+ 1. General Questions2. Setup3. Common Problems4. Troubleshooting5.
+ Security Aspects6. Backup and Data Recovery7. Issues with Specific
+ Versions of cryptsetupA. Contributors
-1. General Questions
-2. Setup
-3. Common Problems
-4. Troubleshooting
-5. Security Aspects
-6. Backup and Data Recovery
-7. Issues with Specific Versions of cryptsetup
-A. Contributors
-
-
-1. General Questions
-
limitations imposed by the LUKS security model BEFORE you face such
a disaster!
limitations imposed by the LUKS security model BEFORE you face such
a disaster!
* Who wrote this?
Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
* Who wrote this?
Current FAQ maintainer is Arno Wagner <arno@wagner.name>. Other
this license is best suited for the purpose at hand and creates the
least problems.
this license is best suited for the purpose at hand and creates the
least problems.
* 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.
* 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?
Instructions on how to subscribe to the mailing-list are at on the
* Is there a mailing-list?
Instructions on how to subscribe to the mailing-list are at on the
with just the word "help" in the subject or message body. Make sure
to send it from your list address.
with just the word "help" in the subject or message body. Make sure
to send it from your list address.
* Can I encrypt an already existing, non-empty partition to use
LUKS?
* Can I encrypt an already existing, non-empty partition to use
LUKS?
For backup, plain GNU tar works well and backs up anything likely
to be in a filesystem.
For backup, plain GNU tar works well and backs up anything likely
to be in a filesystem.
* How do I use LUKS with a loop-device?
Just the same as with any block device. If you want, for example,
* How do I use LUKS with a loop-device?
Just the same as with any block device. If you want, for example,
Afterwards just use /dev/loop0 as a you would use a LUKS partition.
To unmap the file when done, use "losetup -d /dev/loop0".
Afterwards just use /dev/loop0 as a you would use a LUKS partition.
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
then complains about there not being a key-slot with that
passphrase?
* 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?
the already configured key-slots in order to be able to configure a
new key-slot.
the already configured key-slots in order to be able to configure a
new key-slot.
* How do I read a dm-crypt key from file?
Note that the file will still be hashed first, just like keyboard
* How do I read a dm-crypt key from file?
Note that the file will still be hashed first, just like keyboard
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?
What you really do here is to read a passphrase from file, just as
* How do I read a LUKS slot key from file?
What you really do here is to read a passphrase from file, just as
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?
The question you should ask yourself first, is why you would want
* How do I read the LUKS master key from file?
The question you should ask yourself first, is why you would want
good reason, please tell me. If I am convinced, I will add how to
do this here.
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?
A file-stored key or passphrase has the same security requirements
* What are the security requirements for a key read from file?
A file-stored key or passphrase has the same security requirements
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 provide its usual transactional guarantees?
* If I map a journaled file system using dm-crypt/LUKS, does it
still provide its usual transactional guarantees?
better with kernel 2.6.36 and the reworking of filesystem flush
locking. Kernel 2.6.37 is expected to improve this even further.
better with kernel 2.6.36 and the reworking of filesystem flush
locking. Kernel 2.6.37 is expected to improve this even further.
* Can I use LUKS or cryptsetup with a more secure (external) medium
for key storage, e.g. TPM or a smartcard?
* Can I use LUKS or cryptsetup with a more secure (external) medium
for key storage, e.g. TPM or a smartcard?
own tool that in turn gets the key from the more secure key
storage.
own tool that in turn gets the key from the more secure key
storage.
* Can I resize a dm-crypt or LUKS partition?
Yes, you can, as neither dm-crypt nor LUKS stores partition size.
* Can I resize a dm-crypt or LUKS partition?
Yes, you can, as neither dm-crypt nor LUKS stores partition size.
encrypted container sizes larger than 2TiB. Use aes-xts-plain64
for that.
encrypted container sizes larger than 2TiB. Use aes-xts-plain64
for that.
* My dm-crypt/LUKS mapping does not work! What general steps are
there to investigate the problem?
* 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
first. If not, you may want to check the following things.
If you get a specific error message, investigate what it claims
first. If not, you may want to check the following things.
- - Check that "/dev", including "/dev/mapper/control" is there. If it is
- missing, you may have a problem with the "/dev" tree itself or you
- may have broken udev rules.
+ - Check that "/dev", including "/dev/mapper/control" is there. If it
+ is missing, you may have a problem with the "/dev" tree itself or
+ you may have broken udev rules.
- - Check that you have the device mapper and the crypt target in your kernel.
- The output of "dmsetup targets" should list a "crypt" target. If it
- is not there or the command fails, add device mapper and
- crypt-target to the kernel.
-
- - Check that the hash-functions and ciphers you want to use are in the kernel.
- The output of "cat /proc/crypto" needs to list them.
+ - Check that you have the device mapper and the crypt target in your
+ kernel. The output of "dmsetup targets" should list a "crypt"
+ target. If it is not there or the command fails, add device mapper
+ and crypt-target to the kernel.
