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@@ -14,7 +14,7 @@ A. Contributors
1. General Questions 1. General Questions
* What is this? * 1.1 What is this?
This is the FAQ (Frequently Asked Questions) for cryptsetup. It This is the FAQ (Frequently Asked Questions) for cryptsetup. It
covers Linux disk encryption with plain dm-crypt (one passphrase, covers Linux disk encryption with plain dm-crypt (one passphrase,
@@ -25,7 +25,7 @@ A. Contributors
http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions
* WARNINGS * 1.2 WARNINGS
ATTENTION: If you are going to read just one thing, make it the 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 section on Backup and Data Recovery. By far the most questions on
@@ -49,7 +49,7 @@ A. Contributors
installers after a complete backup of all LUKS containers has been installers after a complete backup of all LUKS containers has been
made. made.
NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from 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 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 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 pre-existing LUKS container on the target), as it assumes it is
@@ -73,11 +73,11 @@ A. Contributors
representation. Other characters may have different encoding representation. Other characters may have different encoding
depending on system configuration and your passphrase will not depending on system configuration and your passphrase will not
work with a different encoding. A table of the standardized first work with a different encoding. A table of the standardized first
128 ASCII characters can, e.g. be found on 128 ASCII caracters can, e.g. be found on
http://en.wikipedia.org/wiki/ASCII http://en.wikipedia.org/wiki/ASCII
* System Specific warnings * 1.3 System Specific warnings
- Ubuntu as of 4/2011: It seems the installer offers to create - Ubuntu as of 4/2011: It seems the installer offers to create
LUKS partitions in a way that several people mistook for an offer LUKS partitions in a way that several people mistook for an offer
@@ -90,7 +90,7 @@ A. Contributors
http://launchpad.net/bugs/420080 http://launchpad.net/bugs/420080
* Who wrote this? * 1.4 Who wrote this?
Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
contributors are listed at the end. If you want to contribute, send contributors are listed at the end. If you want to contribute, send
@@ -113,14 +113,14 @@ A. Contributors
least problems. 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/ There is the project website at http://code.google.com/p/cryptsetup/
Please do not post questions there, nobody will read them. Use Please do not post questions there, nobody will read them. Use
the mailing-list instead. 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 Instructions on how to subscribe to the mailing-list are at on the
project website. People are generally helpful and friendly on the project website. People are generally helpful and friendly on the
@@ -143,10 +143,10 @@ A. Contributors
2. Setup 2. Setup
* What is the difference between "plain" and LUKS format? * 2.1 What is the difference between "plain" and LUKS format?
Plain format is just that: It has no metadata on disk, reads all Plain format is just that: It has no metadata on disk, reads all
parameters from the commandline (or the defaults), derives a paramters from the commandline (or the defaults), derives a
master-key from the passphrase and then uses that to de-/encrypt master-key from the passphrase and then uses that to de-/encrypt
the sectors of the device, with a direct 1:1 mapping between the sectors of the device, with a direct 1:1 mapping between
encrypted and decrypted sectors. encrypted and decrypted sectors.
@@ -173,15 +173,15 @@ A. Contributors
properties like key-slot diffusion and salts, etc.. properties like key-slot diffusion and salts, etc..
LUKS format uses a metadata header and 8 key-slot areas that are LUKS format uses a metadata header and 8 key-slot areas that are
being placed at the beginning of the disk, see below under "What being placed ath the begining of the disk, see below under "What
does the LUKS on-disk format looks like?". The passphrases are used does the LUKS on-disk format looks like?". The passphrases are used
to decrypt a single master key that is stored in the anti-forensic to decryt a single master key that is stored in the anti-forensic
stripes. stripes.
Advantages are a higher usability, automatic configuration of Advantages are a higher usability, automatic configuration of
non-default crypto parameters, defenses against low-entropy non-default crypto parameters, defenses against low-entropy
passphrases like salting and iterated PBKDF2 passphrase hashing, passphrases like salting and iterated PBKDF2 passphrase hashing,
the ability to change passphrases, and others. the ability to change passhrases, and others.
