g. automatic
1137 installation on an embedded device without network and other entropy
1140 For the passphrase, don't use a low-entropy passphrase. If your
1141 passphrase is good, then a slow CPU will not matter. If you insist
1142 on a low-entropy passphrase on a slow CPU, use something like
1143 "--iter-time=10" or higher and wait a long time on each LUKS unlock
1144 and pray that the attacker does not find out in which way exactly
1145 your passphrase is low entropy. This also applies to low-entropy
1146 passphrases on fast CPUs. Technology can do only so much to
1147 compensate for problems in front of the keyboard.
1150 * 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
1152 Note: This item applies both to plain dm-crypt and to LUKS
1154 The problem is that cbc-plain has a fingerprint vulnerability, where
1155 a specially crafted file placed into the crypto-container can be
1156 recognized from the outside. The issue here is that for cbc-plain
1157 the initialization vector (IV) is the sector number. The IV gets
1158 XORed to the first data chunk of the sector to be encrypted. If you
1159 make sure that the first data block to be stored in a sector
1160 contains the sector number as well, the first data block to be
1161 encrypted is all zeros and always encrypted to the same ciphertext.
1162 This also works if the first data chunk just has a constant XOR
1163 with the sector number. By having several shifted patterns you can
1164 take care of the case of a non-power-of-two start sector number of
1167 This mechanism allows you to create a pattern of sectors that have
1168 the same first ciphertext block and signal one bit per sector to the
1169 outside, allowing you to e.g. mark media files that way for
1170 recognition without decryption. For large files this is a
1171 practical attack. For small ones, you do not have enough blocks to
1172 signal and take care of different file starting offsets.
1174 In order to prevent this attack, the default was changed to
1175 cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
1176 encryption key as key. This makes the IV unpredictable without
1177 knowing the encryption key and the watermarking attack fails.
1180 * 5.15 Are there any problems with "plain" IV? What is "plain64"?
1182 First, "plain" and "plain64" are both not secure to use with CBC,
1183 see previous FAQ item.
1185 However there are modes, like XTS, that are secure with "plain" IV.
1186 The next limit is that "plain" is 64 bit, with the upper 32 bit set
1187 to zero. This means that on volumes larger than 2TiB, the IV
1188 repeats, creating a vulnerability that potentially leaks some
1189 data. To avoid this, use "plain64", which uses the full sector
1190 number up to 64 bit. Note that "plain64" requires a kernel >=
1191 2.6.33. Also note that "plain64" is backwards compatible for
1192 volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
1193 does not cause any performance penalty compared to "plain".
1196 * 5.16 What about XTS mode?
1198 XTS mode is potentially even more secure than cbc-essiv (but only if
1199 cbc-essiv is insecure in your scenario). It is a NIST standard and
1200 used, e.g. in Truecrypt. At the moment, if you want to use it, you
1201 have to specify it manually as "aes-xts-plain", i.e.
1203 cryptsetup -c aes-xts-plain luksFormat <device>
1205 For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ
1206 item on "plain" and "plain64"):
1208 cryptsetup -c aes-xts-plain64 luksFormat <device>
1210 There is a potential security issue with XTS mode and large blocks.
1211 LUKS and dm-crypt always use 512B blocks and the issue does not
1215 * 5.17 Is LUKS FIPS-140-2 certified?
1217 No. But that is more a problem of FIPS-140-2 than of LUKS. From a
1218 technical point-of-view, LUKS with the right parameters would be
1219 FIPS-140-2 compliant, but in order to make it certified, somebody
1220 has to pay real money for that. And then, whenever cryptsetup is
1221 changed or extended, the certification lapses and has to be
1224 From the aspect of actual security, LUKS with default parameters
1225 should be as good as most things that are FIPS-140-2 certified,
1226 although you may want to make sure to use /dev/random (by
1227 specifying --use-random on luksFormat) as randomness source for
1228 the master key to avoid being potentially insecure in an
1229 entropy-starved situation.
1232 * 5.18 What about Plausible Deniability?
1234 First let me attempt a definition for the case of encrypted
1235 filesystems: Plausible deniability is when you hide encrypted data
1236 inside an encrypted container and it is not possible to prove it is
1237 there. The idea is compelling and on first glance it seems
1238 possible to do it. And from a cryptographic point of view, it
1239 actually is possible.