+ - Check that the hash-functions and ciphers you want to use are in
+ the kernel. The output of "cat /proc/crypto" needs to list them.
* My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
* My dm-crypt mapping suddenly stopped when upgrading cryptsetup.
from 1.0.x to 1.1.x). See under "Issues With Specific Versions of
cryptsetup".
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?
* When I call cryptsetup from cron/CGI, I get errors about unknown
features?
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 you have the
right version.
* Unlocking a LUKS device takes very long. Why?
The iteration time for a key-slot (see Section 5 for an explanation
* Unlocking a LUKS device takes very long. Why?
The iteration time for a key-slot (see Section 5 for an explanation
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.
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.
* "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device.
What is wrong?
* "blkid" sees a LUKS UUID and an ext2/swap UUID on the same device.
What is wrong?
dd if=/dev/zero of=<device> bs=512 seek=2 count=6
dd if=/dev/zero of=<device> bs=512 seek=2 count=6
* cryptsetup segfaults on Gentoo amd64 hardened ...
There seems to be some inteference between the hardening and and
* cryptsetup segfaults on Gentoo amd64 hardened ...
There seems to be some inteference between the hardening and and
other uses, you can apparently specify USE="dynamic" as compile
flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470
other uses, you can apparently specify USE="dynamic" as compile
flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470
* Can a bad RAM module cause problems?
* Can a bad RAM module cause problems?
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.
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?
First you should know that overclocking often makes memory problems
* How do I test RAM?
First you should know that overclocking often makes memory problems
recommend running the following procedure until the first error is
found:
recommend running the following procedure until the first error is
found:
- - Run Memtest86+ for one cycle
- - Run memterster for one cycle (shut down as many other applications as possible)
+ - Run Memtest86+ for one cycle
+
+ - Run memterster for one cycle (shut down as many other applications
+ as possible)
+
- Run Memtest86+ for 24h or more
- Run Memtest86+ for 24h or more
- - Run memtester for 24h or more
+
+ - Run memtester for 24h or more
+
If all that does not produce error messages, your RAM may be sound,
but I have had one weak bit that Memtest86+ needed around 60 hours
to find. If you can reproduce the original problem reliably, a good
If all that does not produce error messages, your RAM may be sound,
but I have had one weak bit that Memtest86+ needed around 60 hours
to find. If you can reproduce the original problem reliably, a good
the settings to the most conservative ones available and try with
that.
the settings to the most conservative ones available and try with
that.
* Should I initialize (overwrite) a new LUKS/dm-crypt partition?
* Should I initialize (overwrite) a new LUKS/dm-crypt partition?
cat /dev/zero > /dev/mapper/e1
dd if=/dev/zero of=/dev/mapper/e1
cat /dev/zero > /dev/mapper/e1
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
* How do I securely erase a LUKS (or other) partition?
For LUKS, if you are in a desperate hurry, overwrite the first few
dd_rescue -w /dev/zero /dev/sda10
dd_rescue -w /dev/zero /dev/sda10
* 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
* 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
after overwrites. Also write-head alignment issues can lead to
data not actually being deleted at all during overwrites.
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?
That depends. See next section.
* What about backup? Does it compromise security?
That depends. See next section.
* Why is all my data gone if I overwrite the LUKS header?
Overwriting the LUKS header in part or in full is the most common
* Why is all my data gone if I overwrite the LUKS header?
Overwriting the LUKS header in part or in full is the most common
the running system. Ask on the mailing-list on how to do that and
make sure nobody switches off the machine.
the running system. Ask on the mailing-list on how to do that and
make sure nobody switches off the machine.
* What is a "salt"?
A salt is a random key-grade value added to the passphrase before
* What is a "salt"?
A salt is a random key-grade value added to the passphrase before
reasonably-sized salt value (256 bit, e.g.) this is quite
infeasible.
reasonably-sized salt value (256 bit, e.g.) this is quite
infeasible.
* Is LUKS secure with a low-entropy (bad) passphrase?
This needs a bit of theory. The quality of your passphrase is
* Is LUKS secure with a low-entropy (bad) passphrase?
This needs a bit of theory. The quality of your passphrase is
either!) If you add 2 or 3 typos or switch some words around, then
this is good passphrase material.
either!) If you add 2 or 3 typos or switch some words around, then
this is good passphrase material.
* What is "iteration count" and why is decreasing it a bad idea?
Iteration count is the number of PBKDF2 iterations a passphrase is
* What is "iteration count" and why is decreasing it a bad idea?
Iteration count is the number of PBKDF2 iterations a passphrase is
vulnerable. Lowering it even further increases this danger
significantly.
vulnerable. Lowering it even further increases this danger
significantly.
* Is LUKS with default parameters less secure on a slow CPU?
Unfortunately, yes. However the only aspect affected is the
* Is LUKS with default parameters less secure on a slow CPU?