Disadvantages are that it is readily obvious there is encrypted Disadvantages are that it is readily obvious there is encrypted
data on disk (but see side note above) and that damage to the data on disk (but see side note above) and that damage to the
@@ -193,7 +193,8 @@ A. Contributors
non-default XTS mode). non-default XTS mode).
* Can I encrypt an already existing, non-empty partition to use LUKS? * 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 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 this is to make a backup of the device in question, securely wipe
@@ -207,7 +208,7 @@ A. Contributors
to be in a filesystem. 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?
This can be very handy for experiments. Setup is just the same as 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 with any block device. If you want, for example, to use a 100MiB
@@ -221,7 +222,7 @@ A. Contributors
To unmap the file when done, use "losetup -d /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 then complains about there not being a key-slot with that
passphrase? passphrase?
@@ -233,17 +234,17 @@ A. Contributors
new key-slot. new key-slot.
* Encryption on top of RAID or the other way round? * 2.5 Encrytion on top of RAID or the other way round?
Unless you have special needs, place encryption between RAID and Unless you have special needs, place encryption between RAID and
filesystem, i.e. encryption on top of RAID. You can do it the other 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 way round, but you have to be aware that you then need to give the
passphrase for each individual disk and RAID autodetection will pasphrase for each individual disk and RAID autotetection will not
not work anymore. Therefore it is better to encrypt the RAID work anymore. Therefore it is better to encrypt the RAID device,
device, e.g. /dev/dm0 . e.g. /dev/dm0 .
* How do I read a dm-crypt key from file? * 2.6 How do I read a dm-crypt key from file?
Note that the file will still be hashed first, just like keyboard Note that the file will still be hashed first, just like keyboard
input. Use the --key-file option, like this: input. Use the --key-file option, like this:
@@ -251,7 +252,7 @@ A. Contributors
cryptsetup create --key-file keyfile e1 /dev/loop0 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 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 you would with manual entry of a passphrase for a key-slot. You can
@@ -277,7 +278,7 @@ A. Contributors
cryptsetup luksOpen --key-file keyfile /dev/loop0 e1 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 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 do this. The only legitimate reason I can think of is if you want
@@ -288,7 +289,7 @@ A. Contributors
do this here. 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 A file-stored key or passphrase has the same security requirements
as one entered interactively, however you can use random bytes and as one entered interactively, however you can use random bytes and
@@ -300,8 +301,8 @@ A. Contributors
head -c 256 /dev/random > keyfile head -c 256 /dev/random > keyfile
* If I map a journaled file system using dm-crypt/LUKS, does it still * 2.10 If I map a journaled file system using dm-crypt/LUKS, does it
provide its usual transactional guarantees? still provide its usual transactional guarantees?
As far as I know it does (but I may be wrong), but please note that 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 these "guarantees" are far weaker than they appear to be. For
@@ -323,8 +324,8 @@ A. Contributors
locking. Kernel 2.6.38 is expected to have more improvements here. locking. Kernel 2.6.38 is expected to have more improvements here.
* Can I use LUKS or cryptsetup with a more secure (external) medium * 2.11 Can I use LUKS or cryptsetup with a more secure (external)
for key storage, e.g. TPM or a smartcard? medium for key storage, e.g. TPM or a smartcard?
Yes, see the answers on using a file-supplied key. You do have to Yes, see the answers on using a file-supplied key. You do have to
write the glue-logic yourself though. Basically you can have write the glue-logic yourself though. Basically you can have
@@ -333,7 +334,7 @@ A. Contributors
storage. 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. Yes, you can, as neither dm-crypt nor LUKS stores partition size.
Whether you should is a different question. Personally I recommend Whether you should is a different question. Personally I recommend
@@ -343,7 +344,7 @@ A. Contributors
LUKS container does not resize the filesystem in it. The backup is LUKS container does not resize the filesystem in it. The backup is
really non-optional here, as a lot can go wrong, resulting in really non-optional here, as a lot can go wrong, resulting in
partial or complete data loss. Using something like gparted to partial or complete data loss. Using something like gparted to
resize an encrypted partition is slow, but typically works. This resize an encrypted partition is slow, but typicaly works. This
will not change the size of the filesystem hidden under the will not change the size of the filesystem hidden under the
encryption though. encryption though.
