4. Troubleshooting
5. Security Aspects
6. Backup and Data Recovery
-7. Issues with Specific Versions of cryptsetup
+7. Interoperability with other Disk Encryption Tools
+8. Issues with Specific Versions of cryptsetup
A. Contributors
1. General Questions
- * What is this?
+ * 1.1 What is this?
This is the FAQ (Frequently Asked Questions) for cryptsetup. It
covers Linux disk encryption with plain dm-crypt (one passphrase,
- no management, no descriptor on disk) and LUKS (multiple user keys
- with one master key, anti-forensics, descriptor block at start of
- device, ...). The latest version should usually be available at
+ no management, no metadata on disk) and LUKS (multiple user keys
+ with one master key, anti-forensic features, metadata block at
+ start of device, ...). The latest version of this FAQ should
+ usually be available at
http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
+
+ * 1.2 WARNINGS
+
ATTENTION: If you are going to read just one thing, make it the
section on Backup and Data Recovery. By far the most questions on
- the cryptsetup mailing list are from people that just managed to
- somehow format or overwrite the start of their LUKS partitions. In
+ the cryptsetup mailing list are from people that managed to damage
+ the start of their LUKS partitions, i.e. the LUKS header. In
most cases, there is nothing that can be done to help these poor
souls recover their data. Make sure you understand the problem and
- limitations imposed by the LUKS security model BEFORE you face such
- a disaster!
-
-
- * Who wrote this?
+ limitations imposed by the LUKS security model BEFORE you face
+ such a disaster! In particular, make sure you have a current header
+ backup before doing any potentially dangerous operations.
+
+ BACKUP: Yes, encrypted disks die, just as normal ones do. A full
+ backup is mandatory, see Section "6. Backup and Data Recovery" on
+ options for doing encrypted backup.
+
+ CLONING/IMAGING: If you clone or image a LUKS container, you make a
+ copy of the LUKS header and the master key will stay the same!
+ That means that if you distribute an image to several machines, the
+ same master key will be used on all of them, regardless of whether
+ you change the passphrases. Do NOT do this! If you do, a root-user
+ on any of the machines with a mapped (decrypted) container or a
+ passphrase on that machine can decrypt all other copies, breaking
+ security. See also Item 6.15.
+
+ DISTRIBUTION INSTALLERS: Some distribution installers offer to
+ create LUKS containers in a way that can be mistaken as activation
+ of an existing container. Creating a new LUKS container on top of
+ an existing one leads to permanent, complete and irreversible data
+ loss. It is strongly recommended to only use distribution
+ installers after a complete backup of all LUKS containers has been
+ made.
+
+ NO WARNING ON NON-INERACTIVE FORMAT: If you feed cryptsetup from
+ STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
+ warning that you are about to format (and e.g. will lose any
+ pre-existing LUKS container on the target), as it assumes it is
+ used from a script. In this scenario, the responsibility for
+ warning the user and possibly checking for an existing LUKS header
+ is shifted to the script. This is a more general form of the
+ previous item.
+
+ LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
+ used in deriving the master key. It is used in decrypting a master
+ key that is randomly selected on header creation. This means that
+ if you create a new LUKS header on top of an old one with
+ exactly the same parameters and exactly the same passphrase as the
+ old one, it will still have a different master key and your data
+ will be permanently lost.
+
+ PASSPHRASE CHARACTER SET: Some people have had difficulties with
+ this when upgrading distributions. It is highly advisable to only
+ use the 94 printable characters from the first 128 characters of
+ the ASCII table, as they will always have the same binary
+ representation. Other characters may have different encoding
+ depending on system configuration and your passphrase will not
+ work with a different encoding. A table of the standardized first
+ 128 ASCII caracters can, e.g. be found on
+ http://en.wikipedia.org/wiki/ASCII
+
+
+ * 1.3 System Specific warnings
+
+ - Ubuntu as of 4/2011: It seems the installer offers to create
+ LUKS partitions in a way that several people mistook for an offer
+ to activate their existing LUKS partition. The installer gives no
+ or an inadequate warning and will destroy your old LUKS header,
+ causing permanent data loss. See also the section on Backup and
+ Data Recovery.
+
+ This issue has been acknowledged by the Ubuntu dev team, see here:
+ http://launchpad.net/bugs/420080
+
+
+ * 1.4 Who wrote this?
Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
contributors are listed at the end. If you want to contribute, send
your article, including a descriptive headline, to the maintainer,
or the dm-crypt mailing list with something like "FAQ ..." in the
- subject. Please note that by contributing to this FAQ, you accept
- the license described below.
+ subject. You can also send more raw information and have me write
+ the section. Please note that by contributing to this FAQ, you
+ accept the license described below.
This work is under the "Attribution-Share Alike 3.0 Unported"
license, which means distribution is unlimited, you may create
least problems.
- * Where is the project website?
+ * 1.5 Where is the project website?
There is the project website at http://code.google.com/p/cryptsetup/
Please do not post questions there, nobody will read them. Use
the mailing-list instead.
- * Is there a mailing-list?
+ * 1.6 Is there a mailing-list?
Instructions on how to subscribe to the mailing-list are at on the
project website. People are generally helpful and friendly on the
with just the word "help" in the subject or message body. Make sure
to send it from your list address.
+ The mailing list archive is here:
+ http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
+
2. Setup
- * Can I encrypt an already existing, non-empty partition to use LUKS?
+ * 2.1 What is the difference between "plain" and LUKS format?
+
+ Plain format is just that: It has no metadata on disk, reads all
+ paramters from the commandline (or the defaults), derives a
+ master-key from the passphrase and then uses that to de-/encrypt
+ the sectors of the device, with a direct 1:1 mapping between
+ encrypted and decrypted sectors.
+
+ Primary advantage is high resilience to damage, as one damaged
+ encrypted sector results in exactly one damaged decrypted sector.
+ Also, it is not readily apparent that there even is encrypted data
+ on the device, as an overwrite with crypto-grade randomness (e.g.
+ from /dev/urandom) looks exactly the same on disk.
+
+ Side-note: That has limited value against the authorities. In
+ civilized countries, they cannot force you to give up a crypto-key
+ anyways. In the US, the UK and dictatorships around the world,
+ they can force you to give up the keys (using imprisonment or worse
+ to pressure you), and in the worst case, they only need a
+ nebulous "suspicion" about the presence of encrypted data. My
+ advice is to either be ready to give up the keys or to not have
+ encrypted data when traveling to those countries, especially when
+ crossing the borders.
+
+ Disadvantages are that you do not have all the nice features that
+ the LUKS metadata offers, like multiple passphrases that can be
+ changed, the cipher being stored in the metadata, anti-forensic
+ properties like key-slot diffusion and salts, etc..
+
+ LUKS format uses a metadata header and 8 key-slot areas that are
+ being placed ath the begining of the disk, see below under "What
+ does the LUKS on-disk format looks like?". The passphrases are used
+ to decryt a single master key that is stored in the anti-forensic
+ stripes.