Unfortunately, yes. However the only aspect affected is the
passphrases on fast CPUs. Technology can do only so much to
compensate for problems in front of the keyboard.
passphrases on fast CPUs. Technology can do only so much to
compensate for problems in front of the keyboard.
* Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
The problem is that cbc-plain has a fingerprint vulnerability, where
* Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
The problem is that cbc-plain has a fingerprint vulnerability, where
encryption key as key. This makes the IV unpredictable without
knowing the encryption key and the watermarking attack fails.
encryption key as key. This makes the IV unpredictable without
knowing the encryption key and the watermarking attack fails.
* Are there any problems with "plain" IV? What is "plain64"?
First, "plain" and "plain64" are both not safe to use with CBC, see
* Are there any problems with "plain" IV? What is "plain64"?
First, "plain" and "plain64" are both not safe to use with CBC, see
volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
does not cause any performance penalty compared to "plain".
volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
does not cause any performance penalty compared to "plain".
* What about XTS mode?
XTS mode is potentially even more secure than cbc-essiv (but only if
* What about XTS mode?
XTS mode is potentially even more secure than cbc-essiv (but only if
LUKS and dm-crypt always use 512B blocks and the issue does not
apply.
LUKS and dm-crypt always use 512B blocks and the issue does not
apply.
-
-6. Backup and Data Recovery
-
+ 6. Backup and Data Recovery
* Does a backup compromise security?
* Does a backup compromise security?
scenarios. (Well, you can burn the tape if it is under your
control...)
scenarios. (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?
* What happens if I overwrite the start of a LUKS partition or
damage the LUKS header or key-slots?
unrecoverable in a cryptographically strong sense, changing about
4-6 bits in random locations of its 128kiB size is quite enough.
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?
I have not tried the different ways to do this, but very likely you
* What happens if I (quick) format a LUKS partition?
I have not tried the different ways to do this, but very likely you
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. You may also damage the key-slots
in part or in full. See also last item.
* What does the on-disk structure of dm-crypt look like?
There is none. dm-crypt takes a block device and gives encrypted
* What does the on-disk structure of dm-crypt look like?
There is none. dm-crypt takes a block device and gives encrypted
dm-crypt device, the damage will be limited to the are you
overwrote.
dm-crypt device, the damage will be limited to the are you
overwrote.
* What does the on-disk structure of LUKS look like?
A LUKS partition consists of a header, followed by 8 key-slot
* What does the on-disk structure of LUKS look like?
A LUKS partition consists of a header, followed by 8 key-slot
The exact specification of the format is here:
http://code.google.com/p/cryptsetup/wiki/Specification
The exact specification of the format is here:
http://code.google.com/p/cryptsetup/wiki/Specification
* How do I backup a LUKS header?
While you could just copy the appropriate number of bytes from the
* How do I backup a LUKS header?
While you could just copy the appropriate number of bytes from the
cryptsetup luksHeaderBackup --header-backup-file h_bak /dev/mapper/c1
cryptsetup luksHeaderBackup --header-backup-file h_bak /dev/mapper/c1
* How do I backup a LUKS partition?
You do a sector-image of the whole partition. This will contain the
* How do I backup a LUKS partition?
You do a sector-image of the whole partition. This will contain the
a sector image of a partition. Note that compression is
ineffective for encrypted data, hence it does not sense to use it.
a sector image of a partition. Note that compression is
ineffective for encrypted data, hence it does not sense to use it.
* Do I need a backup of the full partition? Would the header and
key-slots not be enough?
* Do I need a backup of the full partition? Would the header and
key-slots not be enough?
It is highly advisable to have a complete backup to protect against
this case.
It is highly advisable to have a complete backup to protect against
this case.
* Are there security risks from a backup of the LUKS header or a
whole LUKS partition?
* Are there security risks from a backup of the LUKS header or a
whole LUKS partition?
access, especially if you only erased the LUKS header and not the
whole partition.
access, especially if you only erased the LUKS header and not the
whole partition.
* I think this is overly complicated. Is there an alternative?
Yes, you can use plain dm-crypt. It does not allow multiple
* I think this is overly complicated. Is there an alternative?
Yes, you can use plain dm-crypt. It does not allow multiple
exactly the overwritten parts are lost (rounded up to sector
borders).
exactly the overwritten parts are lost (rounded up to sector
borders).
-
-7. Issues with Specific Versions of cryptsetup
-
+ 7. 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
* When using the create command for plain dm-crypt with cryptsetup
1.1.x, the mapping is incompatible and my data is not accessible
LUKS stores cipher and mode in the metadata on disk, avoiding this
problem.
LUKS stores cipher and mode in the metadata on disk, avoiding this
problem.
* cryptsetup on SLED 10 has problems...
SLED 10 is missing an essential kernel patch for dm-crypt, which
* cryptsetup on SLED 10 has problems...
SLED 10 is missing an essential kernel patch for dm-crypt, which
A. Contributors In no particular order:
- Arno Wagner
A. Contributors In no particular order:
- Arno Wagner
projects / platform / upstream / cryptsetup.git / commitdiff