@@ -356,7 +357,7 @@ A. Contributors
3. Common Problems 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? there to investigate the problem?
If you get a specific error message, investigate what it claims If you get a specific error message, investigate what it claims
@@ -375,15 +376,15 @@ A. Contributors
the kernel. The output of "cat /proc/crypto" needs to list them. 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 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 from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
cryptsetup". cryptsetup".
* When I call cryptsetup from cron/CGI, I get errors about unknown * 3.3 When I call cryptsetup from cron/CGI, I get errors about
features? unknown features?
If you get errors about unknown parameters or the like that are not If you get errors about unknown parameters or the like that are not
present when cryptsetup is called from the shell, make sure you present when cryptsetup is called from the shell, make sure you
@@ -395,7 +396,7 @@ A. Contributors
version gets called. 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 The iteration time for a key-slot (see Section 5 for an explanation
what iteration does) is calculated when setting a passphrase. By what iteration does) is calculated when setting a passphrase. By
@@ -424,8 +425,8 @@ A. Contributors
matter. matter.
* "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device. * 3.5 "blkid" sees a LUKS UUID and an ext2/swap UUID on the same
What is wrong? device. What is wrong?
Some old versions of cryptsetup have a bug where the header does Some old versions of cryptsetup have a bug where the header does
not get completely wiped during LUKS format and an older ext2/swap not get completely wiped during LUKS format and an older ext2/swap
@@ -438,9 +439,9 @@ A. Contributors
cryptsetup luksHeaderRestore --header-backup-file <file> <device> cryptsetup luksHeaderRestore --header-backup-file <file> <device>
* cryptsetup segfaults on Gentoo amd64 hardened ... * 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
There seems to be some interference between the hardening and and There seems to be some inteference between the hardening and and
the way cryptsetup benchmarks PBKDF2. The solution to this is the way cryptsetup benchmarks PBKDF2. The solution to this is
currently not quite clear for an encrypted root filesystem. For currently not quite clear for an encrypted root filesystem. For
other uses, you can apparently specify USE="dynamic" as compile other uses, you can apparently specify USE="dynamic" as compile
@@ -450,17 +451,18 @@ A. Contributors
4. Troubleshooting 4. Troubleshooting
* I get the error "LUKS keyslot x is invalid." What does that mean? * 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 This means that the given keyslot has an offset that points
outside the valid keyslot area. Typically, the reason is a outside the valid keyslot area. Typically, the reason is a
corrupted LUKS header because something was written to the start of corrupted LUKS header because something was written to the start of
the device the LUKS container is on. Refer to Section "Backup and the device the LUKS contaner is on. Refer to Section "Backup and
Data Recovery" and ask on the mailing list if you have trouble Data Recovery" and ask on the mailing list if you have trouble
diagnosing and (if still possible) repairing this. diagnosing and (if still possible) repairing this.
* Can a bad RAM module cause problems? * 4.2 Can a bad RAM module cause problems?
LUKS and dm-crypt can give the RAM quite a workout, especially when LUKS and dm-crypt can give the RAM quite a workout, especially when
combined with software RAID. In particular the combination RAID5 + combined with software RAID. In particular the combination RAID5 +
@@ -502,7 +504,7 @@ A. Contributors
did a verify. did a verify.
* How do I test RAM? * 4.3 How do I test RAM?
First you should know that overclocking often makes memory First you should know that overclocking often makes memory
problems worse. So if you overclock (which I strongly recommend problems worse. So if you overclock (which I strongly recommend
@@ -540,7 +542,7 @@ A. Contributors
5. Security Aspects 5. Security Aspects
* Is LUKS insecure? Everybody can see I have encrypted data! * 5.1 Is LUKS insecure? Everybody can see I have encrypted data!
In practice it does not really matter. In most civilized countries 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 you can just refuse to hand over the keys, no harm done. In some
@@ -565,7 +567,7 @@ A. Contributors
difference between "plain" and LUKS format?" difference between "plain" and LUKS format?"
* Should I initialize (overwrite) a new LUKS/dm-crypt partition? * 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 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 still be there. If the old data is sensitive, you should overwrite
@@ -584,14 +586,14 @@ A. Contributors
dd if=/dev/zero of=/dev/mapper/e1 dd if=/dev/zero of=/dev/mapper/e1
* How do I securely erase a LUKS (or other) partition? * 5.3 How do I securely erase a LUKS (or other) partition?