1241 So, does it work in practice? No, unfortunately. The reasoning used
1242 by its proponents is fundamentally flawed in several ways and the
1243 cryptographic properties fail fatally when colliding with the real
1246 First, why should "I do not have a hidden partition" be any more
1247 plausible than "I forgot my crypto key" or "I wiped that partition
1248 with random data, nothing in there"? I do not see any reason.
1250 Second, there are two types of situations: Either they cannot force
1251 you to give them the key (then you simply do not) or the can. In
1252 the second case, they can always do bad things to you, because they
1253 cannot prove that you have the key in the first place! This means
1254 they do not have to prove you have the key, or that this random
1255 looking data on your disk is actually encrypted data. So the
1256 situation will allow them to waterboard/lock-up/deport you
1257 anyways, regardless of how "plausible" your deniability is. Do not
1258 have a hidden partition you could show to them, but there are
1259 indications you may? Too bad for you. Unfortunately "plausible
1260 deniability" also means you cannot prove there is no hidden data.
1262 Third, hidden partitions are not that hidden. There are basically
1263 just two possibilities: a) Make a large crypto container, but put a
1264 smaller filesystem in there and put the hidden partition into the
1265 free space. Unfortunately this is glaringly obvious and can be
1266 detected in an automated fashion. This means that the initial
1267 suspicion to put you under duress in order to make you reveal you
1268 hidden data is given. b) Make a filesystem that spans the whole
1269 encrypted partition, and put the hidden partition into space not
1270 currently used by that filesystem. Unfortunately that is also
1271 glaringly obvious, as you then cannot write to the filesystem
1272 without a high risk of destroying data in the hidden container.
1273 Have not written anything to the encrypted filesystem in a while?
1274 Too bad, they have the suspicion they need to do unpleasant things
1277 To be fair, if you prepare option b) carefully and directly before
1278 going into danger, it may work. But then, the mere presence of
1279 encrypted data may already be enough to get you into trouble in
1280 those places were they can demand encryption keys.
1282 Here is an additional reference for some problems with plausible
1283 deniability: http://www.schneier.com/paper-truecrypt-dfs.pdf I
1284 strongly suggest you read it.
1286 So, no, I will not provide any instructions on how to do it with
1287 plain dm-crypt or LUKS. If you insist on shooting yourself in the
1288 foot, you can figure out how to do it yourself.
1291 * 5.19 What about SSDs or Flash Drives?
1293 The problem is that you cannot reliably erase parts of these
1294 devices, mainly due to wear-leveling and possibly defect
1297 Basically, when overwriting a sector (of 512B), what the device
1298 does is to move an internal sector (may be 128kB or even larger) to
1299 some pool of discarded, not-yet erased unused sectors, take a
1300 fresh empty sector from the empty-sector pool and copy the old
1301 sector over with the changes to the small part you wrote. This is
1302 done in some fashion so that larger writes do not cause a lot of
1303 small internal updates.
1305 The thing is that the mappings between outside-adressable sectors
1306 and inside sectors is arbitrary (and the vendors are not talking).
1307 Also the discarded sectors are not necessarily erased immediately.
1308 They may linger a long time.
1310 For plain dm-crypt, the consequences are that older encrypted data
1311 may be lying around in some internal pools of the device. Thus may
1312 or may not be a problem and depends on the application. Remember
1313 the same can happen with a filesystem if consecutive writes to the
1314 same area of a file can go to different sectors.
1316 However, for LUKS, the worst case is that key-slots and LUKS
1317 header may end up in these internal pools. This means that password
1318 management functionality is compromised (the old passwords may
1319 still be around, potentially for a very long time) and that fast
1320 erase by overwriting the header and key-slot area is insecure.
1322 Also keep in mind that the discarded/used pool may be large. For
1323 example, a 240GB SSD has about 16GB of spare area in the chips that
1324 it is free to do with as it likes. You would need to make each
1325 individual key-slot larger than that to allow reliable overwriting.
1326 And that assumes the disk thinks all other space is in use.
1327 Reading the internal pools using forensic tools is not that hard,
1328 but may involve some soldering.
1332 If you trust the device vendor (you probably should not...) you can
1333 try an ATA "secure erase" command for SSDs. That does not work for
1334 USB keys though. And if it finishes after a few seconds, it was
1335 possibly faked by the SSD.