+
+ Advantages are a higher usability, automatic configuration of
+ non-default crypto parameters, defenses against low-entropy
+ passphrases like salting and iterated PBKDF2 passphrase hashing,
+ the ability to change passhrases, and others.
+
+ Disadvantages are that it is readily obvious there is encrypted
+ data on disk (but see side note above) and that damage to the
+ header or key-slots usually results in permanent data-loss. See
+ below under "6. Backup and Data Recovery" on how to reduce that
+ risk. Also the sector numbers get shifted by the length of the
+ header and key-slots and there is a loss of that size in capacity
+ (1MB+4096B for defaults and 2MB for the most commonly used
+ non-default XTS mode).
+
+
+ * 2.2 Can I encrypt an already existing, non-empty partition to use
+ LUKS?
There is no converter, and it is not really needed. The way to do
this is to make a backup of the device in question, securely wipe
to be in a filesystem.
- * How do I use LUKS with a loop-device?
+ * 2.3 How do I use LUKS with a loop-device?
- Just the same as with any block device. If you want, for example,
- to use a 100MiB file as LUKS container, do something like this:
+ This can be very handy for experiments. Setup is just the same as
+ with any block device. If you want, for example, to use a 100MiB
+ file as LUKS container, do something like this:
head -c 100M /dev/zero > luksfile # create empty file
losetup /dev/loop0 luksfile # map luksfile to /dev/loop0
To unmap the file when done, use "losetup -d /dev/loop0".
- * When I add a new key-slot to LUKS, it asks for a passphrase but
+ * 2.4 When I add a new key-slot to LUKS, it asks for a passphrase but
then complains about there not being a key-slot with that
passphrase?
new key-slot.
- * How do I read a dm-crypt key from file?
+ * 2.5 Encrytion on top of RAID or the other way round?
+
+ Unless you have special needs, place encryption between RAID and
+ filesystem, i.e. encryption on top of RAID. You can do it the other
+ way round, but you have to be aware that you then need to give the
+ pasphrase for each individual disk and RAID autotetection will not
+ work anymore. Therefore it is better to encrypt the RAID device,
+ e.g. /dev/dm0 .
+
+
+ * 2.6 How do I read a dm-crypt key from file?
Note that the file will still be hashed first, just like keyboard
input. Use the --key-file option, like this:
cryptsetup create --key-file keyfile e1 /dev/loop0
- * How do I read a LUKS slot key from file?
+ * 2.7 How do I read a LUKS slot key from file?
What you really do here is to read a passphrase from file, just as
you would with manual entry of a passphrase for a key-slot. You can
cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
- * How do I read the LUKS master key from file?
+ * 2.8 How do I read the LUKS master key from file?
- The question you should ask yourself first, is why you would want
- to do this. The only legitimate reason I can think of is if you
- want to have two LUKS devices with the same master key. Even then,
- I think it would be preferable to just use key-slots with the same
+ The question you should ask yourself first is why you would want to
+ do this. The only legitimate reason I can think of is if you want
+ to have two LUKS devices with the same master key. Even then, I
+ think it would be preferable to just use key-slots with the same
passphrase, or to use plain dm-crypt instead. If you really have a
good reason, please tell me. If I am convinced, I will add how to
do this here.
- * What are the security requirements for a key read from file?
+ * 2.9 What are the security requirements for a key read from file?
A file-stored key or passphrase has the same security requirements
as one entered interactively, however you can use random bytes and
head -c 256 /dev/random > keyfile
- * If I map a journaled file system using dm-crypt/LUKS, does it still
- provide its usual transactional guarantees?
+ * 2.10 If I map a journaled file system using dm-crypt/LUKS, does it
+ still provide its usual transactional guarantees?
- As far as I know it does (but I may be wrong), but please note that
- these "guarantees" are far weaker than they appear to be. For
- example, you not not get a hard flush to disk surface even on a
- call to fsync. In addition, the HDD itself may do independent
- write reordering. Some other things can go wrong as well. The
- filesystem developers are aware of these problems and typically
- can make it work anyways. That said, dm-crypt/LUKS should not make
- things worse.
+ Yes, it does, unless a very old kernel is used. The required flags
+ come from the filesystem layer and are processed and passed onwards
+ by dm-crypt. A bit more information on the process by which
+ transactional guarantees are implemented can be found here:
- Personally, I have several instances of ext3 on dm-crypt and have
- not noticed any specific problems.
+ http://lwn.net/Articles/400541/
- Update: I did run into frequent small freezes (1-2 sec) when putting
- a vmware image on ext3 over dm-crypt. This does indicate that the
- transactional guarantees are in place, but at a cost. When I went
- back to ext2, the problem went away. This also seems to have gotten
- better with kernel 2.6.36 and the reworking of filesystem flush
- locking. Kernel 2.6.37 is expected to improve this even further.
+ Please note that these "guarantees" are weaker than they appear to
+ be. One problem is that quite a few disks lie to the OS about
+ having flushed their buffers. Some other things can go wrong as
+ well. The filesystem developers are aware of these problems and
+ typically can make it work anyways. That said, dm-crypt/LUKS will
+ not make things worse.
+ One specific problem you can run into though is that you can get
+ short freezes and other slowdowns due to the encryption layer.
+ Encryption takes time and forced flushes will block for that time.
+ For example, I did run into frequent small freezes (1-2 sec) when
+ putting a vmware image on ext3 over dm-crypt. When I went back to
+ ext2, the problem went away. This seems to have gotten better with
+ kernel 2.6.36 and the reworking of filesystem flush locking
+ mechanism (less blocking of CPU activity during flushes). It
+ should improve further and eventually the problem should go away.
- * Can I use LUKS or cryptsetup with a more secure (external) medium
- for key storage, e.g. TPM or a smartcard?
+
+ * 2.11 Can I use LUKS or cryptsetup with a more secure (external)
+ medium for key storage, e.g. TPM or a smartcard?
Yes, see the answers on using a file-supplied key. You do have to
write the glue-logic yourself though. Basically you can have
storage.
- * Can I resize a dm-crypt or LUKS partition?
+ * 2.12 Can I resize a dm-crypt or LUKS partition?
Yes, you can, as neither dm-crypt nor LUKS stores partition size.
Whether you should is a different question. Personally I recommend
backup, recreation of the encrypted partition with new size,
recreation of the filesystem and restore. This gets around the
- tricky business of resizing the filesystem. The backup is really
- non-optional here, as a lot can go wrong, resulting in partial or
- complete data loss. Using something like gparted to resize an
- encrypted partition is slow, but pretty safe and should be fine.
- This will not change the size of the filesystem hidden under the
+ tricky business of resizing the filesystem. Resizing a dm-crypt or
+ LUKS container does not resize the filesystem in it. The backup is
+ really non-optional here, as a lot can go wrong, resulting in
+ partial or complete data loss. Using something like gparted to
+ resize an encrypted partition is slow, but typicaly works. This
+ will not change the size of the filesystem hidden under the
encryption though.
You also need to be aware of size-based limitations. The one
3. Common Problems
- * My dm-crypt/LUKS mapping does not work! What general steps are
+ * 3.1 My dm-crypt/LUKS mapping does not work! What general steps are
there to investigate the problem?
If you get a specific error message, investigate what it claims
the kernel. The output of "cat /proc/crypto" needs to list them.
- * My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
+ * 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
The default cipher, hash or mode may have changed (the mode changed
from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
cryptsetup".
- * When I call cryptsetup from cron/CGI, I get errors about unknown
- features?
+ * 3.3 When I call cryptsetup from cron/CGI, I get errors about
+ unknown features?
If you get errors about unknown parameters or the like that are not
present when cryptsetup is called from the shell, make sure you
have no older version of cryptsetup on your system that then gets
- called by cron/CGI.For example some distributions install
+ called by cron/CGI. For example some distributions install
cryptsetup into /usr/sbin, while a manual install could go to
/usr/local/sbin. As a debugging aid, call "cryptsetup --version"
- from cron/CGI or the non-shell mechanism to be sure you have the
- right version.
+ from cron/CGI or the non-shell mechanism to be sure the right
+ version gets called.
- * Unlocking a LUKS device takes very long. Why?
+ * 3.4 Unlocking a LUKS device takes very long. Why?
The iteration time for a key-slot (see Section 5 for an explanation
what iteration does) is calculated when setting a passphrase. By
However, this operation will not change volume key iteration count
(MK iterations in output of "cryptsetup luksDump"). In order to
change that, you will have to backup the data in the LUKS
- container, luksFormat on the slow machine and restore the data.
- Note that in the original LUKS specification this value was fixed
- to 10, but it is now derived from the PBKDF2 benchmark as well and
- set to iterations in 0.125 sec or 1000, whichever is larger.
+ container (i.e. your encrypted data), luksFormat on the slow
+ machine and restore the data. Note that in the original LUKS
+ specification this value was fixed to 10, but it is now derived
+ from the PBKDF2 benchmark as well and set to iterations in 0.125
+ sec or 1000, whichever is larger. Also note that MK iterations
+ are not very security relevant. But as each key-slot already takes
+ 1 second, spending the additional 0.125 seconds really does not
+ matter.
- * "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device.
- What is wrong?
+ * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
+ device. What is wrong?
Some old versions of cryptsetup have a bug where the header does
not get completely wiped during LUKS format and an older ext2/swap
cryptsetup luksHeaderBackup --header-backup-file <file> <device>
cryptsetup luksHeaderRestore --header-backup-file <file> <device>
- If you cannot use a 1.1.x cryptsetup, you can also do a manual wipe
- of the area in question with the command below. Be very, VERY,
- careful and make sure to do a backup of the header before. If you
- get this wrong, your device may become permanently inaccessible.
-
- dd if=/dev/zero of=<device> bs=512 seek=2 count=6
-
- * cryptsetup segfaults on Gentoo amd64 hardened ...
+ * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
There seems to be some inteference between the hardening and and
the way cryptsetup benchmarks PBKDF2. The solution to this is
4. Troubleshooting
- * Can a bad RAM module cause problems?
+ * 4.1 I get the error "LUKS keyslot x is invalid." What does that
+ mean?
+
+ This means that the given keyslot has an offset that points
+ outside the valid keyslot area. Typically, the reason is a
+ corrupted LUKS header because something was written to the start of
+ the device the LUKS contaner is on. Refer to Section "Backup and
+ Data Recovery" and ask on the mailing list if you have trouble
+ diagnosing and (if still possible) repairing this.
+
+
+ * 4.2 Can a bad RAM module cause problems?
LUKS and dm-crypt can give the RAM quite a workout, especially when
combined with software RAID. In particular the combination RAID5 +
itself when copying large amounts of data, typically several times
larger than your main memory.
- Side note: One thing you should always do on large data movements is
- to run a verify, for example with the "-d" option of "tar" or by
- doing a set of MD5 checksums on the source or target with
+ Side note: One thing you should always do on large data
+ copy/movements is to run a verify, for example with the "-d"
+ option of "tar" or by doing a set of MD5 checksums on the source
+ or target with
find . -type f -exec md5sum \{\} \; > checksum-file
ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
get up to a completely changed 512 byte block per bit error. A
corrupt block causes a lot more havoc than the occasionally
- flipped single bit and can result various obscure errors.
+ flipped single bit and can result in various obscure errors.
- Note however that a verify run on copying between encrypted or
- unencrypted devices can also show you corruption when the copying
- itself did not report any problems. If you find defect RAM, assume
- all backups and copied data to be suspect, unless you did a verify.
+ Note, that a verify run on copying between encrypted or
+ unencrypted devices will reliably detect corruption, even when the
+ copying itself did not report any problems. If you find defect
+ RAM, assume all backups and copied data to be suspect, unless you
+ did a verify.
- * How do I test RAM?
+ * 4.3 How do I test RAM?
- First you should know that overclocking often makes memory problems
- worse. So if you overclock (which I strongly recommend against in a
- system holding data that has some worth), run the tests with the
- overclocking active.
+ First you should know that overclocking often makes memory
+ problems worse. So if you overclock (which I strongly recommend
+ against in a system holding data that has some worth), run the
+ tests with the overclocking active.
There are two good options. One is Memtest86+ and the other is
"memtester" by Charles Cazabon. Memtest86+ requires a reboot and
5. Security Aspects
- * Should I initialize (overwrite) a new LUKS/dm-crypt partition?
+ * 5.1 Is LUKS insecure? Everybody can see I have encrypted data!
+
+ In practice it does not really matter. In most civilized countries
+ you can just refuse to hand over the keys, no harm done. In some
+ countries they can force you to hand over the keys, if they suspect
+ encryption. However the suspicion is enough, they do not have to
+ prove anything. This is for practical reasons, as even the presence
+ of a header (like the LUKS header) is not enough to prove that you
+ have any keys. It might have been an experiment, for example. Or it
+ was used as encrypted swap with a key from /dev/random. So they
+ make you prove you do not have encrypted data. Of course that is
+ just as impossible as the other way round.
+
+ This means that if you have a large set of random-looking data,
+ they can already lock you up. Hidden containers (encryption hidden
+ within encryption), as possible with Truecrypt, do not help
+ either. They will just assume the hidden container is there and
+ unless you hand over the key, you will stay locked up. Don't have
+ a hidden container? Though luck. Anybody could claim that.
+
+ Still, if you are concerned about the LUKS header, use plain
+ dm-crypt with a good passphrase. See also Section 2, "What is the
+ difference between "plain" and LUKS format?"
+
+
+ * 5.2 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
If you just create a filesystem on it, most of the old data will
still be there. If the old data is sensitive, you should overwrite
dd if=/dev/zero of=/dev/mapper/e1
- * How do I securely erase a LUKS (or other) partition?
-
- For LUKS, if you are in a desperate hurry, overwrite the first few
- kilobytes of the LUKS partition. This erases the master key salt
- and makes access impossible. However a LUKS header backup or full
- backup will still grant access to most or all data, so make sure
- that an attacker does not have access to backups or destroy them as
- well.
-
- To do this right, overwrite the whole LUKS partition with a single
- pass of zeros. This is enough for current HDDs. For SSDs you may
- want to erase the whole drive several times to be sure data is not
- retained by wear leveling. This is possibly still insecure as SSD
- technology is not fully understood in this regard. Still, due to
- the anti-forensic properties of the LUKS key-slots, a single
- overwrite of an SSD could be enough. If in doubt, use physical
- destruction in addition. Keep in mind to also erase all backups.
-
- Example for a zero-overwrite erase of partition sda10 done with
+ * 5.3 How do I securely erase a LUKS (or other) partition?
+
+ For LUKS, if you are in a desperate hurry, overwrite the LUKS
+ header and key-slot area. This means overwriting the first
+ (keyslots x stripes x keysize) + offset bytes. For the default
+ parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the
+ LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
+ 512 bit key) this is 2MiB. (The diferent offset stems from
+ differences in the sector alignment of the key-slots.) If in doubt,
+ just be generous and overwrite the first 10MB or so, it will likely
+ still be fast enough. A single overwrite with zeros should be
+ enough. If you anticipate being in a desperate hurry, prepare the
+ command beforehand. Example with /dev/sde1 as the LUKS partition
+ and default parameters:
+
+ head -c 1052672 /dev/zero > /dev/sde1; sync
+
+ A LUKS header backup or full backup will still grant access to
+ most or all data, so make sure that an attacker does not have
+ access to backups or destroy them as well.
+
+ If you have time, overwrite the whole LUKS partition with a single
+ pass of zeros. This is enough for current HDDs. For SSDs or FLASH
+ (USB sticks) you may want to overwrite the whole drive several
+ times to be sure data is not retained by wear leveling. This is
+ possibly still insecure as SSD technology is not fully understood
+ in this regard. Still, due to the anti-forensic properties of the
+ LUKS key-slots, a single overwrite of an SSD or FLASH drive could
+ be enough. If in doubt, use physical destruction in addition. Here
+ is a link to some current reseach results on erasing SSDs and FLASH
+ drives:
+ http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
+
+ Keep in mind to also erase all backups.
+
+ Example for a zero-overwrite erase of partition sde1 done with
dd_rescue:
- dd_rescue -w /dev/zero /dev/sda10
+ dd_rescue -w /dev/zero /dev/sde1
- * How do I securely erase a backup of a LUKS partition or header?
+ * 5.4 How do I securely erase a backup of a LUKS partition or header?
That depends on the medium it is stored on. For HDD and SSD, use
- overwrite with zeros. For an SSD, you may want to overwrite the
- complete SSD several times and use physical destruction in addition,
- see last item. Treat USB flash drives the same as SSDs. For
- re-writable CD/DVD, a single overwrite should also be enough, due
- to the anti-forensic properties of the LUKS keyslots. For
- write-once media, use physical destruction. For low security
- requirements, just cut the CD/DVD into several parts. For high
- security needs, shred or burn the medium. If your backup is on
- magnetic tape, I advise physical destruction by shredding or
- burning. The problem with magnetic tape is that it has a higher
- dynamic range than HDDs and older data may well be recoverable
- after overwrites. Also write-head alignment issues can lead to
- data not actually being deleted at all during overwrites.
+ overwrite with zeros. For an SSD or FLASH drive (USB stick), you
+ may want to overwrite the complete SSD several times and use
+ physical destruction in addition, see last item. For re-writable
+ CD/DVD, a single overwrite should also be enough, due to the
+ anti-forensic properties of the LUKS keyslots. For write-once
+ media, use physical destruction. For low security requirements,
+ just cut the CD/DVD into several parts. For high security needs,
+ shred or burn the medium. If your backup is on magnetic tape, I
+ advise physical destruction by shredding or burning, after
+ overwriting . The problem with magnetic tape is that it has a
+ higher dynamic range than HDDs and older data may well be
+ recoverable after overwrites. Also write-head alignment issues can
+ lead to data not actually being deleted at all during overwrites.
- * What about backup? Does it compromise security?
+ * 5.5 What about backup? Does it compromise security?
- That depends. See next section.
+ That depends. See item 6.7.
- * Why is all my data gone if I overwrite the LUKS header?
+ * 5.6 Why is all my data permanently gone if I overwrite the LUKS
+ header?
Overwriting the LUKS header in part or in full is the most common
reason why access to LUKS containers is lost permanently.
most people found the feedback comforting.
If your header does not contain an intact salt, best go directly
- to the last one ("Acceptance") and think about what to do now.
+ to the last stage ("Acceptance") and think about what to do now.
There is one exception that I know of: If your LUKS container is
still open, then it may be possible to extract the master key from
- the running system. Ask on the mailing-list on how to do that and
- make sure nobody switches off the machine.
+ the running system. See Item "How do I recover the master key from
+ a mapped LUKS container?" in Section "Backup and Data Recovery".
- * What is a "salt"?
+ * 5.7 What is a "salt"?
A salt is a random key-grade value added to the passphrase before
it is processed. It is not kept secret. The reason for using salts
infeasible.
- * Is LUKS secure with a low-entropy (bad) passphrase?
+ * 5.8 Is LUKS secure with a low-entropy (bad) passphrase?
+
+ Note: You should only use the 94 printable characters from 7 bit
+ ASCII code to prevent your passphrase from failing when the
+ character encoding changes, e.g. because of a system upgrade, see
+ also the note at the very start of this FAQ under "WARNINGS".
This needs a bit of theory. The quality of your passphrase is
directly related to its entropy (information theoretic, not
is the time it takes to process a passphrase in order to open the
LUKS container, then an attacker has to spend at maximum
- attack_time_max = 2^n * t
+ attack_time_max = 2^n * t
time for a successful attack and on average half that. There is no
way getting around that relationship. However, there is one thing
passphrase, see next FAQ item.
Still, if you want good security, a high-entropy passphrase is the
- only option. Use at least 64 bits for secret stuff. That is 64
- characters of English text (but only if randomly chosen) or a
- combination of 12 truly random letters and digits.
+ only option. For example, a low-entropy passphrase can never be
+ considered secure against a TLA-level (Three Letter Agency level,
+ i.e. government-level) attacker, no matter what tricks are used in
+ the key-derivation function. Use at least 64 bits for secret stuff.
+ That is 64 characters of English text (but only if randomly chosen)
+ or a combination of 12 truly random letters and digits.
For passphrase generation, do not use lines from very well-known
texts (religious texts, Harry potter, etc.) as they are to easy to
guess. For example, the total Harry Potter has about 1'500'000
words (my estimation). Trying every 64 character sequence starting
and ending at a word boundary would take only something like 20
- days on a single CPU and is entirely feasible.
+ days on a single CPU and is entirely feasible. To put that into
+ perspective, using a number of Amazon EC2 High-CPU Extra Large
+ instances (each gives about 8 real cores), this test costs
+ currently about 50USD/EUR, but can be made to run arbitrarily fast.