For LUKS, if you are in a desperate hurry, overwrite the LUKS For LUKS, if you are in a desperate hurry, overwrite the LUKS
header and key-slot area. This means overwriting the first header and key-slot area. This means overwriting the first
(keyslots x stripes x keysize) + offset bytes. For the default (keyslots x stripes x keysize) + offset bytes. For the default
parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the 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 LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
512 bit key) this is 2MiB. (The different offset stems from 512 bit key) this is 2MiB. (The diferent offset stems from
differences in the sector alignment of the key-slots.) If in doubt, 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 just be generous and overwrite the first 10MB or so, it will likely
still be fast enough. A single overwrite with zeros should be still be fast enough. A single overwrite with zeros should be
@@ -613,8 +615,8 @@ A. Contributors
in this regard. Still, due to the anti-forensic properties of the 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 LUKS key-slots, a single overwrite of an SSD or FLASH drive could
be enough. If in doubt, use physical destruction in addition. Here be enough. If in doubt, use physical destruction in addition. Here
is a link to some current research results on erasing SSDs and is a link to some current reseach results on erasing SSDs and FLASH
FLASH drives: drives:
http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
Keep in mind to also erase all backups. Keep in mind to also erase all backups.
@@ -625,7 +627,7 @@ A. Contributors
dd_rescue -w /dev/zero /dev/sde1 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 That depends on the medium it is stored on. For HDD and SSD, use
overwrite with zeros. For an SSD or FLASH drive (USB stick), you overwrite with zeros. For an SSD or FLASH drive (USB stick), you
@@ -643,12 +645,13 @@ A. Contributors
lead to data not actually being deleted at all during overwrites. 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 permanently 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 Overwriting the LUKS header in part or in full is the most common
reason why access to LUKS containers is lost permanently. reason why access to LUKS containers is lost permanently.
@@ -675,7 +678,7 @@ A. Contributors
a mapped LUKS container?" in Section "Backup and Data Recovery". 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 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 it is processed. It is not kept secret. The reason for using salts
@@ -703,7 +706,7 @@ A. Contributors
infeasible. 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 Note: You should only use the 94 printable characters from 7 bit
ASCII code to prevent your passphrase from failing when the ASCII code to prevent your passphrase from failing when the
@@ -756,7 +759,7 @@ A. Contributors
this is good passphrase material. 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 Iteration count is the number of PBKDF2 iterations a passphrase is
put through before it is used to unlock a key-slot. Iterations are put through before it is used to unlock a key-slot. Iterations are
@@ -821,14 +824,36 @@ A. Contributors
this danger significantly. this danger significantly.
* What about iteration count with plain dm-crypt? * 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?
Simple: There is none. There is also no salting. If you use plain 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 dm-crypt, the only way to be secure is to use a high entropy
passphrase. If in doubt, use LUKS instead. passphrase. If in doubt, use LUKS instead.
* Is LUKS with default parameters less secure on a slow CPU? * 5.12 Is LUKS with default parameters less secure on a slow CPU?
Unfortunately, yes. However the only aspect affected is the Unfortunately, yes. However the only aspect affected is the
protection for low-entropy passphrase or master-key. All other protection for low-entropy passphrase or master-key. All other
@@ -838,7 +863,7 @@ A. Contributors
to give it low entropy. One possibility is to supply the master key to give it low entropy. One possibility is to supply the master key
yourself. If that key is low-entropy, then you get what you yourself. If that key is low-entropy, then you get what you
deserve. The other known possibility is to use /dev/urandom for deserve. The other known possibility is to use /dev/urandom for
key generation in an entropy-starved situation (e.g. automatic key generation in an entropy-startved situation (e.g. automatic
installation on an embedded device without network and other entropy installation on an embedded device without network and other entropy
sources). sources).
@@ -852,7 +877,7 @@ A. Contributors
compensate for problems in front of the keyboard. 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 The problem is that cbc-plain has a fingerprint vulnerability, where
a specially crafted file placed into the crypto-container can be a specially crafted file placed into the crypto-container can be
@@ -880,7 +905,7 @@ A. Contributors
knowing the encryption key and the watermarking attack fails. 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 secure to use with CBC, First, "plain" and "plain64" are both not secure to use with CBC,
see previous FAQ item. see previous FAQ item.