1337 If you can do without password management and are fine with doing
1338 physical destruction for permenently deleting data (allways after
1339 one or several full overwrites!), you can use plain dm-crypt or
1342 If you want or need the original LUKS security features to work,
1343 you can use a detached LUKS header and put that on a conventional,
1344 magnetic disk. That leaves potentially old encrypted data in the
1345 pools on the disk, but otherwise you get LUKS with the same
1346 security as on a magnetic disk.
1348 If you are concerned about your laptop being stolen, you are likely
1349 fine using LUKS on an SSD. An attacker would need to have access
1350 to an old passphrase (and the key-slot for this old passphrase
1351 would actually need to still be somewhere in the SSD) for your
1352 data to be at risk. So unless you pasted your old passphrase all
1353 over the Internet or the attacker has knowledge of it from some
1354 other source and does a targetted laptop theft to get at your
1355 data, you should be fine.
1358 6. Backup and Data Recovery
1361 * 6.1 Why do I need Backup?
1363 First, disks die. The rate for well-treated (!) disk is about 5%
1364 per year, which is high enough to worry about. There is some
1365 indication that this may be even worse for some SSDs. This applies
1366 both to LUKS and plain dm-crypt partitions.
1368 Second, for LUKS, if anything damages the LUKS header or the
1369 key-stripe area then decrypting the LUKS device can become
1370 impossible. This is a frequent occurrence. For example an
1371 accidental format as FAT or some software overwriting the first
1372 sector where it suspects a partition boot sector typically makes a
1373 LUKS partition permanently inaccessible. See more below on LUKS
1376 So, data-backup in some form is non-optional. For LUKS, you may
1377 also want to store a header backup in some secure location. This
1378 only needs an update if you change passphrases.
1381 * 6.2 How do I backup a LUKS header?
1383 While you could just copy the appropriate number of bytes from the
1384 start of the LUKS partition, the best way is to use command option
1385 "luksHeaderBackup" of cryptsetup. This protects also against
1386 errors when non-standard parameters have been used in LUKS
1387 partition creation. Example:
1390 cryptsetup luksHeaderBackup --header-backup-file <file> <device>
1392 To restore, use the inverse command, i.e.
1394 cryptsetup luksHeaderRestore --header-backup-file <file> <device>
1397 * 6.3 How do I test a LUKS header?
1401 cryptsetup -v isLuks <device>
1403 on the device. Without the "-v" it just signals its result via
1404 exit-status. You can also use the more general test
1408 which will also detect other types and give some more info. Omit
1409 "-p" for old versions of blkid that do not support it.
1412 * 6.4 How do I backup a LUKS or dm-crypt partition?
1414 There are two options, a sector-image and a plain file or
1415 filesystem backup of the contents of the partition. The sector
1416 image is already encrypted, but cannot be compressed and contains
1417 all empty space. The filesystem backup can be compressed, can
1418 contain only part of the encrypted device, but needs to be
1419 encrypted separately if so desired.
1421 A sector-image will contain the whole partition in encrypted form,
1422 for LUKS the LUKS header, the keys-slots and the data area. It can
1423 be done under Linux e.g. with dd_rescue (for a direct image copy)
1424 and with "cat" or "dd". Example:
1426 cat /dev/sda10 > sda10.img
1427 dd_rescue /dev/sda10 sda10.img
1429 You can also use any other backup software that is capable of making
1430 a sector image of a partition. Note that compression is
1431 ineffective for encrypted data, hence it does not make sense to
1434 For a filesystem backup, you decrypt and mount the encrypted
1435 partition and back it up as you would a normal filesystem. In this
1436 case the backup is not encrypted, unless your encryption method
1437 does that. For example you can encrypt a backup with "tar" as
1440 tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
1442 And verify the backup like this if you are at "path":
1444 cat backup.tbz2.gpg | gpg - | tar djf -
1446 Note: Always verify backups, especially encrypted ones.
1448 In both cases GnuPG will ask you interactively for your symmetric
1449 key. The verify will only output errors. Use "tar dvjf -" to get
1450 all comparison results. To make sure no data is written to disk
1451 unencrypted, turn off swap if it is not encrypted before doing the
1454 You can of course use different or no compression and you can use
1455 an asymmetric key if you have one and have a backup of the secret
1456 key that belongs to it.
