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
installers after a complete backup of all LUKS containers has been
made.
- NO WARNING ON NON-INERACTIVE FORMAT: If you feed cryptsetup from
+ NO WARNING ON NON-INTERACTIVE 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 caracters can, e.g. be found on
+ 128 ASCII characters can, e.g. be found on
http://en.wikipedia.org/wiki/ASCII
* 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
+ parameters 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 ath the begining of the disk, see below under "What
+ being placed at the beginning 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
+ to decrypt 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.
+ the ability to change passphrases, 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
new key-slot.
- * 2.5 Encrytion on top of RAID or the other way round?
+ * 2.5 Encryption 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 .
+ 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 .
* 2.6 How do I read a dm-crypt key from file?
* 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 may 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:
+
+ http://lwn.net/Articles/400541/
- Personally, I have several instances of ext3 on dm-crypt and have
- not noticed any specific problems.
+ 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.
- 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 2.6.38 is expected to have more improvements here.
+ 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.
* 2.11 Can I use LUKS or cryptsetup with a more secure (external)
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
+ resize an encrypted partition is slow, but typically works. This
will not change the size of the filesystem hidden under the
encryption though.
* 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
- There seems to be some inteference between the hardening and and
+ There seems to be some interference 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
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
+ the device the LUKS container 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.
- Run Memtest86+ for one cycle
- - Run memterster for one cycle (shut down as many other applications
+ - Run memtester for one cycle (shut down as many other applications
as possible)
- Run Memtest86+ for 24h or more
(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
+ 512 bit key) this is 2MiB. (The different 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 reseach results on erasing SSDs and FLASH
- drives:
+ is a link to some current research 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.
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
+ However, the publication usually referred 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
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
+ Assuming 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
+ want. The only protection against a resourceful attacker is a
high-entropy passphrase, see items 5.8 and 5.9.
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-startved situation (e.g. automatic
+ key generation in an entropy-starved situation (e.g. automatic
installation on an embedded device without network and other entropy
sources).
* 5.13 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
+ Note: This item applies both to plain dm-crypt and to LUKS
+
The problem is that cbc-plain has a fingerprint vulnerability, where
a specially crafted file placed into the crypto-container can be
recognized from the outside. The issue here is that for cbc-plain
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.
+
+
+ * 5.16 What about Plausible Deniability?
+
+ First let me attempt a definition for the case of encrypted
+ filesystems: Plausible deniability is when you hide encrypted data
+ inside an encrypted container and it is not possible to prove it is
+ there. The idea is compelling and on first glance it seems
+ possible to do it. And from a cryptographic point of view, it
+ actually is possible.
+
+ So, does it work in practice? No, unfortunately. The reasoning used
+ by its proponents is fundamentally flawed in several ways and the
+ cryptographic properties fail fatally when colliding with the real
+ world.
+
+ First, why should "I do not have a hidden partition" be any more
+ plausible than "I forgot my crypto key" or "I wiped that partition
+ with random data, nothing in there"? I do not see any reason.
+
+ Second, there are two types of situations: Either they cannot force
+ you to give them the key (then you simply do not) or the can. In
+ the second case, they can always do bad things to you, because they
+ cannot prove that you have the key in the first place! This means
+ they do not have to prove you have the key, or that this random
+ looking data on your disk is actually encrypted data. So the
+ situation will allow them to waterboard/lock-up/deport you
+ anyways, regardless of how "plausible" your deniability is. Do not
+ have a hidden partition you could show to them, but there are
+ indications you may? Too bad for you. Unfortunately "plausible
+ deniability" also means you cannot prove there is no hidden data.
+
+ Third, hidden partitions are not that hidden. There are basically
+ just two possibilities: a) Make a large crypto container, but put a
+ smaller filesystem in there and put the hidden partition into the
+ free space. Unfortunately this is glaringly obvious and can be
+ detected in an automated fashion. This means that the initial
+ suspicion to put you under duress in order to make you reveal you
+ hidden data is given. b) Make a filesystem that spans the whole
+ encrypted partition, and put the hidden partition into space not
+ currently used by that filesystem. Unfortunately that is also
+ glaringly obvious, as you then cannot write to the filesystem
+ without a high risk of destroying data in the hidden container.
+ Have not written anything to the encrypted filesystem in a while?
+ Too bad, they have the suspicion they need to do unpleasant things
+ to you.
+
+ To be fair, if you prepare option b) carefully and directly before
+ going into danger, it may work. But then, the mere presence of
+ encrypted data may already be enough to get you into trouble in
+ those places were they can demand encryption keys.
+
+ Here is an additional reference for some problems with plausible
+ deniability: http://www.schneier.com/paper-truecrypt-dfs.pdf I
+ strongly suggest you read it.
+
+ So, no, I will not provide any instructions on how to do it with
+ plain dm-crypt or LUKS. If you insist on shooting yourself in the
+ foot, you can figure out how to do it yourself.
+
+
6. Backup and Data Recovery
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 occuurence. For example an
+ 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
+ LUKS partition permanently inaccessible. See more below on LUKS
header damage.
So, data-backup in some form is non-optional. For LUKS, you may
partition creation. Example:
- cryptsetup luksHeaderBackup --header-backup-file h <device>
+ cryptsetup luksHeaderBackup --header-backup-file <file> <device>
To restore, use the inverse command, i.e.
- cryptsetup luksHeaderRestore --header-backup-file h <device>
+ cryptsetup luksHeaderRestore --header-backup-file <file> <device>
* 6.3 How do I test a LUKS header?
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
+ exit-status. You can also use the more general test
blkid -p <device>
cat backup.tbz2.gpg | gpg - | tar djf -
- Note: Allways verify backups, especially encrypted ones.
+ Note: Always 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 filestem-level backup that can be used when
+ A second option for a filesystem-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.
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 methid that encrypts the whole backup in
+ 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 data) or
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
- irretrivable, unless you have a LUKS header backup. You may also
+ irretrievable, unless you have a LUKS header backup. You may also
damage the key-slots in part or in full. See also last item.
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
+ First, there is a script by Milan that automates the whole
process, except generating a new LUKS header with the old master
key (it prints the command for that though):
- Do a luksFormat to create a new LUKS header.
NOTE: If your header is intact and you just forgot the
- passphrase, you can just set a new passphrase, see next subitem.
+ passphrase, you can just set a new passphrase, see next
+ sub-item.
Unmap the device before you do that (luksClose). Then do
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,
+ result in alignment to 2k sectors and typical Flash/SSD sectors,
which is highly desirable for a number of reasons. Changing the
- alignment is not recomended.
+ alignment is not recommended.
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 conatiner size of 260kiB + 512B or
+ bytes, with a minimal LUKS container size of 260kiB + 512B or
266752 bytes.
borders).
+ * 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 issues 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
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.
+ losetup. This sometimes 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.
+ audience may be interested, please email the FAQ maintainer.
* 7.2 loop-aes: General observations.
loop-aes is a patch for losetup. Possible problems and deviations
from cryptsetup option syntax include:
- - Offsets specifed in bytes (cryptsetup: 512 byte sectors)
+ - Offsets specified in bytes (cryptsetup: 512 byte sectors)
- The need to specify an IV offset
give you the information you need.
- * 7.3 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 losetupp
- command
+ takes it in sectors of 512 bytes each. Example: The losetup command
losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
- mount /dev/loop0 mountpoint
+ mount /dev/loop0 mount-point
translates to
cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
- mount /dev/mapper/e1 mountpoint
+ mount /dev/mapper/e1 mount-point
* 7.4 loop-aes with 160 bit key
projects / platform / upstream / cryptsetup.git / blobdiff