@@ -896,7 +921,7 @@ A. Contributors
does not cause any performance penalty compared to "plain". 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 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 cbc-essiv is insecure in your scenario). It is a NIST standard and
@@ -918,7 +943,7 @@ A. Contributors
6. Backup and Data Recovery 6. Backup and Data Recovery
* Why do I need Backup? * 6.1 Why do I need Backup?
First, disks die. The rate for well-treated (!) disk is about 5% First, disks die. The rate for well-treated (!) disk is about 5%
per year, which is high enough to worry about. There is some per year, which is high enough to worry about. There is some
@@ -927,10 +952,10 @@ A. Contributors
Second, for LUKS, if anything damages the LUKS header or the Second, for LUKS, if anything damages the LUKS header or the
key-stripe area then decrypting the LUKS device can become key-stripe area then decrypting the LUKS device can become
impossible. This is a frequent occurrence. For example an impossible. This is a frequent occuurence. For example an
accidental format as FAT or some software overwriting the first accidental format as FAT or some software overwriting the first
sector where it suspects a partition boot sector typically makes a sector where it suspects a partition boot sector typically makes a
LUKS partition permanently inaccessible. See more below on LUKS LUKS partition permanently inacessible. See more below on LUKS
header damage. header damage.
So, data-backup in some form is non-optional. For LUKS, you may So, data-backup in some form is non-optional. For LUKS, you may
@@ -938,7 +963,7 @@ A. Contributors
only needs an update if you change passphrases. only needs an update if you change passphrases.
* How do I backup a LUKS header? * 6.2 How do I backup a LUKS header?
While you could just copy the appropriate number of bytes from the 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 start of the LUKS partition, the best way is to use command option
@@ -954,14 +979,14 @@ A. Contributors
cryptsetup luksHeaderRestore --header-backup-file h <device> cryptsetup luksHeaderRestore --header-backup-file h <device>
* How do I test a LUKS header? * 6.3 How do I test a LUKS header?
Use Use
cryptsetup -v isLuks <device> cryptsetup -v isLuks <device>
on the device. Without the "-v" it just signals its result via on the device. Without the "-v" it just signals its result via
exit-status. You can also use the more general test exit-status. You can alos use the more general test
blkid -p <device> blkid -p <device>
@@ -969,7 +994,7 @@ A. Contributors
"-p" for old versions of blkid that do not support it. "-p" for old versions of blkid that do not support it.
* How do I backup a LUKS or dm-crypt partition? * 6.4 How do I backup a LUKS or dm-crypt partition?
There are two options, a sector-image and a plain file or There are two options, a sector-image and a plain file or
filesystem backup of the contents of the partition. The sector filesystem backup of the contents of the partition. The sector
@@ -1003,7 +1028,7 @@ A. Contributors
cat backup.tbz2.gpg | gpg - | tar djf - cat backup.tbz2.gpg | gpg - | tar djf -
Note: Always verify backups, especially encrypted ones. Note: Allways verify backups, especially encrypted ones.
In both cases GnuPG will ask you interactively for your symmetric In both cases GnuPG will ask you interactively for your symmetric
key. The verify will only output errors. Use "tar dvjf -" to get key. The verify will only output errors. Use "tar dvjf -" to get
@@ -1015,13 +1040,13 @@ A. Contributors
an asymmetric key if you have one and have a backup of the secret an asymmetric key if you have one and have a backup of the secret
key that belongs to it. key that belongs to it.
A second option for a filesystem-level backup that can be used A second option for a filestem-level backup that can be used when
when the backup is also on local disk (e.g. an external USB drive) the backup is also on local disk (e.g. an external USB drive) is
is to use a LUKS container there and copy the files to be backed up to use a LUKS container there and copy the files to be backed up
between both mounted containers. Also see next item. between both mounted containers. Also see next item.
* Do I need a backup of the full partition? Would the header and * 6.5 Do I need a backup of the full partition? Would the header and
key-slots not be enough? key-slots not be enough?
Backup protects you against two things: Disk loss or corruption Backup protects you against two things: Disk loss or corruption
@@ -1037,7 +1062,7 @@ A. Contributors
backup to protect against this case. backup to protect against this case.
* *What do I need to backup if I use "decrypt_derived"? * *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 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. a key derived from the master key of an already decrypted device.
@@ -1054,7 +1079,7 @@ A. Contributors
device with the new LUKS header. device with the new LUKS header.
* Does a backup compromise security? * 6.7 Does a backup compromise security?
Depends on how you do it. However if you do not have one, you are Depends on how you do it. However if you do not have one, you are
going to eventually lose your encrypted data. going to eventually lose your encrypted data.
@@ -1075,7 +1100,7 @@ A. Contributors
In both cases, there is an additional (usually small) risk with In both cases, there is an additional (usually small) risk with
binary backups: An attacker can see how many sectors and which binary backups: An attacker can see how many sectors and which
ones have been changed since the backup. To prevent this, use a ones have been changed since the backup. To prevent this, use a
filesystem level backup method that encrypts the whole backup in filesystem level backup methid that encrypts the whole backup in
one go, e.g. as described above with tar and GnuPG. one go, e.g. as described above with tar and GnuPG.
My personal advice is to use one USB disk (low value data) or My personal advice is to use one USB disk (low value data) or
@@ -1090,8 +1115,8 @@ A. Contributors
is under your control...) is under your control...)
* What happens if I overwrite the start of a LUKS partition or damage * 6.8 What happens if I overwrite the start of a LUKS partition or
the LUKS header or key-slots? damage the LUKS header or key-slots?
There are two critical components for decryption: The salt values There are two critical components for decryption: The salt values
in the header itself and the key-slots. If the salt values are in the header itself and the key-slots. If the salt values are
@@ -1104,16 +1129,16 @@ A. Contributors
4-6 bits in random locations of its 128kiB size is quite enough. 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 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 will have written a new boot-sector, which in turn overwrites the
LUKS header, including the salts, making your data permanently LUKS header, including the salts, making your data permanently
irretrievable, unless you have a LUKS header backup. You may also irretrivable, unless you have a LUKS header backup. You may also
damage the key-slots in part or in full. See also last item. damage the key-slots in part or in full. See also last item.
* How do I recover the master key from a mapped LUKS container? * 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 This is typically only needed if you managed to damage your LUKS
header, but the container is still mapped, i.e. "luksOpen"ed. It header, but the container is still mapped, i.e. "luksOpen"ed. It
@@ -1131,7 +1156,7 @@ A. Contributors
key afterwards. Changing the master key requires a full data key afterwards. Changing the master key requires a full data
backup, luksFormat and then restore of the backup. backup, luksFormat and then restore of the backup.
First, there is a script by Milan that automates the whole First, there is a script by Milan that automatizes the whole
process, except generating a new LUKS header with the old master process, except generating a new LUKS header with the old master
key (it prints the command for that though): key (it prints the command for that though):
@@ -1183,7 +1208,7 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
You may want to disable the old one afterwards. You may want to disable the old one afterwards.
* What does the on-disk structure of dm-crypt look like? * 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 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 access to each of its blocks with a key derived from the passphrase
@@ -1196,7 +1221,7 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
overwrote. 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 A LUKS partition consists of a header, followed by 8 key-slot
descriptors, followed by 8 key slots, followed by the encrypted descriptors, followed by 8 key slots, followed by the encrypted
@@ -1235,13 +1260,13 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
http://code.google.com/p/cryptsetup/wiki/Specification http://code.google.com/p/cryptsetup/wiki/Specification
* What is the smallest possible LUKS container? * 6.13 What is the smallest possible LUKS container?
Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
modern Linux partitioning tools that also align to 1MB, this will modern Linux partitioning tools that also align to 1MB, this will
result in alignment to 2k sectors and typical Flash/SSD sectors, result in aligmnet to 2k secors and typical Flash/SSD sectors,
which is highly desirable for a number of reasons. Changing the which is highly desirable for a number of reasons. Changing the
alignment is not recommended. alignment is not recomended.