1458 A second option for a filesystem-level backup that can be used when
1459 the backup is also on local disk (e.g. an external USB drive) is
1460 to use a LUKS container there and copy the files to be backed up
1461 between both mounted containers. Also see next item.
1464 * 6.5 Do I need a backup of the full partition? Would the header and
1465 key-slots not be enough?
1467 Backup protects you against two things: Disk loss or corruption
1468 and user error. By far the most questions on the dm-crypt mailing
1469 list about how to recover a damaged LUKS partition are related
1470 to user error. For example, if you create a new filesystem on a
1471 LUKS partition, chances are good that all data is lost
1474 For this case, a header+key-slot backup would often be enough. But
1475 keep in mind that a well-treated (!) HDD has roughly a failure
1476 risk of 5% per year. It is highly advisable to have a complete
1477 backup to protect against this case.
1480 * *6.6 What do I need to backup if I use "decrypt_derived"?
1482 This is a script in Debian, intended for mounting /tmp or swap with
1483 a key derived from the master key of an already decrypted device.
1484 If you use this for an device with data that should be persistent,
1485 you need to make sure you either do not lose access to that master
1486 key or have a backup of the data. If you derive from a LUKS
1487 device, a header backup of that device would cover backing up the
1488 master key. Keep in mind that this does not protect against disk
1491 Note: If you recreate the LUKS header of the device you derive from
1492 (using luksFormat), the master key changes even if you use the same
1493 passphrase(s) and you will not be able to decrypt the derived
1494 device with the new LUKS header.
1497 * 6.7 Does a backup compromise security?
1499 Depends on how you do it. However if you do not have one, you are
1500 going to eventually lose your encrypted data.
1502 There are risks introduced by backups. For example if you
1503 change/disable a key-slot in LUKS, a binary backup of the partition
1504 will still have the old key-slot. To deal with this, you have to
1505 be able to change the key-slot on the backup as well, securely
1506 erase the backup or do a filesystem-level backup instead of a binary
1509 If you use dm-crypt, backup is simpler: As there is no key
1510 management, the main risk is that you cannot wipe the backup when
1511 wiping the original. However wiping the original for dm-crypt
1512 should consist of forgetting the passphrase and that you can do
1513 without actual access to the backup.
1515 In both cases, there is an additional (usually small) risk with
1516 binary backups: An attacker can see how many sectors and which
1517 ones have been changed since the backup. To prevent this, use a
1518 filesystem level backup method that encrypts the whole backup in
1519 one go, e.g. as described above with tar and GnuPG.
1521 My personal advice is to use one USB disk (low value data) or
1522 three disks (high value data) in rotating order for backups, and
1523 either use independent LUKS partitions on them, or use encrypted
1524 backup with tar and GnuPG.
1526 If you do network-backup or tape-backup, I strongly recommend to
1527 go the filesystem backup path with independent encryption, as you
1528 typically cannot reliably delete data in these scenarios,
1529 especially in a cloud setting. (Well, you can burn the tape if it
1530 is under your control...)
1533 * 6.8 What happens if I overwrite the start of a LUKS partition or
1534 damage the LUKS header or key-slots?
1536 There are two critical components for decryption: The salt values
1537 in the header itself and the key-slots. If the salt values are
1538 overwritten or changed, nothing (in the cryptographically strong
1539 sense) can be done to access the data, unless there is a backup
1540 of the LUKS header. If a key-slot is damaged, the data can still
1541 be read with a different key-slot, if there is a remaining
1542 undamaged and used key-slot. Note that in order to make a key-slot
1543 unrecoverable in a cryptographically strong sense, changing about
1544 4-6 bits in random locations of its 128kiB size is quite enough.
1547 * 6.9 What happens if I (quick) format a LUKS partition?
1549 I have not tried the different ways to do this, but very likely you
1550 will have written a new boot-sector, which in turn overwrites the
1551 LUKS header, including the salts, making your data permanently
1552 irretrievable, unless you have a LUKS header backup. You may also
1553 damage the key-slots in part or in full. See also last item.
1556 * 6.10 How do I recover the master key from a mapped LUKS container?
1558 This is typically only needed if you managed to damage your LUKS
1559 header, but the container is still mapped, i.e. "luksOpen"ed. It
1560 also helps if you have a mapped container that you forgot or do not
1561 know a passphrase for (e.g. on a long running server.)