On the other hand, choosing 1.5 lines from, say, the Wheel of Time
is in itself not more secure, but the book selection adds quite a
this is good passphrase material.
- * What is "iteration count" and why is decreasing it a bad idea?
+ * 5.9 What is "iteration count" and why is decreasing it a bad idea?
Iteration count is the number of PBKDF2 iterations a passphrase is
put through before it is used to unlock a key-slot. Iterations are
0.0001 seconds on a modern CPU.
Example 2: The user did a bit better and has 32 chars of English
- text. That would give use about 32 bits of entropy. With 1 second
+ text. That would be about 32 bits of entropy. With 1 second
iteration, that means an attacker on the same CPU needs around 136
years. That is pretty impressive for such a weak passphrase.
Without the iterations, it would be more like 50 days on a modern
CPU, and possibly far less.
In addition, the attacker can both parallelize and use special
- hardware like GPUs to speed up the attack. The attack can also
- happen quite some time after the luksFormat operation and CPUs can
- have become faster and cheaper. For that reason you want a bit of
- extra security. Anyways, in Example 1 your are screwed. In example
- 2, not necessarily. Even if the attack is faster, it still has a
- certain cost associated with it, say 10000 EUR/USD with iteration
- and 1 EUR/USD without iteration. The first can be prohibitively
- expensive, while the second is something you try even without
- solid proof that the decryption will yield something useful.
+ hardware like GPUs or FPGAs to speed up the attack. The attack can
+ also happen quite some time after the luksFormat operation and CPUs
+ can have become faster and cheaper. For that reason you want a
+ bit of extra security. Anyways, in Example 1 your are screwed.
+ In example 2, not necessarily. Even if the attack is faster, it
+ still has a certain cost associated with it, say 10000 EUR/USD
+ with iteration and 1 EUR/USD without iteration. The first can be
+ prohibitively expensive, while the second is something you try
+ even without solid proof that the decryption will yield something
+ useful.
The numbers above are mostly made up, but show the idea. Of course
the best thing is to have a high-entropy passphrase.
very bad idea, unless you know and can enforce that the users only
use high-entropy passphrases. If you decrease the iteration time
without ensuring that, then you put your users at increased risk,
- and considering how often LUKS containers are unlocked in a
+ and considering how rarely LUKS containers are unlocked in a
typical work-flow, you do so without a good reason. Don't do it.
- The iteration time is already low enough that some users will
- still chose passphrases with entropy low enough that they are
- vulnerable. Lowering it even further increases this danger
- significantly.
+ The iteration time is already low enough that users with entropy
+ low passphrases are vulnerable. Lowering it even further increases
+ this danger significantly.
+
+
+ * 5.10 Some people say PBKDF2 is insecure?
+
+ There is some discussion that a hash-function should have a "large
+ memory" property, i.e. that it should require a lot of memory to be
+ computed. This serves to prevent attacks using special programmable
+ circuits, like FPGAs, and attacks using graphics cards. PBKDF2
+ does not need a lot of memory and is vulnerable to these attacks.
+ However, the publication usually refered in these discussions is
+ not very convincing in proving that the presented hash really is
+ "large memory" (that may change, email the FAQ maintainer when it
+ does) and it is of limited usefulness anyways. Attackers that use
+ clusters of normal PCs will not be affected at all by a "large
+ memory" property. For example the US Secret Service is known to
+ use the off-hour time of all the office PCs of the Treasury for
+ password breaking. The Treasury has about 110'000 employees.
+ Asuming every one has an office PC, that is significant computing
+ power, all of it with plenty of memory for computing "large
+ memory" hashes. Bot-net operators also have all the memory they
+ want. The only protection against a resouceful attacker is a
+ high-entropy passphrase, see items 5.8 and 5.9.
+
+ * 5.11 What about iteration count with plain dm-crypt?
- * Is LUKS with default parameters less secure on a slow CPU?
+ Simple: There is none. There is also no salting. If you use plain
+ dm-crypt, the only way to be secure is to use a high entropy
+ passphrase. If in doubt, use LUKS instead.
+
+
+ * 5.12 Is LUKS with default parameters less secure on a slow CPU?
Unfortunately, yes. However the only aspect affected is the
protection for low-entropy passphrase or master-key. All other
compensate for problems in front of the keyboard.
- * Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
+ * 5.13 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
The problem is that cbc-plain has a fingerprint vulnerability, where
a specially crafted file placed into the crypto-container can be
knowing the encryption key and the watermarking attack fails.
- * Are there any problems with "plain" IV? What is "plain64"?
+ * 5.14 Are there any problems with "plain" IV? What is "plain64"?
- First, "plain" and "plain64" are both not safe to use with CBC, see
- previous FAQ item.
+ First, "plain" and "plain64" are both not secure to use with CBC,
+ see previous FAQ item.
However there are modes, like XTS, that are secure with "plain" IV.
The next limit is that "plain" is 64 bit, with the upper 32 bit set
does not cause any performance penalty compared to "plain".
- * What about XTS mode?
+ * 5.15 What about XTS mode?
XTS mode is potentially even more secure than cbc-essiv (but only if
cbc-essiv is insecure in your scenario). It is a NIST standard and
apply.
+ * 5.16 Is LUKS FIPS-140-2 certified?
+
+ No. But that is more a problem of FIPS-140-2 than of LUKS. From a
+ technical point-of-view, LUKS with the right parameters would be
+ FIPS-140-2 compliant, but in order to make it certified, somebody
+ has to pay real money for that. And then, whenever cryptsetup is
+ changed or extended, the certification lapses and has to be
+ obtained again.
+
+ From the aspect of actual security, LUKS with default parameters
+ should be as good as most things that are FIPS-140-2 certified,
+ although you may want to make sure to use /dev/random (by
+ specifying --use-random on luksFormat) as randomness source for
+ the master key to avoid being potentially insecure in an
+ entropy-starved situation.
+
+
6. Backup and Data Recovery
- * Does a backup compromise security?
+ * 6.1 Why do I need Backup?
+
+ First, disks die. The rate for well-treated (!) disk is about 5%
+ per year, which is high enough to worry about. There is some
+ indication that this may be even worse for some SSDs. This applies
+ both to LUKS and plain dm-crypt partitions.
+
+ Second, for LUKS, if anything damages the LUKS header or the
+ key-stripe area then decrypting the LUKS device can become
+ impossible. This is a frequent occurrence. For example an
+ accidental format as FAT or some software overwriting the first
+ sector where it suspects a partition boot sector typically makes a
+ LUKS partition permanently inacessible. See more below on LUKS
+ header damage.
+
+ So, data-backup in some form is non-optional. For LUKS, you may
+ also want to store a header backup in some secure location. This
+ only needs an update if you change passphrases.
+
+
+ * 6.2 How do I backup a LUKS header?
+
+ While you could just copy the appropriate number of bytes from the
+ start of the LUKS partition, the best way is to use command option
+ "luksHeaderBackup" of cryptsetup. This protects also against
+ errors when non-standard parameters have been used in LUKS
+ partition creation. Example:
+
+
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
+
+ To restore, use the inverse command, i.e.
+
+ cryptsetup luksHeaderRestore --header-backup-file <file> <device>
+
+
+ * 6.3 How do I test a LUKS header?
+
+ Use
+
+ cryptsetup -v isLuks <device>
+
+ on the device. Without the "-v" it just signals its result via
+ exit-status. You can alos use the more general test
+
+ blkid -p <device>
+
+ which will also detect other types and give some more info. Omit
+ "-p" for old versions of blkid that do not support it.
+
+
+ * 6.4 How do I backup a LUKS or dm-crypt partition?
+
+ There are two options, a sector-image and a plain file or
+ filesystem backup of the contents of the partition. The sector
+ image is already encrypted, but cannot be compressed and contains
+ all empty space. The filesystem backup can be compressed, can
+ contain only part of the encrypted device, but needs to be
+ encrypted separately if so desired.
+
+ A sector-image will contain the whole partition in encrypted form,
+ for LUKS the LUKS header, the keys-slots and the data area. It can
+ be done under Linux e.g. with dd_rescue (for a direct image copy)
+ and with "cat" or "dd". Example:
+
+ cat /dev/sda10 > sda10.img
+ dd_rescue /dev/sda10 sda10.img
+
+ You can also use any other backup software that is capable of making
+ a sector image of a partition. Note that compression is
+ ineffective for encrypted data, hence it does not make sense to
+ use it.
+
+ For a filesystem backup, you decrypt and mount the encrypted
+ partition and back it up as you would a normal filesystem. In this
+ case the backup is not encrypted, unless your encryption method
+ does that. For example you can encrypt a backup with "tar" as
+ follows with GnuPG:
+
+ tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
+
+ And verify the backup like this if you are at "path":
+
+ cat backup.tbz2.gpg | gpg - | tar djf -
+
+ Note: Allways verify backups, especially encrypted ones.
+
+ In both cases GnuPG will ask you interactively for your symmetric
+ key. The verify will only output errors. Use "tar dvjf -" to get
+ all comparison results. To make sure no data is written to disk
+ unencrypted, turn off swap if it is not encrypted before doing the
+ backup.
+
+ You can of course use different or no compression and you can use
+ an asymmetric key if you have one and have a backup of the secret
+ key that belongs to it.
+
+ A second option for a filestem-level backup that can be used when
+ the backup is also on local disk (e.g. an external USB drive) is
+ to use a LUKS container there and copy the files to be backed up
+ between both mounted containers. Also see next item.
+
+
+ * 6.5 Do I need a backup of the full partition? Would the header and
+ key-slots not be enough?
+
+ Backup protects you against two things: Disk loss or corruption
+ and user error. By far the most questions on the dm-crypt mailing
+ list about how to recover a damaged LUKS partition are related
+ to user error. For example, if you create a new filesystem on a
+ LUKS partition, chances are good that all data is lost
+ permanently.
+
+ For this case, a header+key-slot backup would often be enough. But
+ keep in mind that a well-treated (!) HDD has roughly a failure
+ risk of 5% per year. It is highly advisable to have a complete
+ backup to protect against this case.
+
+
+ * *6.6 What do I need to backup if I use "decrypt_derived"?
+
+ This is a script in Debian, intended for mounting /tmp or swap with
+ a key derived from the master key of an already decrypted device.
+ If you use this for an device with data that should be persistent,
+ you need to make sure you either do not lose access to that master
+ key or have a backup of the data. If you derive from a LUKS
+ device, a header backup of that device would cover backing up the
+ master key. Keep in mind that this does not protect against disk
+ loss.
+
+ Note: If you recreate the LUKS header of the device you derive from
+ (using luksFormat), the master key changes even if you use the same
+ passphrase(s) and you will not be able to decrypt the derived
+ device with the new LUKS header.
+
+
+ * 6.7 Does a backup compromise security?
- Depends on how you do it. First, a backup is non-optional with
- encrypted data just the same way it is with non-encrypted data.
- Disks do break and they do not care whether they make plain or
- encrypted data inaccessible.
+ Depends on how you do it. However if you do not have one, you are
+ going to eventually lose your encrypted data.
- However there are risks introduced by backups. For example if you
+ There are risks introduced by backups. For example if you
change/disable a key-slot in LUKS, a binary backup of the partition
will still have the old key-slot. To deal with this, you have to
- be able to change the key-slot on the backup as well, or use a
- different set-up. One option is to have a different passphrase on
- the backup and to make the backup with both containers open.
- Another one is to make a backup of the original, opened container to
- a single file, e.g. with tar, and to encrypt that file with
- public-key-cryptography, e.g. with GnuPG. You can then keep the
- secret key in a safe place, because it is only used to decrypt a
- backup. The key the backup is encrypted with can be stored without
- special security measures, as long as an attacker cannot replace
- it with his own key.
+ be able to change the key-slot on the backup as well, securely
+ erase the backup or do a filesystem-level backup instead of a binary
+ one.
If you use dm-crypt, backup is simpler: As there is no key
management, the main risk is that you cannot wipe the backup when
should consist of forgetting the passphrase and that you can do
without actual access to the backup.
- In both cases, there is an additional (usually small) risk: An
- attacker can see how many sectors and which ones have been changed
- since the backup. This is not possible with the public-key method
- though.
+ In both cases, there is an additional (usually small) risk with
+ binary backups: An attacker can see how many sectors and which
+ ones have been changed since the backup. To prevent this, use a
+ filesystem level backup method that encrypts the whole backup in
+ one go, e.g. as described above with tar and GnuPG.
- My personal advice is to use one USB disk (low value date) or three
- disks (high value data) in rotating order for backups, and either
- use different passphrases or keep them easily accessible in case
- you need to disable a key-slot. If you do network-backup or
- tape-backup, I strongly recommend to go the public-key path,
- especially as you typically cannot reliably delete data in these
- scenarios. (Well, you can burn the tape if it is under your
- control...)
+ My personal advice is to use one USB disk (low value data) or
+ three disks (high value data) in rotating order for backups, and
+ either use independent LUKS partitions on them, or use encrypted
+ backup with tar and GnuPG.
+ If you do network-backup or tape-backup, I strongly recommend to
+ go the filesystem backup path with independent encryption, as you
+ typically cannot reliably delete data in these scenarios,
+ especially in a cloud setting. (Well, you can burn the tape if it
+ is under your control...)
- * What happens if I overwrite the start of a LUKS partition or damage
- the LUKS header or key-slots?
+
+ * 6.8 What happens if I overwrite the start of a LUKS partition or
+ damage the LUKS header or key-slots?