That said, with default parameters, the data area starts at That said, with default parameters, the data area starts at
exactly 2MB offset (at 0x101000 for cryptsetup versions before exactly 2MB offset (at 0x101000 for cryptsetup versions before
@@ -1283,11 +1308,11 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0 cryptsetup luksFormat -c blowfish -s 64 --align-payload=8 /dev/loop0
This results in a data offset of 0x41000, i.e. 260kiB or 266240 This results in a data offset of 0x41000, i.e. 260kiB or 266240
bytes, with a minimal LUKS container size of 260kiB + 512B or bytes, with a minimal LUKS conatiner size of 260kiB + 512B or
266752 bytes. 266752 bytes.
* I think this is overly complicated. Is there an alternative? * 6.14 I think this is overly complicated. Is there an alternative?
Not really. Encryption comes at a price. You can use plain Not really. Encryption comes at a price. You can use plain
dm-crypt to simplify things a bit. It does not allow multiple dm-crypt to simplify things a bit. It does not allow multiple
@@ -1300,27 +1325,27 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
7. Interoperability with other Disk Encryption Tools 7. Interoperability with other Disk Encryption Tools
* What is this section about? * 7.1 What is this section about?
Cryptsetup for plain dm-crypt can be used to access a number of Cryptsetup for plain dm-crypt can be used to access a number of
on-disk formats created by tools like loop-aes patched into on-disk formats created by tools like loop-aes patched into
losetup. This sometimes works and sometimes does not. This losetup. This somtimes works and sometimes does not. This section
section collects insights into what works, what does not and where collects insights into what works, what does not and where more
more information is required. information is required.
Additional information may be found in the mailing-list archives, Additional information may be found in the mailing-list archives,
mentioned at the start of this FAQ document. If you have a mentioned at the start of this FAQ document. If you have a
solution working that is not yet documented here and think a wider solution working that is not yet documented here and think a wider
audience may be interested, please email the FAQ maintainer. audience may be intertested, please email the FAQ maintainer.
* loop-aes: General observations. * 7.2 loop-aes: General observations.
One problem is that there are different versions of losetup around. One problem is that there are different versions of losetup around.
loop-aes is a patch for losetup. Possible problems and deviations loop-aes is a patch for losetup. Possible problems and deviations
from cryptsetup option syntax include: from cryptsetup option syntax include:
- Offsets specified in bytes (cryptsetup: 512 byte sectors) - Offsets specifed in bytes (cryptsetup: 512 byte sectors)
- The need to specify an IV offset - The need to specify an IV offset
@@ -1338,11 +1363,12 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
give you the information you need. give you the information you need.
* loop-aes patched into losetup on Debian 5.x, kernel 2.6.32 * 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 In this case, the main problem seems to be that this variant of
losetup takes the offset (-o option) in bytes, while cryptsetup losetup takes the offset (-o option) in bytes, while cryptsetup
takes it in sectors of 512 bytes each. Example: The losetup command takes it in sectors of 512 bytes each. Example: The losetupp
command
losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1 losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
mount /dev/loop0 mountpoint mount /dev/loop0 mountpoint
@@ -1353,7 +1379,7 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
mount /dev/mapper/e1 mountpoint mount /dev/mapper/e1 mountpoint
* loop-aes with 160 bit key * 7.4 loop-aes with 160 bit key
This seems to be sometimes used with twofish and blowfish and This seems to be sometimes used with twofish and blowfish and
represents a 160 bit ripemed160 hash output padded to 196 bit key represents a 160 bit ripemed160 hash output padded to 196 bit key
@@ -1365,9 +1391,9 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
8. Issues with Specific Versions of cryptsetup 8. Issues with Specific Versions of cryptsetup
* When using the create command for plain dm-crypt with cryptsetup * 8.1 When using the create command for plain dm-crypt with
1.1.x, the mapping is incompatible and my data is not accessible cryptsetup 1.1.x, the mapping is incompatible and my data is not
anymore! accessible anymore!
With cryptsetup 1.1.x, the distro maintainer can define different With cryptsetup 1.1.x, the distro maintainer can define different
default encryption modes for LUKS and plain devices. You can check default encryption modes for LUKS and plain devices. You can check
@@ -1386,7 +1412,7 @@ http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-ac
problem. 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 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 is broken in its kernel as a result. There may be a very old