1563 WARNING: Things go wrong, do a full backup before trying this!
1565 WARNING: This exposes the master key of the LUKS container. Note
1566 that both ways to recreate a LUKS header with the old master key
1567 described below will write the master key to disk. Unless you are
1568 sure you have securely erased it afterwards, e.g. by writing it to
1569 an encrypted partition, RAM disk or by erasing the filesystem you
1570 wrote it to by a complete overwrite, you should change the master
1571 key afterwards. Changing the master key requires a full data
1572 backup, luksFormat and then restore of the backup.
1574 First, there is a script by Milan that automates the whole
1575 process, except generating a new LUKS header with the old master
1576 key (it prints the command for that though):
1578 http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
1580 You can also do this manually. Here is how:
1582 - Get the master key from the device mapper. This is done by the
1583 following command. Substitute c5 for whatever you mapped to:
1585 # dmsetup table --target crypt --showkey /dev/mapper/c5
1587 0 200704 crypt aes-cbc-essiv:sha256
1588 a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
1591 The result is actually one line, wrapped here for clarity. The long
1592 hex string is the master key.
1594 - Convert the master key to a binary file representation. You can
1595 do this manually, e.g. with hexedit. You can also use the tool
1596 "xxd" from vim like this:
1598 echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
1600 - Do a luksFormat to create a new LUKS header.
1602 NOTE: If your header is intact and you just forgot the
1603 passphrase, you can just set a new passphrase, see next
1606 Unmap the device before you do that (luksClose). Then do
1608 cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
1610 Note that if the container was created with other than the default
1611 settings of the cryptsetup version you are using, you need to give
1612 additional parameters specifying the deviations. If in doubt, try
1613 the script by Milan. It does recover the other parameters as well.
1615 Side note: This is the way the decrypt_derived script gets at the
1616 master key. It just omits the conversion and hashes the master key
1619 - If the header is intact and you just forgot the passphrase, just
1620 set a new passphrase like this:
1622 cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
1624 You may want to disable the old one afterwards.
1627 * 6.11 What does the on-disk structure of dm-crypt look like?
1629 There is none. dm-crypt takes a block device and gives encrypted
1630 access to each of its blocks with a key derived from the passphrase
1631 given. If you use a cipher different than the default, you have to
1632 specify that as a parameter to cryptsetup too. If you want to
1633 change the password, you basically have to create a second
1634 encrypted device with the new passphrase and copy your data over.
1635 On the plus side, if you accidentally overwrite any part of a
1636 dm-crypt device, the damage will be limited to the are you
1640 * 6.12 What does the on-disk structure of LUKS look like?
1642 A LUKS partition consists of a header, followed by 8 key-slot
1643 descriptors, followed by 8 key slots, followed by the encrypted
1646 Header and key-slot descriptors fill the first 592 bytes. The
1647 key-slot size depends on the creation parameters, namely on the
1648 number of anti-forensic stripes, key material offset and master
1651 With the default parameters, each key-slot is a bit less than
1652 128kiB in size. Due to sector alignment of the key-slot start,
1653 that means the key block 0 is at offset 0x1000-0x20400, key
1654 block 1 at offset 0x21000-0x40400, and key block 7 at offset
1655 0xc1000-0xe0400. The space to the next full sector address is
1656 padded with zeros. Never used key-slots are filled with what the
1657 disk originally contained there, a key-slot removed with
1658 "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Due to
1659 2MiB default alignment, start of the data area for cryptsetup 1.3
1660 and later is at 2MiB, i.e. at 0x200000. For older versions, it is
1661 at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes
1662 from the start of the partition. Incidentally, "luksHeaderBackup"
1663 for a LUKS container created with default parameters dumps exactly
1664 the first 2MiB (or 1'052'672 bytes for headers created with
1665 cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
1668 For non-default parameters, you have to figure out placement
1669 yourself. "luksDump" helps. See also next item. For the most common
1670 non-default settings, namely aes-xts-plain with 512 bit key, the
1671 offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
1672 0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, ..., and start of
1673 bulk data at 0x200000.
1675 The exact specification of the format is here:
1676 http://code.google.com/p/cryptsetup/wiki/Specification
1679 * 6.13 What is the smallest possible LUKS container?