There are two critical components for decryption: The salt values
in the header itself and the key-slots. If the salt values are
overwritten or changed, nothing (in the cryptographically strong
- sense) can be done to access the data, unless there is a backup of
- the LUKS header. If a key-slot is damaged, the data can still be
- read with a different key-slot, if there is a remaining undamaged
- and used key-slot. Note that in order to make a key-slot
+ sense) can be done to access the data, unless there is a backup
+ of the LUKS header. If a key-slot is damaged, the data can still
+ be read with a different key-slot, if there is a remaining
+ undamaged and used key-slot. Note that in order to make a key-slot
unrecoverable in a cryptographically strong sense, changing about
4-6 bits in random locations of its 128kiB size is quite enough.
- * What happens if I (quick) format a LUKS partition?
+ * 6.9 What happens if I (quick) format a LUKS partition?
I have not tried the different ways to do this, but very likely you
will have written a new boot-sector, which in turn overwrites the
- LUKS header, including the salts. You may also damage the key-slots
- in part or in full. See also last item.
+ LUKS header, including the salts, making your data permanently
+ irretrivable, unless you have a LUKS header backup. You may also
+ damage the key-slots in part or in full. See also last item.
+
+
+ * 6.10 How do I recover the master key from a mapped LUKS container?
+
+ This is typically only needed if you managed to damage your LUKS
+ header, but the container is still mapped, i.e. "luksOpen"ed. It
+ also helps if you have a mapped container that you forgot or do not
+ know a passphrase for (e.g. on a long running server.)
+
+ WARNING: Things go wrong, do a full backup before trying this!
+ WARNING: This exposes the master key of the LUKS container. Note
+ that both ways to recreate a LUKS header with the old master key
+ described below will write the master key to disk. Unless you are
+ sure you have securely erased it afterwards, e.g. by writing it to
+ an encrypted partition, RAM disk or by erasing the filesystem you
+ wrote it to by a complete overwrite, you should change the master
+ key afterwards. Changing the master key requires a full data
+ backup, luksFormat and then restore of the backup.
+
+ First, there is a script by Milan that automatizes the whole
+ process, except generating a new LUKS header with the old master
+ key (it prints the command for that though):
+
+http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-active
+
+ You can also do this manually. Here is how:
+
+ - Get the master key from the device mapper. This is done by the
+ following command. Substitute c5 for whatever you mapped to:
+
+ # dmsetup table --target crypt --showkey /dev/mapper/c5
+ Result:
+ 0 200704 crypt aes-cbc-essiv:sha256
+ a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
+ 0 7:0 4096
+
+ The result is actually one line, wrapped here for clarity. The long
+ hex string is the master key.
+
+ - Convert the master key to a binary file representation. You can
+ do this manually, e.g. with hexedit. You can also use the tool
+ "xxd" from vim like this:
+
+ echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
+
+ - Do a luksFormat to create a new LUKS header.
- * What does the on-disk structure of dm-crypt look like?
+ NOTE: If your header is intact and you just forgot the
+ passphrase, you can just set a new passphrase, see next subitem.
+
+ Unmap the device before you do that (luksClose). Then do
+
+ cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
+
+ Note that if the container was created with other than the default
+ settings of the cryptsetup version you are using, you need to give
+ additional parameters specifying the deviations. If in doubt, try
+ the script by Milan. It does recover the other parameters as well.
+
+ Side note: This is the way the decrypt_derived script gets at the
+ master key. It just omits the conversion and hashes the master key
+ string.
+
+ - If the header is intact and you just forgot the passphrase, just
+ set a new passphrase like this:
+
+ cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
+
+ You may want to disable the old one afterwards.
+
+
+ * 6.11 What does the on-disk structure of dm-crypt look like?
There is none. dm-crypt takes a block device and gives encrypted
access to each of its blocks with a key derived from the passphrase
overwrote.
- * What does the on-disk structure of LUKS look like?
+ * 6.12 What does the on-disk structure of LUKS look like?
A LUKS partition consists of a header, followed by 8 key-slot
descriptors, followed by 8 key slots, followed by the encrypted
Header and key-slot descriptors fill the first 592 bytes. The
key-slot size depends on the creation parameters, namely on the
- number of anti-forensic stripes and on key block alignment.
+ number of anti-forensic stripes, key material offset and master
+ key size.
- With 4000 stripes (the default), each key-slot is a bit less than
+ With the default parameters, each key-slot is a bit less than
128kiB in size. Due to sector alignment of the key-slot start,
- that means the key block 0 is at offset 0x1000-0x20400, key block
- 1 at offset 0x21000-0x40400, and key block 7 at offset
+ that means the key block 0 is at offset 0x1000-0x20400, key
+ block 1 at offset 0x21000-0x40400, and key block 7 at offset
0xc1000-0xe0400. The space to the next full sector address is
padded with zeros. Never used key-slots are filled with what the
disk originally contained there, a key-slot removed with
- "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Start of
- bulk data (with the default 4000 stripes and 8 key-slots) is at
- 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes from
- the start of the partition. This is also the value given by command
- "luksDump" with "Payload offset: 2056", just multiply by the sector
- size (512 bytes). Incidentally, "luksHeaderBackup" dumps exactly
- the first 1'052'672 bytes to file and "luksHeaderRestore" restores
+ "luksRemoveKey" or "luksKillSlot" gets filled with 0xff. Due to
+ 2MiB default alignment, start of the data area for cryptsetup 1.3
+ and later is at 2MiB, i.e. at 0x200000. For older versions, it is
+ at 0x101000, i.e. at 1'052'672 bytes, i.e. at 1MiB + 4096 bytes
+ from the start of the partition. Incidentally, "luksHeaderBackup"
+ for a LUKS container created with default parameters dumps exactly
+ the first 2MiB (or 1'052'672 bytes for headers created with
+ cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
them.
+ For non-default parameters, you have to figure out placement
+ yourself. "luksDump" helps. See also next item. For the most common
+ non-default settings, namely aes-xts-plain with 512 bit key, the
+ offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
+ 0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, ..., and start of
+ bulk data at 0x200000.
+
The exact specification of the format is here:
http://code.google.com/p/cryptsetup/wiki/Specification
- * How do I backup a LUKS header?
+ * 6.13 What is the smallest possible LUKS container?
- While you could just copy the appropriate number of bytes from the
- start of the LUKS partition, the best way is to use command option
- "luksHeaderBackup" of cryptsetup. This protects also against errors
- when non-standard parameters have been used in LUKS partition
- creation. Example:
+ Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
+ modern Linux partitioning tools that also align to 1MB, this will
+ result in aligmnet to 2k secors and typical Flash/SSD sectors,
+ which is highly desirable for a number of reasons. Changing the
+ alignment is not recomended.
-
- cryptsetup luksHeaderBackup --header-backup-file h /dev/mapper/c1
-
+ That said, with default parameters, the data area starts at
+ exactly 2MB offset (at 0x101000 for cryptsetup versions before
+ 1.3). The smallest data area you can have is one sector of 512
+ bytes. Data areas of 0 bytes can be created, but fail on mapping.
- * How do I backup a LUKS partition?
-
- You do a sector-image of the whole partition. This will contain the
- LUKS header, the keys-slots and the data ares. It can be done
- under Linux e.g. with dd_rescue (for a direct image copy) and with
- "cat" or "dd". Example:
-
- cat /dev/sda10 > sda10.img
- dd_rescue /dev/sda10 sda10.img
-
- You can also use any other backup software that is capable of making
- a sector image of a partition. Note that compression is
- ineffective for encrypted data, hence it does not sense to use it.
+ While you cannot put a filesystem into something this small, it may
+ still be used to contain, for example, key. Note that with current
+ formatting tools, a partition for a container this size will be
+ 3MiB anyways. If you put the LUKS container into a file (via
+ losetup and a loopback device), the file needs to be 2097664 bytes
+ in size, i.e. 2MiB + 512B.
+ There two ways to influence the start of the data area are key-size
+ and alignment.
- * Do I need a backup of the full partition? Would the header and
- key-slots not be enough?
+ For alignment, you can go down to 1 on the parameter. This will
+ still leave you with a data-area starting at 0x101000, i.e.
+ 1MiB+4096B (default parameters) as alignment will be rounded up to
+ the next multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run
+ on a larger file and dump the LUKS header to get actual
+ information.
- Backup protects you against two things: Disk loss or corruption and
- user error. By far the most questions on the dm-crypt mailing list
- about how to recover a damaged LUKS partition are related to user
- error. For example, if you create a new filesystem on a LUKS
- partition, chances are good that all data is lost permanently.
+ For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
+ (e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
+ You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
+ 128 bit has to be considered insecure today.
- For this case, a header+key-slot backup would often be enough. But
- keep in mind that a HDD has roughly a failure risk of 5% per year.
- It is highly advisable to have a complete backup to protect against
- this case.
+ Example 1 - AES 128 bit with CBC:
+ cryptsetup luksFormat -s 128 --align-payload=8 <device>
+
+ This results in a data offset of 0x81000, i.e. 516KiB or 528384
+ bytes. Add one 512 byte sector and the smallest LUKS container size
+ with these parameters is 516KiB + 512B or 528896 bytes.
- * Are there security risks from a backup of the LUKS header or a
- whole LUKS partition?
+ Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
- Yes. One risk is that if you remove access rights for specific
- key-slots by deleting their contents, the data can still be
- accessed with invalidated passphrase and the backup. The other risk
- is that if you erase a LUKS partition, a backup could still grant
- access, especially if you only erased the LUKS header and not the
- whole partition.
+ cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
+
+ This results in a data offset of 0x41000, i.e. 260kiB or 266240
+ bytes, with a minimal LUKS conatiner size of 260kiB + 512B or
+ 266752 bytes.
- * I think this is overly complicated. Is there an alternative?
+ * 6.14 I think this is overly complicated. Is there an alternative?
- Yes, you can use plain dm-crypt. It does not allow multiple
+ Not really. Encryption comes at a price. You can use plain
+ dm-crypt to simplify things a bit. It does not allow multiple
passphrases, but on the plus side, it has zero on disk description
and if you overwrite some part of a plain dm-crypt partition,
exactly the overwritten parts are lost (rounded up to sector
borders).
-7. Issues with Specific Versions of cryptsetup
+ * 6.15 Can I clone a LUKS container?
+
+ You can, but it breaks security, because the cloned container has
+ the same header and hence the same master key. You cannot change
+ the master key on a LUKS container, even if you change the
+ passphrase(s), the master key stays the same. That means whoever
+ has access to one of the clones can decrypt them all, completely
+ bypassing the passphrases.
+
+ The right way to do this is to first luksFormat the target
+ container, then to clone the contents of the source container, with
+ both containers mapped, i.e. decrypted. You can clone the decrypted
+ contents of a LUKS container in binary mode, although you may run
+ into secondary issuses with GUIDs in filesystems, partition tables,
+ RAID-components and the like. These are just the normal problems
+ binary cloning causes.
+
+ Note that if you need to ship (e.g.) cloned LUKS containers with a
+ default passphrase, that is fine as long as each container was
+ individually created (and hence has its own master key). In this
+ case, changing the default passphrase will make it secure again.
+
+
+7. Interoperability with other Disk Encryption Tools
+
+
+ * 7.1 What is this section about?
+
+ Cryptsetup for plain dm-crypt can be used to access a number of
+ on-disk formats created by tools like loop-aes patched into
+ losetup. This somtimes works and sometimes does not. This section
+ collects insights into what works, what does not and where more
+ information is required.
+
+ Additional information may be found in the mailing-list archives,
+ mentioned at the start of this FAQ document. If you have a
+ solution working that is not yet documented here and think a wider
+ audience may be intertested, please email the FAQ maintainer.
+
+
+ * 7.2 loop-aes: General observations.
+
+ One problem is that there are different versions of losetup around.
+ loop-aes is a patch for losetup. Possible problems and deviations
+ from cryptsetup option syntax include:
+
+ - Offsets specifed in bytes (cryptsetup: 512 byte sectors)
+
+ - The need to specify an IV offset
+
+ - Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
+
+ - Key size needs specifying (e.g. "-s 128" for 128 bit keys)
+
+ - Passphrase hash algorithm needs specifying
+
+ Also note that because plain dm-crypt and loop-aes format does not
+ have metadata, autodetection, while feasible in most cases, would
+ be a lot of work that nobody really wants to do. If you still have
+ the old set-up, using a verbosity option (-v) on mapping with the
+ old tool or having a look into the system logs after setup could
+ give you the information you need.
+
+
+ * 7.3 loop-aes patched into losetup on debian 5.x, kernel 2.6.32
+
+ In this case, the main problem seems to be that this variant of
+ losetup takes the offset (-o option) in bytes, while cryptsetup
+ takes it in sectors of 512 bytes each. Example: The losetupp
+ command
+
+ losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
+ mount /dev/loop0 mountpoint
+
+ translates to
+
+ cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
+ mount /dev/mapper/e1 mountpoint
+
+
+ * 7.4 loop-aes with 160 bit key
+
+ This seems to be sometimes used with twofish and blowfish and
+ represents a 160 bit ripemed160 hash output padded to 196 bit key
+ length. It seems the corresponding options for cryptsetup are
+
+ --cipher twofish-cbc-null -s 192 -h ripemd160:20
+
+
+8. Issues with Specific Versions of cryptsetup
- * When using the create command for plain dm-crypt with cryptsetup
- 1.1.x, the mapping is incompatible and my data is not accessible
- anymore!
+ * 8.1 When using the create command for plain dm-crypt with
+ cryptsetup 1.1.x, the mapping is incompatible and my data is not
+ accessible anymore!
With cryptsetup 1.1.x, the distro maintainer can define different
default encryption modes for LUKS and plain devices. You can check
problem.
- * cryptsetup on SLED 10 has problems...
+ * 8.2 cryptsetup on SLED 10 has problems...
SLED 10 is missing an essential kernel patch for dm-crypt, which
is broken in its kernel as a result. There may be a very old
projects / platform / upstream / cryptsetup.git / blobdiff