such a disaster! In particular, make sure you have a current header
backup before doing any potentially dangerous operations.
+ SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
+ unclear how to get LUKS or plain dm-crypt to run on them with the
+ full set of security features intact. This may or may not be a
+ problem, depending on the attacher model. See Section 5.17.
+
BACKUP: Yes, encrypted disks die, just as normal ones do. A full
backup is mandatory, see Section "6. Backup and Data Recovery" on
options for doing encrypted backup.
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
PASSPHRASE CHARACTER SET: Some people have had difficulties with
this when upgrading distributions. It is highly advisable to only
- use the 94 printable characters from the first 128 characters of
+ use the 95 printable characters from the first 128 characters of
the ASCII table, as they will always have the same binary
representation. Other characters may have different encoding
depending on system configuration and your passphrase will not
work with a different encoding. A table of the standardized first
- 128 ASCII caracters can, e.g. be found on
+ 128 ASCII characters can, e.g. be found on
http://en.wikipedia.org/wiki/ASCII
- * 1.3 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
This issue has been acknowledged by the Ubuntu dev team, see here:
http://launchpad.net/bugs/420080
+ Update 7/2012: I am unsure whether this has been fixed by now, best
+ be careful.
+
+
+ * 1.4 My LUKS-device is broken! Help!
+
+ First: Do not panic! In many cases the data is still recoverable.
+ Do not do anything hasty! Steps:
+
+ - Take some deep breaths. Maybe add some relaxing music. This may
+ sound funny, but I am completely serious. Often, critical damage is
+ done only after the initial problem.
+
+ - Do not reboot. The keys mays still be in the kernel if the device
+ is mapped.
+
+ - Make sure others do not reboot the system.
+
+ - Do not write to your disk without a clear understanding why this
+ will not make matters worse. Do a sector-level backup before any
+ writes. Often you do not need to write at all to get enough access
+ to make a backup of the data.
- * 1.4 Who wrote this?
+ - Relax some more.
+
+ - Read section 6 of this FAQ.
+
+ - Ask on the mailing-list if you need more help.
+
+
+ * 1.5 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
* 2.1 What is the difference between "plain" and LUKS format?
+ First, unless you happen to understand the cryptographic background
+ well, you should use LUKS. It does protect the user from a lot of
+ common mistakes. Plain dm-crypt is for experts.
+
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?
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
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.
+
+
+ * 5.17 What about SSDs or Flash Drives?
+
+ The problem is that you cannot reliably erase parts of these
+ devices, mainly due to wear-leveling and possibly defect
+ management.
+
+ Basically, when overwriting a sector (of 512B), what the device
+ does is to move an internal sector (may be 128kB or even larger) to
+ some pool of discarded, not-yet erased unused sectors, take a
+ fresh empty sector from the empty-sector pool and copy the old
+ sector over with the changes to the small part you wrote. This is
+ done in some fashion so that larger writes do not cause a lot of
+ small internal updates.
+
+ The thing is that the mappings between outside-adressable sectors
+ and inside sectors is arbitrary (and the vendors are not talking).
+ Also the discarded sectors are not necessarily erased immediately.
+ They may linger a long time.
+
+ For plain dm-crypt, the consequences are that older encrypted data
+ may be lying around in some internal pools of the device. Thus may
+ or may not be a problem and depends on the application. Remember
+ the same can happen with a filesystem if consecutive writes to the
+ same area of a file can go to different sectors.
+
+ However, for LUKS, the worst case is that key-slots and LUKS
+ header may end up in these internal pools. This means that password
+ management functionality is compromised (the old passwords may
+ still be around, potentially for a very long time) and that fast
+ erase by overwriting the header and key-slot area is insecure.
+
+ Also keep in mind that the discarded/used pool may be large. For
+ example, a 240GB SSD has about 16GB of spare area in the chips that
+ it is free to do with as it likes. You would need to make each
+ individual key-slot larger than that to allow reliable overwriting.
+ And that assumes the disk thinks all other space is in use.
+ Reading the internal pools using forensic tools is not that hard,
+ but may involve some soldering.
+
+ What to do?
+
+ If you trust the device vendor (you probably should not...) you can
+ try an ATA "secure erase" command for SSDs. That does not work for
+ USB keys though. And if it finishes after a few seconds, it was
+ possibly faked by the SSD.
+
+ If you can do without password management and are fine with doing
+ physical destruction for permenently deleting data (allways after
+ one or several full overwrites!), you can use plain dm-crypt or
+ LUKS.
+
+ If you want or need the original LUKS security features to work,
+ you can use a detached LUKS header and put that on a conventional,
+ magnetic disk. That leaves potentially old encrypted data in the
+ pools on the disk, but otherwise you get LUKS with the same
+ security as on a magnetic disk.
+
+ If you are concerned about your laptop being stolen, you are likely
+ fine using LUKS on an SSD. An attacker would need to have access
+ to an old passphrase (and the key-slot for this old passphrase
+ would actually need to still be somewhere in the SSD) for your
+ data to be at risk. So unless you pasted your old passphrase all
+ over the Internet or the attacker has knowledge of it from some
+ other source and does a targetted laptop theft to get at your
+ data, you should be fine.
+
+
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):
-http://code.google.com/p/cryptsetup/source/browse/trunk/misc/luks-header-from-active
+http://code.google.com/p/cryptsetup/source/browse/misc/luks-header-from-active
You can also do this manually. Here is how:
- 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.
container, then to clone the contents of the source container, with
both containers mapped, i.e. decrypted. You can clone the decrypted
contents of a LUKS container in binary mode, although you may run
- into secondary issuses with GUIDs in filesystems, partition tables,
+ into secondary issues with GUIDs in filesystems, partition tables,
RAID-components and the like. These are just the normal problems
binary cloning causes.
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