1681 Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
1682 modern Linux partitioning tools that also align to 1MB, this will
1683 result in alignment to 2k sectors and typical Flash/SSD sectors,
1684 which is highly desirable for a number of reasons. Changing the
1685 alignment is not recommended.
1687 That said, with default parameters, the data area starts at
1688 exactly 2MB offset (at 0x101000 for cryptsetup versions before
1689 1.3). The smallest data area you can have is one sector of 512
1690 bytes. Data areas of 0 bytes can be created, but fail on mapping.
1692 While you cannot put a filesystem into something this small, it may
1693 still be used to contain, for example, key. Note that with current
1694 formatting tools, a partition for a container this size will be
1695 3MiB anyways. If you put the LUKS container into a file (via
1696 losetup and a loopback device), the file needs to be 2097664 bytes
1697 in size, i.e. 2MiB + 512B.
1699 There two ways to influence the start of the data area are key-size
1702 For alignment, you can go down to 1 on the parameter. This will
1703 still leave you with a data-area starting at 0x101000, i.e.
1704 1MiB+4096B (default parameters) as alignment will be rounded up to
1705 the next multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run
1706 on a larger file and dump the LUKS header to get actual
1709 For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
1710 (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
1711 You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
1712 128 bit has to be considered insecure today.
1714 Example 1 - AES 128 bit with CBC:
1716 cryptsetup luksFormat -s 128 --align-payload=8 <device>
1718 This results in a data offset of 0x81000, i.e. 516KiB or 528384
1719 bytes. Add one 512 byte sector and the smallest LUKS container size
1720 with these parameters is 516KiB + 512B or 528896 bytes.
1722 Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
1724 cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
1726 This results in a data offset of 0x41000, i.e. 260kiB or 266240
1727 bytes, with a minimal LUKS container size of 260kiB + 512B or
1731 * 6.14 I think this is overly complicated. Is there an alternative?
1733 Not really. Encryption comes at a price. You can use plain
1734 dm-crypt to simplify things a bit. It does not allow multiple
1735 passphrases, but on the plus side, it has zero on disk description
1736 and if you overwrite some part of a plain dm-crypt partition,
1737 exactly the overwritten parts are lost (rounded up to sector
1741 * 6.15 Can I clone a LUKS container?
1743 You can, but it breaks security, because the cloned container has
1744 the same header and hence the same master key. You cannot change
1745 the master key on a LUKS container, even if you change the
1746 passphrase(s), the master key stays the same. That means whoever
1747 has access to one of the clones can decrypt them all, completely
1748 bypassing the passphrases.
1750 The right way to do this is to first luksFormat the target
1751 container, then to clone the contents of the source container, with
1752 both containers mapped, i.e. decrypted. You can clone the decrypted
1753 contents of a LUKS container in binary mode, although you may run
1754 into secondary issues with GUIDs in filesystems, partition tables,
1755 RAID-components and the like. These are just the normal problems
1756 binary cloning causes.
1758 Note that if you need to ship (e.g.) cloned LUKS containers with a
1759 default passphrase, that is fine as long as each container was
1760 individually created (and hence has its own master key). In this
1761 case, changing the default passphrase will make it secure again.
1764 7. Interoperability with other Disk Encryption Tools
1767 * 7.1 What is this section about?
1769 Cryptsetup for plain dm-crypt can be used to access a number of
1770 on-disk formats created by tools like loop-aes patched into
1771 losetup. This sometimes works and sometimes does not. This
1772 section collects insights into what works, what does not and where
1773 more information is required.
1775 Additional information may be found in the mailing-list archives,
1776 mentioned at the start of this FAQ document. If you have a
1777 solution working that is not yet documented here and think a wider
1778 audience may be interested, please email the FAQ maintainer.
1781 * 7.2 loop-aes: General observations.
1783 One problem is that there are different versions of losetup around.
1784 loop-aes is a patch for losetup. Possible problems and deviations
1785 from cryptsetup option syntax include:
1787 - Offsets specified in bytes (cryptsetup: 512 byte sectors)
1789 - The need to specify an IV offset
1791 - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
1793 - Key size needs specifying (e.g. "-s 128" for 128 bit keys)
1795 - Passphrase hash algorithm needs specifying
1797 Also note that because plain dm-crypt and loop-aes format does not
1798 have metadata, and while the loopAES extension for cryptsetup tries
1799 autodetection (see command loopaesOpen), it may not always work.
