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,
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
section on Backup and Data Recovery. By far the most questions on
installers after a complete backup of all LUKS containers has been
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
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
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 characters can, e.g. be found on
+ 128 ASCII caracters can, e.g. be found on
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
LUKS partitions in a way that several people mistook for an offer
http://launchpad.net/bugs/420080
- * Who wrote this?
+ * 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
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
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
- 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
the sectors of the device, with a direct 1:1 mapping between
encrypted and decrypted sectors.
properties like key-slot diffusion and salts, etc..
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
- 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.
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 passphrases, and others.
+ 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
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
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?
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
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.
- * 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
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
- passphrase for each individual disk and RAID autodetection will
- not work anymore. Therefore it is better to encrypt the RAID
- device, e.g. /dev/dm0 .
+ 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 .
- * 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
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
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
locking. Kernel 2.6.38 is expected to have more improvements here.
- * 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
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 typically works. This
+ resize an encrypted partition is slow, but typicaly works. This
will not change the size of the filesystem hidden under the
encryption though.
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
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
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 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
currently not quite clear for an encrypted root filesystem. For
other uses, you can apparently specify USE="dynamic" as compile
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
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 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
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
combined with software RAID. In particular the combination RAID5 +
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
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
you can just refuse to hand over the keys, no harm done. In some
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
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?
+ * 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 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,
just be generous and overwrite the first 10MB or so, it will likely
still be fast enough. A single overwrite with zeros should be
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 research results on erasing SSDs and
- FLASH drives:
+ 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.
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 or FLASH drive (USB stick), you
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
reason why access to LUKS containers is lost permanently.
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
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
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
dm-crypt, the only way to be secure is to use a high entropy
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
protection for low-entropy passphrase or master-key. All other
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
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
sources).
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 secure to use with CBC,
see previous FAQ item.
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
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%
per year, which is high enough to worry about. There is some
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
+ impossible. This is a frequent occuurence. 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 inaccessible. See more below on LUKS
+ LUKS partition permanently inacessible. See more below on LUKS
header damage.
So, data-backup in some form is non-optional. For LUKS, you may
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
start of the LUKS partition, the best way is to use command option
cryptsetup luksHeaderRestore --header-backup-file h <device>
- * How do I test a LUKS header?
+ * 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 also use the more general test
+ exit-status. You can alos use the more general test
blkid -p <device>
"-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
filesystem backup of the contents of the partition. The sector
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
key. The verify will only output errors. Use "tar dvjf -" to get
an asymmetric key if you have one and have a backup of the secret
key that belongs to it.
- A second option for a filesystem\7f-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
+ 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.
- * 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?
Backup protects you against two things: Disk loss or corruption
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
a key derived from the master key of an already decrypted device.
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
going to eventually lose your encrypted data.
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
+ filesystem level backup methid 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 data) or
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
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, 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.
- * 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
header, but the container is still mapped, i.e. "luksOpen"ed. It
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 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
key (it prints the command for that though):
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
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
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
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
- alignment is not recommended.
+ alignment is not recomended.
That said, with default parameters, the data area starts at
exactly 2MB offset (at 0x101000 for cryptsetup versions before
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 container size of 260kiB + 512B or
+ 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?
Not really. Encryption comes at a price. You can use plain
dm-crypt to simplify things a bit. It does not allow multiple
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
on-disk formats created by tools like loop-aes patched into
- losetup. This sometimes works and sometimes does not. This
- section collects insights into what works, what does not and where
- more information is required.
+ 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 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.
loop-aes is a patch for losetup. Possible problems and deviations
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
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
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
mount /dev/loop0 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
represents a 160 bit ripemed160 hash output padded to 196 bit key
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 / commitdiff