1800 If you still have the old set-up, using a verbosity option (-v)
1801 on mapping with the old tool or having a look into the system logs
1802 after setup could give you the information you need. Below, there
1803 are also some things that worked for somebody.
1806 * 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32
1808 In this case, the main problem seems to be that this variant of
1809 losetup takes the offset (-o option) in bytes, while cryptsetup
1810 takes it in sectors of 512 bytes each. Example: The losetup command
1812 losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
1813 mount /dev/loop0 mount-point
1817 cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
1818 mount /dev/mapper/e1 mount-point
1821 * 7.4 loop-aes with 160 bit key
1823 This seems to be sometimes used with twofish and blowfish and
1824 represents a 160 bit ripemed160 hash output padded to 196 bit key
1825 length. It seems the corresponding options for cryptsetup are
1827 --cipher twofish-cbc-null -s 192 -h ripemd160:20
1830 * 7.5 loop-aes v1 format OpenSUSE
1832 Apparently this is done by older OpenSUSE distros and stopped
1833 working from OpenSUSE 12.1 to 12.2. One user had success with the
1836 cryptsetup create <target> <device> -c aes -s 128 -h sha256
1839 * 7.6 Kernel encrypted loop device (cryptoloop)
1841 There are a number of different losetup implementations for using
1842 encrypted loop devices so getting this to work may need a bit of
1845 NOTE: Do NOT use this for new containers! Some of the existing
1846 implementations are insecure and future support is uncertain.
1848 Example for a compatible mapping:
1850 losetup -e twofish -N /dev/loop0 /image.img
1854 cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain
1856 with the mapping being done to /dev/mapper/image_plain instead of
1861 Cipher, mode and pasword hash (or no hash):
1863 -e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256]
1864 -e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256]
1866 Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512
1870 -o 2560 => -o 5 -p 5 # 2560/512 = 5
1872 There is no replacement for --pass-fd, it has to be emulated using
1873 keyfiles, see the cryptsetup man-page.
1876 8. Issues with Specific Versions of cryptsetup
1879 * 8.1 When using the create command for plain dm-crypt with
1880 cryptsetup 1.1.x, the mapping is incompatible and my data is not
1883 With cryptsetup 1.1.x, the distro maintainer can define different
1884 default encryption modes for LUKS and plain devices. You can check
1885 these compiled-in defaults using "cryptsetup --help". Moreover, the
1886 plain device default changed because the old IV mode was
1887 vulnerable to a watermarking attack.
1889 If you are using a plain device and you need a compatible mode, just
1890 specify cipher, key size and hash algorithm explicitly. For
1891 compatibility with cryptsetup 1.0.x defaults, simple use the
1894 cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
1896 LUKS stores cipher and mode in the metadata on disk, avoiding this
1900 * 8.2 cryptsetup on SLED 10 has problems...
1902 SLED 10 is missing an essential kernel patch for dm-crypt, which
1903 is broken in its kernel as a result. There may be a very old
1904 version of cryptsetup (1.0.x) provided by SLED, which should also
1905 not be used anymore as well. My advice would be to drop SLED 10.
1908 9. References and Further Reading
1911 * Purpose of this Section
1913 The purpose of this section is to collect references to all
1914 materials that do not fit the FAQ but are relevant in some fashion.
1915 This can be core topics like the LUKS spec or disk encryption, but
1916 it can also be more tangential, like secure storage management or
1917 cryptography used in LUKS. It should still have relevance to
1918 cryptsetup and its applications.
1920 If you wan to see something added here, send email to the
1921 maintainer (or the cryptsetup mailing list) giving an URL, a
1922 description (1-3 lines preferred) and a section to put it in. You
1923 can also propose new sections.
1925 At this time I would like to limit the references to things that
1926 are available on the web.
1931 - LUKS on-disk format spec:
1932 http://code.google.com/p/cryptsetup/wiki/Specification
1937 - Some code examples are in the source package under docs/examples
1940 * Brute-forcing passphrases
1943 http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
1946 http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
1952 * SSD and Flash Disk Related
1958 * Attacks Against Disk Encryption
1961 * Risk Management as Relevant for Disk Encryption
1969 A. Contributors In no particular order: