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Frequently Asked Questions.
Sections
1. General Questions
2. Setup
3. Common Problems
4. Troubleshooting
5. Security Aspects
6. Backup and Data Recovery
7. Interoperability with other Disk Encryption Tools
8. Issues with Specific Versions of cryptsetup
9. The Initrd question
10. References and Further Reading
A. Contributors
1. General Questions
* 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, no
management, no metadata on disk) and LUKS (multiple user keys with
one master key, anti-forensic features, metadata block at start of
device, ...). The latest version of this FAQ should usually be
available at
https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions
* 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
the cryptsetup mailing list are from people that managed to damage
the start of their LUKS partitions, i.e. the LUKS header. In most
cases, there is nothing that can be done to help these poor souls
recover their data. Make sure you understand the problem and
limitations imposed by the LUKS security model BEFORE you face such a
disaster! In particular, make sure you have a current header backup
before doing any potentially dangerous operations.
DEBUG COMMANDS: While the --debug option does not leak data, "strace"
and the like can leak your full passphrase. Do not post an strace
output with the correct passphrase to a mailing-list or online! See
Item 4.5 for more explanation.
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 attacker model. See Section 5.19.
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.
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 an
existing one leads to permanent, complete and irreversible data loss.
It is strongly recommended to only use distribution installers after
a complete backup of all LUKS containers has been made.
UBUNTU INSTALLER: In particular the Ubuntu installer seems to be
quite willing to kill LUKS containers in several different ways.
Those responsible at Ubuntu seem not to care very much (it is very
easy to recognize a LUKS container), so treat the process of
installing Ubuntu as a severe hazard to any LUKS container you may
have.
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 used
from a script. In this scenario, the responsibility for warning the
user and possibly checking for an existing LUKS header is shifted to
the script. This is a more general form of the previous item.
LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
used in deriving the master key. It is used in decrypting a master
key that is randomly selected on header creation. This means that if
you create a new LUKS header on top of an old one with exactly the
same parameters and exactly the same passphrase as the old one, it
will still have a different master key and your data will be
permanently lost.
PASSPHRASE CHARACTER SET: Some people have had difficulties with this
when upgrading distributions. It is highly advisable to only 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 characters
can, e.g. be found on http://en.wikipedia.org/wiki/ASCII
KEYBOARD NUM-PAD: Apparently some pre-boot authentication
environments (these are done by the distro, not by cryptsetup, so
complain there) treat digits entered on the num-pad and ones entered
regularly different. This may be because the BIOS USB keyboard
driver is used and that one may have bugs on some computers. If you
cannot open your device in pre-boot, try entering the digits over the
regular digit keys.
* 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 to
activate their existing LUKS partition. The installer gives no or an
inadequate warning and will destroy your old LUKS header, causing
permanent data loss. See also the section on Backup and Data
Recovery.
This issue has been acknowledged by the Ubuntu dev team, see
here: http://launchpad.net/bugs/420080
Update 4/2013: I am still unsure whether this has been fixed by now,
best be careful. They also seem to have added even more LUKS killer
functionality to the Ubuntu installer. I can only strongly
recommended to not install Ubuntu on a system with existing LUKS
containers without complete backups.
Update 11/2014: There seem to be other problems with existing LUKS
containers and Ubuntu as well, be extra careful when using LUKS
on Ubuntu in any way, but exactly as the Ubuntu installer does.
* 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 may 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.
- 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>. If you want
to send me encrypted email, my current PGP key is DSA key CB5D9718,
fingerprint 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718.
Other contributors are listed at the end. If you want to contribute,
send your article, including a descriptive headline, to the
maintainer, or the dm-crypt mailing list with something like "FAQ
..." in the subject. You can also send more raw information and have
me write the section. Please note that by contributing to this FAQ,
you accept the license described below.
This work is under the "Attribution-Share Alike 3.0 Unported"
license, which means distribution is unlimited, you may create
derived works, but attributions to original authors and this license
statement must be retained and the derived work must be under the
same license. See http://creativecommons.org/licenses/by-sa/3.0/ for
more details of the license.
Side note: I did text license research some time ago and I think this
license is best suited for the purpose at hand and creates the least
problems.
* 1.6 Where is the project website?
There is the project website at
https://gitlab.com/cryptsetup/cryptsetup/ Please do not post
questions there, nobody will read them. Use the mailing-list
instead.
* 1.7 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
list.
The question of how to unsubscribe from the list does crop up
sometimes. For this you need your list management URL, which is sent
to you initially and once at the start of each month. Go to the URL
mentioned in the email and select "unsubscribe". This page also
allows you to request a password reminder.
Alternatively, you can send an Email to dm-crypt-request@saout.de
with just the word "help" in the subject or message body. Make sure
to send it from your list address.
The mailing list archive is here:
https://marc.info/?l=dm-crypt
* 1.8 Unsubscribe from the mailing-list
Send mail to dm-crypt-unsubscribe@saout.de from the subscribed
account. You will get an email with instructions.
Basically, you just have to respond to it unmodified to get
unsubscribed. The listserver admin functions are not very fast. It
can take 15 minutes or longer for a reply to arrive (I suspect
greylisting is in use), so be patient.
Also note that nobody on the list can unsubscribe you, sending
demands to be unsubscribed to the list just annoys people that are
entirely blameless for you being subscribed.
If you are subscribed, a subscription confirmation email was sent to
your email account and it had to be answered before the subscription
went active. The confirmation emails from the listserver have
subjects like these (with other numbers):
Subject: confirm 9964cf10.....
and are sent from dm-crypt-request@saout.de. You should check whether
you have anything like it in your sent email folder. If you find
nothing and are sure you did not confirm, then you should look into a
possible compromise of your email account.
2. Setup
* 2.1 LUKS Container Setup mini-HOWTO
This item tries to give you a very brief list of all the steps you
should go though when creating a new LUKS encrypted container, i.e.
encrypted disk, partition or loop-file.
01) All data will be lost, if there is data on the target, make a
backup.
02) Make very sure you have the right target disk, partition or
loop-file.
03) If the target was in use previously, it is a good idea to wipe it
before creating the LUKS container in order to remove any trace of
old file systems and data. For example, some users have managed to
run e2fsck on a partition containing a LUKS container, possibly
because of residual ext2 superblocks from an earlier use. This can
do arbitrary damage up to complete and permanent loss of all data in
the LUKS container.
To just quickly wipe file systems (old data may remain), use
wipefs -a <target device>
To wipe file system and data, use something like
cat /dev/zero > <target device>
This can take a while. To get a progress indicator, you can use the
tool dd_rescue (->google) instead or use my stream meter "wcs"
(source here: http://www.tansi.org/tools/index.html) in the following
fashion:
cat /dev/zero | wcs > <target device>
Be very sure you have the right target, all data will be lost!
Note that automatic wiping is on the TODO list for cryptsetup, so at
some time in the future this will become unnecessary.
Alternatively, plain dm-crypt can be used for a very fast wipe with
crypto-grade randomness, see Item 2.19
04) Create the LUKS container:
cryptsetup luksFormat <target device>
Just follow the on-screen instructions.
Note: Passphrase iteration is determined by cryptsetup depending on
CPU power. On a slow device, this may be lower than you want. I
recently benchmarked this on a Raspberry Pi and it came out at about
1/15 of the iteration count for a typical PC. If security is
paramount, you may want to increase the time spent in iteration, at
the cost of a slower unlock later. For the Raspberry Pi, using
cryptsetup luksFormat -i 15000 <target device>
gives you an iteration count and security level equal to an average
PC for passphrase iteration and master-key iteration. If in doubt,
check the iteration counts with
cryptsetup luksDump <target device>
and adjust the iteration count accordingly by creating the container
again with a different iteration time (the number after '-i' is the
iteration time in milliseconds) until your requirements are met.
05) Map the container. Here it will be mapped to /dev/mapper/c1:
cryptsetup luksOpen <target device> c1
06) (Optionally) wipe the container (make sure you have the right
target!):
cat /dev/zero > /dev/mapper/c1
Note that this creates a small information leak, as an attacker can
determine whether a 512 byte block is zero if the attacker has access
to the encrypted container multiple times. Typically a competent
attacker that has access multiple times can install a passphrase
sniffer anyways, so this leakage is not very significant. For
getting a progress indicator, see step 03.
Note that at some time in the future, cryptsetup will do this for
you, but currently it is a TODO list item.
07) Create a file system in the mapped container, for example an
ext3 file system (any other file system is possible):
mke2fs -j /dev/mapper/c1
08) Mount your encrypted file system, here on /mnt:
mount /dev/mapper/c1 /mnt
Done. You can now use the encrypted file system to store data. Be
sure to read though the rest of the FAQ, these are just the very
basics. In particular, there are a number of mistakes that are easy
to make, but will compromise your security.
* 2.2 LUKS on partitions or raw disks?
This is a complicated question, and made more so by the availability
of RAID and LVM. I will try to give some scenarios and discuss
advantages and disadvantages. Note that I say LUKS for simplicity,
but you can do all the things described with plain dm-crypt as well.
Also note that your specific scenario may be so special that most or
even all things I say below do not apply.
Be aware that if you add LVM into the mix, things can get very
complicated. Same with RAID but less so. In particular, data
recovery can get exceedingly difficult. Only do so if you have a
really good reason and always remember KISS is what separates an
engineer from an amateur. Of course, if you really need the added
complexity, KISS is satisfied. But be very sure as there is a price
to pay for it. In engineering, complexity is always the enemy and
needs to be fought without mercy when encountered.
Also consider using RAID instead of LVM, as at least with the old
superblock format 0.90, the RAID superblock is in the place (end of
disk) where the risk of it permanently damaging the LUKS header is
smallest and you can have your array assembled by the RAID controller
(i.e. the kernel), as it should be. Use partition type 0xfd for
that. I recommend staying away from superblock formats 1.0, 1.1 and
1.2 unless you really need them. Be aware that you lose
autodetection with them and have to fall back to some user-space
script to do it.
Scenarios:
(1) Encrypted partition: Just make a partition to your liking, and
put LUKS on top of it and a filesystem into the LUKS container. This
gives you isolation of differently-tasked data areas, just as
ordinary partitioning does. You can have confidential data,
non-confidential data, data for some specific applications,
user-homes, root, etc. Advantages are simplicity as there is a 1:1
mapping between partitions and filesystems, clear security
functionality and the ability to separate data into different,
independent (!) containers.
Note that you cannot do this for encrypted root, that requires an
initrd. On the other hand, an initrd is about as vulnerable to a
competent attacker as a non-encrypted root, so there really is no
security advantage to doing it that way. An attacker that wants to
compromise your system will just compromise the initrd or the kernel
itself. The better way to deal with this is to make sure the root
partition does not store any critical data and move that to
additional encrypted partitions. If you really are concerned your
root partition may be sabotaged by somebody with physical access
(that would however strangely not, say, sabotage your BIOS, keyboard,
etc.), protect it in some other way. The PC is just not set-up for a
really secure boot-chain (whatever some people may claim).
(2) Fully encrypted raw block device: For this, put LUKS on the raw
device (e.g. /dev/sdb) and put a filesystem into the LUKS container,
no partitioning whatsoever involved. This is very suitable for
things like external USB disks used for backups or offline
data-storage.
(3) Encrypted RAID: Create your RAID from partitions and/or full
devices. Put LUKS on top of the RAID device, just if it were an
ordinary block device. Applications are just the same as above, but
you get redundancy. (Side note as many people seem to be unaware of
it: You can do RAID1 with an arbitrary number of components in
Linux.) See also Item 2.8.
(4) Now, some people advocate doing the encryption below the RAID
layer. That has several serious problems. One is that suddenly
debugging RAID issues becomes much harder. You cannot do automatic
RAID assembly anymore. You need to keep the encryption keys for the
components in sync or manage them somehow. The only possible
advantage is that things may run a little faster as more CPUs do the
encryption, but if speed is a priority over security and simplicity,
you are doing this wrong anyways. A good way to mitigate a speed
issue is to get a CPU that does hardware AES.
* 2.3 How do I set up encrypted swap?
As things that are confidential can end up in swap (keys,
passphrases, etc. are usually protected against being swapped to
disk, but other things may not be), it may be advisable to do
something about the issue. One option is to run without swap, which
generally works well in a desktop-context. It may cause problems in
a server-setting or under special circumstances. The solution to
that is to encrypt swap with a random key at boot-time.
NOTE: This is for Debian, and should work for Debian-derived
distributions. For others you may have to write your own startup
script or use other mechanisms.
01) Add the swap partition to /etc/crypttab. A line like the
following should do it:
swap /dev/<partition> /dev/urandom swap,noearly
Warning: While Debian refuses to overwrite partitions with a
filesystem or RAID signature on it, if your disk IDs may change
(adding or removing disks, failure of disk during boot, etc.), you
may want to take additional precautions. Yes, this means that your
kernel device names like sda, sdb, ... can change between reboots!
This is not a concern if you have only one disk. One possibility is
to make sure the partition number is not present on additional disks
or also swap there. Another is to encapsulate the swap partition (by
making it a 1-disk RAID1 or by using LVM), so that it gets a
persistent identifier. Specifying it directly by UUID does not work,
unfortunately, as the UUID is part of the swap signature and that is
not visible from the outside due to the encryption and in addition
changes on each reboot with this setup.
Note: Use /dev/random if you are paranoid or in a potential
low-entropy situation (embedded system, etc.). This may cause the
operation to take a long time during boot. If you are in a "no
entropy" situation, you cannot encrypt swap securely. In this
situation you should find some entropy, also because nothing else
using crypto will be secure, like ssh, ssl or GnuPG.
Note: The "noearly" option makes sure things like LVM, RAID, etc.
are running. As swap is non-critical for boot, it is fine to start
it late.
02) Add the swap partition to /etc/fstab. A line like the following
should do it:
/dev/mapper/swap none swap sw 0 0
That is it. Reboot or start it manually to activate encrypted swap.
Manual start would look like this:
/etc/init.d/crypdisks start
swapon /dev/mapper/swap
* 2.4 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
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.
Primary advantage is high resilience to damage, as one damaged
encrypted sector results in exactly one damaged decrypted sector.
Also, it is not readily apparent that there even is encrypted data on
the device, as an overwrite with crypto-grade randomness (e.g. from
/dev/urandom) looks exactly the same on disk.
Side-note: That has limited value against the authorities. In
civilized countries, they cannot force you to give up a crypto-key
anyways. In quite a few countries around the world, they can force
you to give up the keys (using imprisonment or worse to pressure you,
sometimes without due process), and in the worst case, they only need
a nebulous "suspicion" about the presence of encrypted data.
Sometimes this applies to everybody, sometimes only when you are
suspected of having "illicit data" (definition subject to change) and
sometimes specifically when crossing a border. Note that this is
going on in countries like the US and the UK, to different degrees
and sometimes with courts restricting what the authorities can
actually demand.
My advice is to either be ready to give up the keys or to not have
encrypted data when traveling to those countries, especially when
crossing the borders. The latter also means not having any
high-entropy (random) data areas on your disk, unless you can explain
them and demonstrate that explanation. Hence doing a zero-wipe of
all free space, including unused space, may be a good idea.
Disadvantages are that you do not have all the nice features that the
LUKS metadata offers, like multiple passphrases that can be changed,
the cipher being stored in the metadata, anti-forensic 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 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
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.
Disadvantages are that it is readily obvious there is encrypted data
on disk (but see side note above) and that damage to the header or
key-slots usually results in permanent data-loss. See below under
"6. Backup and Data Recovery" on how to reduce that risk. Also the
sector numbers get shifted by the length of the header and key-slots
and there is a loss of that size in capacity (1MB+4096B for defaults
and 2MB for the most commonly used non-default XTS mode).
* 2.5 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 the
device (as LUKS device initialization does not clear away old data),
do a luksFormat, optionally overwrite the encrypted device, create a
new filesystem and restore your backup on the now encrypted device.
Also refer to sections "Security Aspects" and "Backup and Data
Recovery".
For backup, plain GNU tar works well and backs up anything likely
to be in a filesystem.
* 2.6 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
file as LUKS container, do something like this:
head -c 100M /dev/zero > luksfile # create empty file
losetup /dev/loop0 luksfile # map luksfile to /dev/loop0
cryptsetup luksFormat /dev/loop0 # create LUKS on loop device
Afterwards just use /dev/loop0 as a you would use a LUKS partition.
To unmap the file when done, use "losetup -d /dev/loop0".
* 2.7 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?
That is as intended. You are asked a passphrase of an existing
key-slot first, before you can enter the passphrase for the new
key-slot. Otherwise you could break the encryption by just adding a
new key-slot. This way, you have to know the passphrase of one of
the already configured key-slots in order to be able to configure a
new key-slot.
* 2.8 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
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 .
This means that the typical layering looks like this:
Filesystem <- top
|
Encryption
|
RAID
|
Raw partitions
|
Raw disks <- bottom
The big advantage is that you can manage the RAID container just like
any RAID container, it does not care that what is in it is encrypted.
* 2.9 How do I read a dm-crypt key from file?
Use the --key-file option, like this:
cryptsetup create --key-file keyfile e1 /dev/loop0
This will read the binary key from file, i.e. no hashing or
transformation will be applied to the keyfile before its bits are
used as key. Extra bits (beyond the length of the key) at the end
are ignored. Note that if you read from STDIN, the data will still
be hashed, just as a key read interactively from the terminal. See
the man-page sections "NOTES ON PASSPHRASE PROCESSING..." for more
detail.
* 2.10 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
add a new passphrase to a free key-slot, set the passphrase of an
specific key-slot or put an already configured passphrase into a
file. In the last case make sure no trailing newline (0x0a) is
contained in the key file, or the passphrase will not work because
the whole file is used as input.
To add a new passphrase to a free key slot from file, use something
like this:
cryptsetup luksAddKey /dev/loop0 keyfile
To add a new passphrase to a specific key-slot, use something
like this:
cryptsetup luksAddKey --key-slot 7 /dev/loop0 keyfile
To supply a key from file to any LUKS command, use the --key-file
option, e.g. like this:
cryptsetup luksOpen --key-file keyfile /dev/loop0 e1
* 2.11 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 to
have two LUKS devices with the same master key. Even then, I think
it would be preferable to just use key-slots with the same
passphrase, or to use plain dm-crypt instead. If you really have a
good reason, please tell me. If I am convinced, I will add how to do
this here.
* 2.12 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
thereby use bytes you cannot type on the keyboard. You can use any
file you like as key file, for example a plain text file with a human
readable passphrase. To generate a file with random bytes, use
something like this:
head -c 256 /dev/random > keyfile
* 2.13 If I map a journaled file system using dm-crypt/LUKS, does
it still provide its usual transactional guarantees?
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/
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.
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.
* 2.14 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
cryptsetup read the key from STDIN and write it there with your own
tool that in turn gets the key from the more secure key storage.
For TPM support, you may want to have a look at tpm-luks at
https://github.com/shpedoikal/tpm-luks. Note that tpm-luks is not
related to the cryptsetup project.
* 2.15 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
backup, recreation of the encrypted partition with new size,
recreation of the filesystem and restore. This gets around the
tricky business of resizing the filesystem. Resizing a dm-crypt or
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 will not
change the size of the filesystem hidden under the encryption though.
You also need to be aware of size-based limitations. The one
currently relevant is that aes-xts-plain should not be used for
encrypted container sizes larger than 2TiB. Use aes-xts-plain64 for
that.
* 2.16 How do I Benchmark the Ciphers, Hashes and Modes?
Since version 1.60 cryptsetup supports the "benchmark" command.
Simply run as root:
cryptsetup benchmark
It will output first iterations/second for the key-derivation
function PBKDF2 parameterized with different hash-functions, and then
the raw encryption speed of ciphers with different modes and
key-sizes. You can get more than the default benchmarks, see the
man-page for the relevant parameters. Note that XTS mode takes two
keys, hence the listed key sizes are double that for other modes and
half of it is the cipher key, the other half is the XTS key.
* 2.17 How do I Verify I have an Authentic cryptsetup Source Package?
Current maintainer is Milan Broz and he signs the release packages
with his PGP key. The key he currently uses is the "RSA key ID
D93E98FC", fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B D93E
98FC. While I have every confidence this really is his key and that
he is who he claims to be, don't depend on it if your life is at
stake. For that matter, if your life is at stake, don't depend on me
being who I claim to be either.
That said, as cryptsetup is under good version control, a malicious
change should be noticed sooner or later, but it may take a while.
Also, the attacker model makes compromising the sources in a
non-obvious way pretty hard. Sure, you could put the master-key
somewhere on disk, but that is rather obvious as soon as somebody
looks as there would be data in an empty LUKS container in a place it
should not be. Doing this in a more nefarious way, for example
hiding the master-key in the salts, would need a look at the sources
to be discovered, but I think that somebody would find that sooner or
later as well.
That said, this discussion is really a lot more complicated and
longer as an FAQ can sustain. If in doubt, ask on the mailing list.
* 2.18 Is there a concern with 4k Sectors?
Not from dm-crypt itself. Encryption will be done in 512B blocks, but
if the partition and filesystem are aligned correctly and the
filesystem uses multiples of 4kiB as block size, the dm-crypt layer
will just process 8 x 512B = 4096B at a time with negligible
overhead. LUKS does place data at an offset, which is 2MiB per
default and will not break alignment. See also Item 6.12 of this FAQ
for more details. Note that if your partition or filesystem is
misaligned, dm-crypt can make the effect worse though.
* 2.19 How can I wipe a device with crypto-grade randomness?
The conventional recommendation if you want to not just do a
zero-wipe is to use something like
cat /dev/urandom > <taget-device>
That is very slow and painful at 10-20MB/s on a fast computer.
Using cryptsetup and a plain dm-crypt device with a random key,
it is much faster and gives you the same level of security. The
defaults are quite enough.
For device set-up, do the following:
cryptsetup open --type plain -d /dev/urandom /dev/<block-device> to_be_wiped
This maps the container as plain under /dev/mapper/to_be_wiped with a
random password. For the actual wipe you have several options.
Simple wipe without progress-indicator:
cat /dev/zero > /dev/mapper/to_be_wiped
Progress-indicator by dd_rescue:
dd_rescue -w /dev/zero /dev/mapper/to_be_wiped
Progress-indicator by my "wcs" stream meter (available from
http://www.tansi.org/tools/index.html ):
cat /dev/zero | wcs > /dev/mapper/to_be_wiped
Remove the mapping at the end and you are done.
* 2.20 How to I wipe only the LUKS header?
This is not the emergency wipe procedure. That is in Item 5.4. This procedure
is intended to be used when the data should stay intact, e.g. when you change
your LUKS container to use a detached header and want to remove the old one.
Most safe way is this (backup is still a good idea):
01) Determine header size in 512 Byte sectors with "luksDump":
cryptsetup luksDump <device with LUKS container>
-> ...
Payload offset: <number>
...
02) Take the result number and write number * 512 zeros to the start of the
device, e.g. like this:
dd bs=512 count=<number> if=/dev/zero of=<device>
That is it.
3. Common Problems
* 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
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 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.
* 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".
* 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 have
no older version of cryptsetup on your system that then gets called
by cron/CGI. For example some distributions install cryptsetup into
/usr/sbin, while a manual install could go to /usr/local/sbin. As a
debugging aid, call "cryptsetup --version" from cron/CGI or the
non-shell mechanism to be sure the right version gets called.
* 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
default it is 1 second on the machine where the passphrase is set.
If you set a passphrase on a fast machine and then unlock it on a
slow machine, the unlocking time can be much longer. Also take into
account that up to 8 key-slots have to be tried in order to find the
right one.
If this is problem, you can add another key-slot using the slow
machine with the same passphrase and then remove the old key-slot.
The new key-slot will have an iteration count adjusted to 1 second on
the slow machine. Use luksKeyAdd and then luksKillSlot or
luksRemoveKey.
However, this operation will not change volume key iteration count
(MK iterations in output of "cryptsetup luksDump"). In order to
change that, you will have to backup the data in the LUKS container
(i.e. your encrypted data), luksFormat on the slow machine and
restore the data. Note that in the original LUKS specification this
value was fixed 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. Also note that MK iterations are not very
security relevant. But as each key-slot already takes 1 second,
spending the additional 0.125 seconds really does not matter.
* 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
signature remains on the device. This confuses blkid.
Fix: Wipe the unused header areas by doing a backup and restore of
the header with cryptsetup 1.1.x:
cryptsetup luksHeaderBackup --header-backup-file <file> <device>
cryptsetup luksHeaderRestore --header-backup-file <file> <device>
* 3.6 cryptsetup segfaults on Gentoo amd64 hardened ...
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 flag, see
http://bugs.gentoo.org/show_bug.cgi?id=283470
4. Troubleshooting
* 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 Data Recovery"
and ask on the mailing list if you have trouble diagnosing and (if
still possible) repairing this.
* 4.2 I cannot unlock my LUKS container! What could be the problem?
First, make sure you have a correct passphrase. Then make sure you
have the correct key-map and correct keyboard. And then make sure
you have the correct character set and encoding, see also "PASSPHRASE
CHARACTER SET" under Section 1.2.
If you are sure you are entering the passphrase right, there is the
possibility that the respective key-slot has been damaged. There is
no way to recover a damaged key-slot, except from a header backup
(see Section 6). For security reasons, there is also no checksum in
the key-slots that could tell you whether a key-slot has been
damaged. The only checksum present allows recognition of a correct
passphrase, but that only works if the passphrase is correct and the
respective key-slot is intact.
In order to find out whether a key-slot is damaged one has to look
for "non-random looking" data in it. There is a tool that
automates this in the cryptsetup distribution from version 1.6.0
onwards. It is located in misc/keyslot_checker/. Instructions how
to use and how to interpret results are in the README file. Note
that this tool requires a libcryptsetup from cryptsetup 1.6.0 or
later (which means libcryptsetup.so.4.5.0 or later). If the tool
complains about missing functions in libcryptsetup, you likely have
an earlier version from your distribution still installed. You can
either point the symbolic link(s) from libcryptsetup.so.4 to the new
version manually, or you can uninstall the distribution version of
cryptsetup and re-install that from cryptsetup >= 1.6.0 again to fix
this.
* 4.3 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 +
LUKS + XFS seems to uncover RAM problems that never caused obvious
problems before. Symptoms vary, but often the problem manifest
itself when copying large amounts of data, typically several times
larger than your main memory.
Side note: One thing you should always do on large data
copy/movements is to run a verify, for example with the "-d" option
of "tar" or by doing a set of MD5 checksums on the source or target
with
find . -type f -exec md5sum \{\} \; > checksum-file
and then a "md5sum -c checksum-file" on the other side. If you get
mismatches here, RAM is the primary suspect. A lesser suspect is an
overclocked CPU. I have found countless hardware problems in verify
runs after copying or making backups. Bit errors are much more
common than most people think.
Some RAM issues are even worse and corrupt structures in one of the
layers. This typically results in lockups, CPU state dumps in the
system logs, kernel panic or other things. It is quite possible to
have the problem with an encrypted device, but not with an otherwise
the same unencrypted device. The reason for that is that encryption
has an error amplification property: You flip one bit in an encrypted
data block, and the decrypted version has half of its bits flipped.
This is an important security property for modern ciphers. With the
usual modes in cryptsetup (CBC, ESSIV, XTS), you get up to a
completely changed 512 byte block per bit error. A corrupt block
causes a lot more havoc than the occasionally flipped single bit and
can result in various obscure errors.
Note that a verify run on copying between encrypted or unencrypted
devices will reliably detect corruption, even when the copying 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.
* 4.4 How do I test RAM?
First you should know that overclocking often makes memory problems
worse. So if you overclock (which I strongly recommend against in a
system holding data that has some worth), run the tests with the
overclocking active.
There are two good options. One is Memtest86+ and the other is
"memtester" by Charles Cazabon. Memtest86+ requires a reboot and
then takes over the machine, while memtester runs from a root-shell.
Both use different testing methods and I have found problems fast
with each one that the other needed long to find. I recommend
running the following procedure until the first error is found:
- Run Memtest86+ for one cycle
- Run memtester for one cycle (shut down as many other applications
as possible)
- Run Memtest86+ 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
additional test may be to remove half of the RAM (if you have more
than one module) and try whether the problem is still there and if
so, try with the other half. If you just have one module, get a
different one and try with that. If you do overclocking, reduce the
settings to the most conservative ones available and try with that.
* 4.5 Is there a risk using debugging tools like strace?
There most definitely is. An dump from strace and friends can contain
all data entered, including the full passphrase. Example with strace
and passphrase "test":
> strace cryptsetup luksOpen /dev/sda10 c1
...
read(6, "test\n", 512) = 5
...
Depending on different factors and the tool used, the passphrase may
also be encoded and not plainly visible. Hence it is never a good
idea to give such a trace from a live container to anybody. Recreate
the problem with a test container or set a temporary passphrase like
"test" and use that for the trace generation. Item 2.6 explains how
to create a loop-file backed LUKS container that may come in handy
for this purpose.
See also Item 6.10 for another set of data you should not give to
others.
5. Security Aspects
* 5.1 How long is a secure passphrase ?
This is just the short answer. For more info and explanation of some
of the terms used in this item, read the rest of Section 5. The
actual recommendation is at the end of this item.
First, passphrase length is not really the right measure, passphrase
entropy is. For example, a random lowercase letter (a-z) gives you
4.7 bit of entropy, one element of a-z0-9 gives you 5.2 bits of
entropy, an element of a-zA-Z0-9 gives you 5.9 bits and
a-zA-Z0-9!@#$%\^&:-+ gives you 6.2 bits. On the other hand, a random
English word only gives you 0.6...1.3 bits of entropy per character.
Using sentences that make sense gives lower entropy, series of random
words gives higher entropy. Do not use sentences that can be tied to
you or found on your computer. This type of attack is done routinely
today.
That said, it does not matter too much what scheme you use, but it
does matter how much entropy your passphrase contains, because an
attacker has to try on average
1/2 * 2^(bits of entropy in passphrase)
different passphrases to guess correctly.
Historically, estimations tended to use computing time estimates, but
more modern approaches try to estimate cost of guessing a passphrase.
As an example, I will try to get an estimate from the numbers in
http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
More references can be found a the end of this document. Note that
these are estimates from the defender side, so assuming something is
easier than it actually is is fine. An attacker may still have
vastly higher cost than estimated here.
LUKS uses SHA1 for hashing per default. The claim in the reference is
63 billion tries/second for SHA1. We will leave aside the check
whether a try actually decrypts a key-slot. Now, the machine has 25
GPUs, which I will estimate at an overall lifetime cost of USD/EUR
1000 each, and an useful lifetime of 2 years. (This is on the low
side.) Disregarding downtime, the machine can then break
N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18
passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed
once with SHA1 for EUR/USD 25k. Note that as this can be
parallelized, it can be done faster than 2 years with several of
these machines.
For plain dm-crypt (no hash iteration) this is it. This gives (with
SHA1, plain dm-crypt default is ripemd160 which seems to be slightly
slower than SHA1):
Passphrase entropy Cost to break
60 bit EUR/USD 6k
65 bit EUR/USD 200K
70 bit EUR/USD 6M
75 bit EUR/USD 200M
80 bit EUR/USD 6B
85 bit EUR/USD 200B
... ...
For LUKS, you have to take into account hash iteration in PBKDF2.
For a current CPU, there are about 100k iterations (as can be queried
with ''cryptsetup luksDump''.
The table above then becomes:
Passphrase entropy Cost to break
50 bit EUR/USD 600k
55 bit EUR/USD 20M
60 bit EUR/USD 600M
65 bit EUR/USD 20B
70 bit EUR/USD 600B
75 bit EUR/USD 20T
... ...
Recommendation:
To get reasonable security for the next 10 years, it is a good idea
to overestimate by a factor of at least 1000.
Then there is the question of how much the attacker is willing to
spend. That is up to your own security evaluation. For general use,
I will assume the attacker is willing to spend up to 1 million
EUR/USD. Then we get the following recommendations:
Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
or a random English sentence of > 135 characters length.
LUKS: Use > 65 bit. That is e.g. 14 random chars from a-z or a random
English sentence of > 108 characters length.
If paranoid, add at least 20 bit. That is roughly four additional
characters for random passphrases and roughly 32 characters for a
random English sentence.
* 5.2 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
countries they can force you to hand over the keys, if they suspect
encryption. However the suspicion is enough, they do not have to
prove anything. This is for practical reasons, as even the presence
of a header (like the LUKS header) is not enough to prove that you
have any keys. It might have been an experiment, for example. Or it
was used as encrypted swap with a key from /dev/random. So they make
you prove you do not have encrypted data. Of course that is just as
impossible as the other way round.
This means that if you have a large set of random-looking data, they
can already lock you up. Hidden containers (encryption hidden within
encryption), as possible with Truecrypt, do not help either. They
will just assume the hidden container is there and unless you hand
over the key, you will stay locked up. Don't have a hidden
container? Though luck. Anybody could claim that.
Still, if you are concerned about the LUKS header, use plain dm-crypt
with a good passphrase. See also Section 2, "What is the difference
between "plain" and LUKS format?"
* 5.3 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
it before encrypting. In any case, not initializing will leave the
old data there until the specific sector gets written. That may
enable an attacker to determine how much and where on the partition
data was written. If you think this is a risk, you can prevent this
by overwriting the encrypted device (here assumed to be named "e1")
with zeros like this:
dd_rescue -w /dev/zero /dev/mapper/e1
or alternatively with one of the following more standard commands:
cat /dev/zero > /dev/mapper/e1
dd if=/dev/zero of=/dev/mapper/e1
* 5.4 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 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 enough. If you anticipate
being in a desperate hurry, prepare the command beforehand. Example
with /dev/sde1 as the LUKS partition and default parameters:
head -c 1052672 /dev/zero > /dev/sde1; sync
A LUKS header backup or full backup will still grant access to most
or all data, so make sure that an attacker does not have access to
backups or destroy them as well.
If you have time, overwrite the whole LUKS partition with a single
pass of zeros. This is enough for current HDDs. For SSDs or FLASH
(USB sticks) you may want to overwrite the whole drive several times
to be sure data is not retained by wear leveling. This is possibly
still insecure as SSD technology is not fully understood 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: http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
Keep in mind to also erase all backups.
Example for a zero-overwrite erase of partition sde1 done with
dd_rescue:
dd_rescue -w /dev/zero /dev/sde1
* 5.5 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 may
want to overwrite the complete SSD several times and use physical
destruction in addition, see last item. For re-writable CD/DVD, a
single overwrite should also be enough, due to the anti-forensic
properties of the LUKS keyslots. For write-once media, use physical
destruction. For low security requirements, just cut the CD/DVD into
several parts. For high security needs, shred or burn the medium.
If your backup is on magnetic tape, I advise physical destruction by
shredding or burning, after overwriting . The problem with magnetic
tape is that it has a higher dynamic range than HDDs and older data
may well be recoverable after overwrites. Also write-head alignment
issues can lead to data not actually being deleted at all during
overwrites.
* 5.6 What about backup? Does it compromise security?
That depends. See item 6.7.
* 5.7 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.
Overwriting can be done in a number of fashions, like creating a new
filesystem on the raw LUKS partition, making the raw partition part
of a raid array and just writing to the raw partition.
The LUKS header contains a 256 bit "salt" per key-slot and without
that no decryption is possible. While the salts are not secret, they
are key-grade material and cannot be reconstructed. This is a
cryptographically strong "cannot". From observations on the
cryptsetup mailing-list, people typically go though the usual stages
of grief (Denial, Anger, Bargaining, Depression, Acceptance) when
this happens to them. Observed times vary between 1 day and 2 weeks
to complete the cycle. Seeking help on the mailing-list is fine.
Even if we usually cannot help with getting back your data, most
people found the feedback comforting.
If your header does not contain an intact key-slot salt, best go
directly to the last stage ("Acceptance") and think about what to do
now. There is one exception that I know of: If your LUKS container
is still open, then it may be possible to extract the master key from
the running system. See Item "How do I recover the master key from a
mapped LUKS container?" in Section "Backup and Data Recovery".
* 5.8 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 is
as follows: If an attacker wants to crack the password for a single
LUKS container, then every possible passphrase has to be tried.
Typically an attacker will not try every binary value, but will try
words and sentences from a dictionary.
If an attacker wants to attack several LUKS containers with the same
dictionary, then a different approach makes sense: Compute the
resulting slot-key for each dictionary element and store it on disk.
Then the test for each entry is just the slow unlocking with the slot
key (say 0.00001 sec) instead of calculating the slot-key first (1
sec). For a single attack, this does not help. But if you have more
than one container to attack, this helps tremendously, also because
you can prepare your table before you even have the container to
attack! The calculation is also very simple to parallelize. You
could, for example, use the night-time unused CPU power of your
desktop PCs for this.
This is where the salt comes in. If the salt is combined with the
passphrase (in the simplest form, just appended to it), you suddenly
need a separate table for each salt value. With a reasonably-sized
salt value (256 bit, e.g.) this is quite infeasible.
* 5.9 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 character
encoding changes, e.g. because of a system upgrade, see also the
note at the very start of this FAQ under "WARNINGS".
This needs a bit of theory. The quality of your passphrase is
directly related to its entropy (information theoretic, not
thermodynamic). The entropy says how many bits of "uncertainty" or
"randomness" are in you passphrase. In other words, that is how
difficult guessing the passphrase is.
Example: A random English sentence has about 1 bit of entropy per
character. A random lowercase (or uppercase) character has about 4.7
bit of entropy.
Now, if n is the number of bits of entropy in your passphrase and t
is the time it takes to process a passphrase in order to open the
LUKS container, then an attacker has to spend at maximum
attack_time_max = 2^n * t
time for a successful attack and on average half that. There is no
way getting around that relationship. However, there is one thing
that does help, namely increasing t, the time it takes to use a
passphrase, see next FAQ item.
Still, if you want good security, a high-entropy passphrase is the
only option. For example, a low-entropy passphrase can never be
considered secure against a TLA-level (Three Letter Agency level,
i.e. government-level) attacker, no matter what tricks are used in
the key-derivation function. Use at least 64 bits for secret stuff.
That is 64 characters of English text (but only if randomly chosen)
or a combination of 12 truly random letters and digits.
For passphrase generation, do not use lines from very well-known
texts (religious texts, Harry potter, etc.) as they are to easy to
guess. For example, the total Harry Potter has about 1'500'000 words
(my estimation). Trying every 64 character sequence starting and
ending at a word boundary would take only something like 20 days on a
single CPU and is entirely feasible. To put that into perspective,
using a number of Amazon EC2 High-CPU Extra Large instances (each
gives about 8 real cores), this test costs currently about 50USD/EUR,
but can be made to run arbitrarily fast.
On the other hand, choosing 1.5 lines from, say, the Wheel of Time
is in itself not more secure, but the book selection adds quite
a bit of entropy. (Now that I have mentioned it here, don't use
tWoT either!) If you add 2 or 3 typos or switch some words around,
then this is good passphrase material.
* 5.10 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
done with the explicit purpose to increase the time that it takes to
unlock a key-slot. This provides some protection against use of
low-entropy passphrases.
The idea is that an attacker has to try all possible passphrases.
Even if the attacker knows the passphrase is low-entropy (see last
item), it is possible to make each individual try take longer. The
way to do this is to repeatedly hash the passphrase for a certain
time. The attacker then has to spend the same time (given the same
computing power) as the user per try. With LUKS, the default is 1
second of PBKDF2 hashing.
Example 1: Lets assume we have a really bad passphrase (e.g. a
girlfriends name) with 10 bits of entropy. With the same CPU, an
attacker would need to spend around 500 seconds on average to break
that passphrase. Without iteration, it would be more like 0.0001
seconds on a modern CPU.
Example 2: The user did a bit better and has 32 chars of English
text. That would be about 32 bits of entropy. With 1 second
iteration, that means an attacker on the same CPU needs around 136
years. That is pretty impressive for such a weak passphrase.
Without the iterations, it would be more like 50 days on a modern
CPU, and possibly far less.
In addition, the attacker can both parallelize and use special
hardware like GPUs or FPGAs to speed up the attack. The attack can
also happen quite some time after the luksFormat operation and CPUs
can have become faster and cheaper. For that reason you want a bit
of extra security. Anyways, in Example 1 your are screwed. In
example 2, not necessarily. Even if the attack is faster, it still
has a certain cost associated with it, say 10000 EUR/USD with
iteration and 1 EUR/USD without iteration. The first can be
prohibitively expensive, while the second is something you try even
without solid proof that the decryption will yield something useful.
The numbers above are mostly made up, but show the idea. Of course
the best thing is to have a high-entropy passphrase.
Would a 100 sec iteration time be even better? Yes and no.
Cryptographically it would be a lot better, namely 100 times better.
However, usability is a very important factor for security technology
and one that gets overlooked surprisingly often. For LUKS, if you
have to wait 2 minutes to unlock the LUKS container, most people will
not bother and use less secure storage instead. It is better to have
less protection against low-entropy passphrases and people actually
use LUKS, than having them do without encryption altogether.
Now, what about decreasing the iteration time? This is generally a
very bad idea, unless you know and can enforce that the users only
use high-entropy passphrases. If you decrease the iteration time
without ensuring that, then you put your users at increased risk, and
considering how rarely LUKS containers are unlocked in a typical
work-flow, you do so without a good reason. Don't do it. The
iteration time is already low enough that users with entropy low
passphrases are vulnerable. Lowering it even further increases this
danger significantly.
* 5.11 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 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 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. 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
resourceful attacker is a high-entropy passphrase, see items 5.9 and
5.10.
* 5.12 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.
* 5.13 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
security aspects are independent of CPU speed.
The master key is less critical, as you really have to work at it 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 installation on an
embedded device without network and other entropy sources).
For the passphrase, don't use a low-entropy passphrase. If your
passphrase is good, then a slow CPU will not matter. If you insist
on a low-entropy passphrase on a slow CPU, use something like
"--iter-time=10000" or higher and wait a long time on each LUKS
unlock and pray that the attacker does not find out in which way
exactly your passphrase is low entropy. This also applies to
low-entropy passphrases on fast CPUs. Technology can do only so much
to compensate for problems in front of the keyboard.
Also note that power-saving modes will make your CPU slower. This
will reduce iteration count on LUKS container creation. It will keep
unlock times at the expected values though at this CPU speed.
* 5.14 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
the initialization vector (IV) is the sector number. The IV gets
XORed to the first data chunk of the sector to be encrypted. If you
make sure that the first data block to be stored in a sector contains
the sector number as well, the first data block to be encrypted is
all zeros and always encrypted to the same ciphertext. This also
works if the first data chunk just has a constant XOR with the sector
number. By having several shifted patterns you can take care of the
case of a non-power-of-two start sector number of the file.
This mechanism allows you to create a pattern of sectors that have
the same first ciphertext block and signal one bit per sector to the
outside, allowing you to e.g. mark media files that way for
recognition without decryption. For large files this is a practical
attack. For small ones, you do not have enough blocks to signal and
take care of different file starting offsets.
In order to prevent this attack, the default was changed to
cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
encryption key as key. This makes the IV unpredictable without
knowing the encryption key and the watermarking attack fails.
* 5.15 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.
However there are modes, like XTS, that are secure with "plain" IV.
The next limit is that "plain" is 64 bit, with the upper 32 bit set
to zero. This means that on volumes larger than 2TiB, the IV
repeats, creating a vulnerability that potentially leaks some data.
To avoid this, use "plain64", which uses the full sector number up to
64 bit. Note that "plain64" requires a kernel 2.6.33 or more recent.
Also note that "plain64" is backwards compatible for volume sizes of
maximum size 2TiB, but not for those > 2TiB. Finally, "plain64" does
not cause any performance penalty compared to "plain".
* 5.16 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
used, e.g. in Truecrypt. From version 1.6.0 of cryptsetup onwards,
aes-xts-plain64 is the default for LUKS. If you want to use it with
a cryptsetup before version 1.6.0 or with plain dm-crypt, you have to
specify it manually as "aes-xts-plain", i.e.
cryptsetup -c aes-xts-plain luksFormat <device>
For volumes >2TiB and kernels >= 2.6.33 use "plain64" (see FAQ item
on "plain" and "plain64"):
cryptsetup -c aes-xts-plain64 luksFormat <device>
There is a potential security issue with XTS mode and large blocks.
LUKS and dm-crypt always use 512B blocks and the issue does not
apply.
* 5.17 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.
* 5.18 What about Plausible Deniability?
First let me attempt a definition for the case of encrypted
filesystems: Plausible deniability is when you store data
inside an encrypted container and it is not possible to prove it is
there without having a special passphrase. And at the same time
it must be "plausible" that there actually is no hidden data there.
As a simple entropy-analysis will show that here may be data there,
the second part is what makes it tricky.
There seem to be a lot of misunderstandings what that
means, so let me make clear that this refers to the situation where
the attackers can prove that there is data that may be random or
may be part of a plausible-deniability scheme, they just cannot
prove which one it is. Hence a plausible-deniability
scheme must hold up when the attackers know there is
something potentially fishy. If you just hide data and rely on
it not being found, that is just simple deniability, not "plausible"
deniability and I am not talking about that in the following.
Simple deniability against a low-competence attacker may
be as simple as renaming a file or putting data into an unused
part of a disk. Simple deniability against a high-skill attacker
with time to invest is usually pointless though unless you go
for advanced steganographic techniques, which have their own
drawbacks, such as low data capacity.
Now, the idea of plausible deniability 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 they 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.
* 5.19 What about SSDs, Flash and Hybrid 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-addressable 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 may or may not be secure for a hybrid drive. If
it finishes on an SSD after a few seconds, it was possibly faked.
Unfortunately, for hybrid drives that indicator does not work, as the
drive may well take the time to truly erase the magnetic part, but
only mark the SSD/Flash part as erased while data is still in there.
If you can do without password management and are fine with doing
physical destruction for permanently deleting data (always after one
or several full overwrites!), you can use plain dm-crypt or LUKS.
If you want or need all 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 or hybrid drive. 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 targeted laptop theft to get at your data,
you should be fine.
* 5.20 LUKS is broken! It uses SHA-1!
No, it is not. SHA-1 is (academically) broken for finding collisions,
but not for using it in a key-derivation function. And that
collision vulnerability is for non-iterated use only. And you need
the hash-value in verbatim.
This basically means that if you already have a slot-key, and you
have set the PBKDF2 iteration count to 1 (it is > 10'000 normally),
you could (maybe) derive a different passphrase that gives you the
the same slot-key. But if you have the slot-key, you can already
unlock the key-slot and get the master key, breaking everything. So
basically, this SHA-1 vulnerability allows you to open a LUKS
container with high effort when you already have it open.
The real problem here is people that do not understand crypto and
claim things are broken just because some mechanism is used that has
been broken for a specific different use. The way the mechanism is
used matters very much. A hash that is broken for one use can be
completely secure for other uses and here it is.
* 5.21 Why is there no "Nuke-Option"?
A "Nuke-Option" or "Kill-switch" is a password that when entered upon
unlocking instead wipes the header and all passwords. So when
somebody forces you to enter your password, you can destroy the data
instead.
While this sounds attractive at first glance, it does not make sense
once a real security analysis is done. One problem is that you have
to have some kind of HSM (Hardware Security Module) in order to
implement it securely. In the movies, a HSM starts to smoke and melt
once the Nuke-Option has been activated. In reality, it just wipes
some battery-backed RAM cells. A proper HSM costs something like
20'000...100'000 EUR/USD and there a Nuke-Option may make some sense.
BTW, a chipcard or a TPM is not a HSM, although some vendors are
promoting that myth.
Now, a proper HSMs will have a wipe option but not a Nuke-Option,
i.e. you can explicitly wipe the HSM, but by a different process
than unlocking it takes. Why is that? Simple: If somebody can force
you to reveal passwords, then they can also do bad things to you if
you do not or if you enter a nuke password instead. Think locking
you up for a few years for "destroying evidence" or for far longer
and without trial for being a "terrorist suspect". No HSM maker will
want to expose its customers to that risk.
Now think of the typical LUKS application scenario, i.e. disk
encryption. Usually the ones forcing you to hand over your password
will have access to the disk as well, and, if they have any real
suspicion, they will mirror your disk before entering anything
supplied by you. This neatly negates any Nuke-Option. If they have
no suspicion (just harassing people that cross some border for
example), the Nuke-Option would work, but see above about likely
negative consequences and remember that a Nuke-Option may not work
reliably on SSD and hybrid drives anyways.
Hence my advice is to never take data that you do not want to reveal
into any such situation in the first place. There is no need to
transfer data on physical carriers today. The Internet makes it
quite possible to transfer data between arbitrary places and modern
encryption makes it secure. If you do it right, nobody will even be
able to identify source or destination. (How to do that is out of
scope of this document. It does require advanced skills in this age
of pervasive surveillance.)
Hence, LUKS has not kill option because it would do much more harm
than good.
Still, if you have a good use-case (i.e. non-abstract real-world
situation) where a Nuke-Option would actually be beneficial, please
let me know.
* 5.22 Does cryptsetup open network connections to websites, etc. ?
This question seems not to make much sense at first glance, but here
is an example form the real world: The TrueCrypt GUI has a "Donation"
button. Press it, and a web-connection to the TrueCrypt website is
opened via the default browser, telling everybody that listens that
you use TrueCrypt. In the worst case, things like this can get
people tortured or killed.
So: Cryptsetup will never open any network connections except the
local netlink socket it needs to talk to the kernel crypto API.
In addition, the installation package should contain all
documentation, including this FAQ, so that you do not have to go to a
web-site to read it. (If your distro cuts the docu, please complain
to them.) In security software, any connection initiated to anywhere
outside your machine should always be the result of an explicit
request for such a connection by the user and cryptsetup will stay
true to that principle.
6. Backup and Data Recovery
* 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 indication
that this may be even worse for some SSDs. This applies both to LUKS
and plain dm-crypt partitions.
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
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
header damage.
So, data-backup in some form is non-optional. For LUKS, you may also
want to store a header backup in some secure location. This only
needs an update if you change passphrases.
* 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
"luksHeaderBackup" of cryptsetup. This protects also against errors
when non-standard parameters have been used in LUKS partition
creation. Example:
cryptsetup luksHeaderBackup --header-backup-file <file> <device>
To restore, use the inverse command, i.e.
cryptsetup luksHeaderRestore --header-backup-file <file> <device>
If you are unsure about a header to be restored, make a backup of the
current one first! You can also test the header-file without restoring
it by using the --header option for a detached header like this:
cryptsetup --header <file> luksOpen <device> </dev/mapper/ -name>
If that unlocks your keys-lot, you are good. Do not forget to close
the device again.
Under some circumstances (damaged header), this fails. Then use the
following steps:
First determine the master-key size:
cryptsetup luksDump <device>
gives a line of the form
MK bits: <bits>
with bits equal to 256 for the old defaults and 512 for the new
defaults. 256 bits equals a total header size of 1'052'672 Bytes and
512 bits one of 2MiB. (See also Item 6.12) If luksDump fails, assume
2MiB, but be aware that if you restore that, you may also restore the
first 1M or so of the filesystem. Do not change the filesystem if
you were unable to determine the header size! With that, restoring a
too-large header backup is still safe.
Second, dump the header to file. There are many ways to do it, I
prefer the following:
head -c 1052672 <device> > header_backup.dmp
or
head -c 2M <device> > header_backup.dmp
for a 2MiB header. Verify the size of the dump-file to be sure.
To restore such a backup, you can try luksHeaderRestore or do a more
basic
cat header_backup.dmp > <device>
* 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
blkid -p <device>
which will also detect other types and give some more info. Omit
"-p" for old versions of blkid that do not support it.
* 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 image is already
encrypted, but cannot be compressed and contains all empty space.
The filesystem backup can be compressed, can contain only part of the
encrypted device, but needs to be encrypted separately if so desired.
A sector-image will contain the whole partition in encrypted form,
for LUKS the LUKS header, the keys-slots and the data area. It can
be done under Linux e.g. with dd_rescue (for a direct image copy)
and with "cat" or "dd". Example:
cat /dev/sda10 > sda10.img
dd_rescue /dev/sda10 sda10.img
You can also use any other backup software that is capable of making
a sector image of a partition. Note that compression is ineffective
for encrypted data, hence it does not make sense to use it.
For a filesystem backup, you decrypt and mount the encrypted
partition and back it up as you would a normal filesystem. In this
case the backup is not encrypted, unless your encryption method does
that. For example you can encrypt a backup with "tar" as follows
with GnuPG:
tar cjf - <path> | gpg --cipher-algo AES -c - > backup.tbz2.gpg
And verify the backup like this if you are at "path":
cat backup.tbz2.gpg | gpg - | tar djf -
Note: Always verify backups, especially encrypted ones!
There is one problem with verifying like this: The kernel may still
have some files cached and in fact verify them against RAM or may
even verify RAM against RAM, which defeats the purpose of the
exercise. The following command empties the kernel caches:
echo 3 > /proc/sys/vm/drop_caches
Run it after backup and before verify.
In both cases GnuPG will ask you interactively for your symmetric
key. The verify will only output errors. Use "tar dvjf -" to get
all comparison results. To make sure no data is written to disk
unencrypted, turn off swap if it is not encrypted before doing the
backup.
Restore works like certification with the 'd' ('difference') replaced
by 'x' ('eXtract'). Refer to the man-page of tar for more
explanations and instructions. Note that with default options tar
will overwrite already existing files without warning. If you are
unsure about how to use tar, experiment with it in a location where
you cannot do damage.
You can of course use different or no compression and you can use 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-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.
* 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 and
user error. By far the most questions on the dm-crypt mailing list
about how to recover a damaged LUKS partition are related to user
error. For example, if you create a new filesystem on a LUKS
partition, chances are good that all data is lost permanently.
For this case, a header+key-slot backup would often be enough. But
keep in mind that a well-treated (!) HDD has roughly a failure risk
of 5% per year. It is highly advisable to have a complete backup to
protect against this case.
* 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. If
you use this for an device with data that should be persistent, you
need to make sure you either do not lose access to that master key or
have a backup of the data. If you derive from a LUKS device, a
header backup of that device would cover backing up the master key.
Keep in mind that this does not protect against disk loss.
Note: If you recreate the LUKS header of the device you derive from
(using luksFormat), the master key changes even if you use the same
passphrase(s) and you will not be able to decrypt the derived device
with the new LUKS header.
* 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.
There are risks introduced by backups. For example if you
change/disable a key-slot in LUKS, a binary backup of the partition
will still have the old key-slot. To deal with this, you have to be
able to change the key-slot on the backup as well, securely erase the
backup or do a filesystem-level backup instead of a binary one.
If you use dm-crypt, backup is simpler: As there is no key
management, the main risk is that you cannot wipe the backup when
wiping the original. However wiping the original for dm-crypt should
consist of forgetting the passphrase and that you can do without
actual access to the backup.
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 one
go, e.g. as described above with tar and GnuPG.
My personal advice is to use one USB disk (low value data) or three
disks (high value data) in rotating order for backups, and either use
independent LUKS partitions on them, or use encrypted backup with tar
and GnuPG.
If you do network-backup or tape-backup, I strongly recommend to go
the filesystem backup path with independent encryption, as you
typically cannot reliably delete data in these scenarios, especially
in a cloud setting. (Well, you can burn the tape if it is under your
control...)
* 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 key-slot descriptors of the header and the key-slots. If the
salt values are overwritten or changed, nothing (in the
cryptographically strong sense) can be done to access the data,
unless there is a backup of the LUKS header. If a key-slot is
damaged, the data can still be read with a different key-slot, if
there is a remaining undamaged and used key-slot. Note that in order
to make a key-slot unrecoverable in a cryptographically strong sense,
changing about 4-6 bits in random locations of its 128kiB size is
quite enough.
* 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
damage the key-slots in part or in full. See also last item.
* 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
also helps if you have a mapped container that you forgot or do not
know a passphrase for (e.g. on a long running server.)
WARNING: Things go wrong, do a full backup before trying this!
WARNING: This exposes the master key of the LUKS container. Note
that both ways to recreate a LUKS header with the old master key
described below will write the master key to disk. Unless you are
sure you have securely erased it afterwards, e.g. by writing it to
an encrypted partition, RAM disk or by erasing the filesystem you
wrote it to by a complete overwrite, you should change the master 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 process,
except generating a new LUKS header with the old master key (it
prints the command for that though):
https://gitlab.com/cryptsetup/cryptsetup/blob/master/misc/luks-header-from-active
You can also do this manually. Here is how:
- Get the master key from the device mapper. This is done by the
following command. Substitute c5 for whatever you mapped to:
# dmsetup table --target crypt --showkey /dev/mapper/c5
Result:
0 200704 crypt aes-cbc-essiv:sha256
a1704d9715f73a1bb4db581dcacadaf405e700d591e93e2eaade13ba653d0d09
0 7:0 4096
The result is actually one line, wrapped here for clarity. The long
hex string is the master key.
- Convert the master key to a binary file representation. You can do
this manually, e.g. with hexedit. You can also use the tool "xxd"
from vim like this:
echo "a1704d9....53d0d09" | xxd -r -p > <master-key-file>
- 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 sub-item.
Unmap the device before you do that (luksClose). Then do
cryptsetup luksFormat --master-key-file=<master-key-file> <luks device>
Note that if the container was created with other than the default
settings of the cryptsetup version you are using, you need to give
additional parameters specifying the deviations. If in doubt, try
the script by Milan. It does recover the other parameters as well.
Side note: This is the way the decrypt_derived script gets at the
master key. It just omits the conversion and hashes the master key
string.
- If the header is intact and you just forgot the passphrase, just
set a new passphrase like this:
cryptsetup luksAddKey --master-key-file=<master-key-file> <luks device>
You may want to disable the old one afterwards.
* 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
given. If you use a cipher different than the default, you have to
specify that as a parameter to cryptsetup too. If you want to change
the password, you basically have to create a second encrypted device
with the new passphrase and copy your data over. On the plus side,
if you accidentally overwrite any part of a dm-crypt device, the
damage will be limited to the area you overwrote.
* 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 data
area.
Header and key-slot descriptors fill the first 592 bytes. The
key-slot size depends on the creation parameters, namely on the
number of anti-forensic stripes, key material offset and master key
size.
With the default parameters, each key-slot is a bit less than 128kiB
in size. Due to sector alignment of the key-slot start, that means
the key block 0 is at offset 0x1000-0x20400, key block 1 at offset
0x21000-0x40400, and key block 7 at offset 0xc1000-0xe0400. The
space to the next full sector address is padded with zeros. Never
used key-slots are filled with what the disk originally contained
there, a key-slot removed with "luksRemoveKey" or "luksKillSlot" gets
filled with 0xff. Due to 2MiB default alignment, start of the data
area for cryptsetup 1.3 and later is at 2MiB, i.e. at 0x200000. For
older versions, it is at 0x101000, i.e. at 1'052'672 bytes, i.e. at
1MiB + 4096 bytes from the start of the partition. Incidentally,
"luksHeaderBackup" for a LUKS container created with default
parameters dumps exactly the first 2MiB (or 1'052'672 bytes for
headers created with cryptsetup versions < 1.3) to file and
"luksHeaderRestore" restores them.
For non-default parameters, you have to figure out placement
yourself. "luksDump" helps. See also next item. For the most
common non-default settings, namely aes-xts-plain with 512 bit key,
the offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, ..., and start of bulk
data at 0x200000.
The exact specification of the format is here:
https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
For your convenience, here is the LUKS header with hex offsets.
NOTE: The spec counts key-slots from 1 to 8, but the cryptsetup tool
counts from 0 to 7. The numbers here refer to the cryptsetup
numbers.
Refers to LUKS On-Disk Format Specification Version 1.2.1
LUKS header:
offset length name data type description
-----------------------------------------------------------------------
0x0000 0x06 magic byte[] 'L','U','K','S', 0xba, 0xbe
0 6
0x0006 0x02 version uint16_t LUKS version
6 3
0x0008 0x20 cipher-name char[] cipher name spec.
8 32
0x0028 0x20 cipher-mode char[] cipher mode spec.
40 32
0x0048 0x20 hash-spec char[] hash spec.
72 32
0x0068 0x04 payload-offset uint32_t bulk data offset in sectors
104 4 (512 bytes per sector)
0x006c 0x04 key-bytes uint32_t number of bytes in key
108 4
0x0070 0x14 mk-digest byte[] master key checksum
112 20 calculated with PBKDF2
0x0084 0x20 mk-digest-salt byte[] salt for PBKDF2 when
132 32 calculating mk-digest
0x00a4 0x04 mk-digest-iter uint32_t iteration count for PBKDF2
164 4 when calculating mk-digest
0x00a8 0x28 uuid char[] partition UUID
168 40
0x00d0 0x30 key-slot-0 key slot key slot 0
208 48
0x0100 0x30 key-slot-1 key slot key slot 1
256 48
0x0130 0x30 key-slot-2 key slot key slot 2
304 48
0x0160 0x30 key-slot-3 key slot key slot 3
352 48
0x0190 0x30 key-slot-4 key slot key slot 4
400 48
0x01c0 0x30 key-slot-5 key slot key slot 5
448 48
0x01f0 0x30 key-slot-6 key slot key slot 6
496 48
0x0220 0x30 key-slot-7 key slot key slot 7
544 48
Key slot:
offset length name data type description
-------------------------------------------------------------------------
0x0000 0x04 active uint32_t key slot enabled/disabled
0 4
0x0004 0x04 iterations uint32_t PBKDF2 iteration count
4 4
0x0008 0x20 salt byte[] PBKDF2 salt
8 32
0x0028 0x04 key-material-offset uint32_t key start sector
40 4 (512 bytes/sector)
0x002c 0x04 stripes uint32_t number of anti-forensic
44 4 stripes
* 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,
which is highly desirable for a number of reasons. Changing the
alignment is not recommended.
That said, with default parameters, the data area starts at exactly
2MB offset (at 0x101000 for cryptsetup versions before 1.3). The
smallest data area you can have is one sector of 512 bytes. Data
areas of 0 bytes can be created, but fail on mapping.
While you cannot put a filesystem into something this small, it may
still be used to contain, for example, key. Note that with current
formatting tools, a partition for a container this size will be 3MiB
anyways. If you put the LUKS container into a file (via losetup and
a loopback device), the file needs to be 2097664 bytes in size, i.e.
2MiB + 512B.
The two ways to influence the start of the data area are key-size and
alignment.
For alignment, you can go down to 1 on the parameter. This will still
leave you with a data-area starting at 0x101000, i.e. 1MiB+4096B
(default parameters) as alignment will be rounded up to the next
multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run on a
larger file and dump the LUKS header to get actual information.
For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
(e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
128 bit has to be considered insecure today.
Example 1 - AES 128 bit with CBC:
cryptsetup luksFormat -s 128 --align-payload=8 <device>
This results in a data offset of 0x81000, i.e. 516KiB or 528384
bytes. Add one 512 byte sector and the smallest LUKS container size
with these parameters is 516KiB + 512B or 528896 bytes.
Example 2 - Blowfish 64 bit with CBC (WARNING: insecure):
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 266752
bytes.
* 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 passphrases,
but on the plus side, it has zero on disk description and if you
overwrite some part of a plain dm-crypt partition, exactly the
overwritten parts are lost (rounded up to sector 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
* 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.
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.
* 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)
- The need to specify an IV offset
- Encryption mode needs specifying (e.g. "-c twofish-cbc-plain")
- Key size needs specifying (e.g. "-s 128" for 128 bit keys)
- Passphrase hash algorithm needs specifying
Also note that because plain dm-crypt and loop-aes format does not
have metadata, and while the loopAES extension for cryptsetup tries
autodetection (see command loopaesOpen), it may not always work. If
you still have the old set-up, using a verbosity option (-v) on
mapping with the old tool or having a look into the system logs after
setup could give you the information you need. Below, there are also
some things that worked for somebody.
* 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
losetup -e twofish -o 2560 /dev/loop0 /dev/sdb1
mount /dev/loop0 mount-point
translates to
cryptsetup create -c twofish -o 5 --skip 5 e1 /dev/sdb1
mount /dev/mapper/e1 mount-point
* 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
length. It seems the corresponding options for cryptsetup are
--cipher twofish-cbc-null -s 192 -h ripemd160:20
* 7.5 loop-aes v1 format OpenSUSE
Apparently this is done by older OpenSUSE distros and stopped working
from OpenSUSE 12.1 to 12.2. One user had success with the following:
cryptsetup create <target> <device> -c aes -s 128 -h sha256
* 7.6 Kernel encrypted loop device (cryptoloop)
There are a number of different losetup implementations for using
encrypted loop devices so getting this to work may need a bit of
experimentation.
NOTE: Do NOT use this for new containers! Some of the existing
implementations are insecure and future support is uncertain.
Example for a compatible mapping:
losetup -e twofish -N /dev/loop0 /image.img
translates to
cryptsetup create image_plain /image.img -c twofish-cbc-plain -H plain
with the mapping being done to /dev/mapper/image_plain instead of
to /dev/loop0.
More details:
Cipher, mode and password hash (or no hash):
-e cipher [-N] => -c cipher-cbc-plain -H plain [-s 256]
-e cipher => -c cipher-cbc-plain -H ripemd160 [-s 256]
Key size and offsets (losetup: bytes, cryptsetuop: sectors of 512 bytes):
-k 128 => -s 128
-o 2560 => -o 5 -p 5 # 2560/512 = 5
There is no replacement for --pass-fd, it has to be emulated using
keyfiles, see the cryptsetup man-page.
8. Issues with Specific Versions of cryptsetup
* 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. You can check the compiled-in defaults
using "cryptsetup --help". Moreover, the plain device default
changed because the old IV mode was vulnerable to a watermarking
attack.
If you are using a plain device and you need a compatible mode, just
specify cipher, key size and hash algorithm explicitly. For
compatibility with cryptsetup 1.0.x defaults, simple use the
following:
cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160 <name> <dev>
LUKS stores cipher and mode in the metadata on disk, avoiding this
problem.
* 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 version of
cryptsetup (1.0.x) provided by SLED, which should also not be used
anymore as well. My advice would be to drop SLED 10.
* 8.3 Gcrypt 1.6.x and later break Whirlpool
It is the other way round: In gcrypt 1.5.x, Whirlpool is broken and
it was fixed in 1.6.0 and later. If you selected whirlpool as hash
on creation of a LUKS container, it does not work anymore with the
fixed library. This shows one serious risk of using rarely used
settings.
Note that at the time this FAQ item was written, 1.5.4 was the latest
1.5.x version and it has the flaw, i.e. works with the old Whirlpool
version. Possibly later 1.5.x versions will work as well. If not,
please let me know.
The only two ways to access older LUKS containers created with
Whirlpool are to either decrypt with an old gcrypt version that has
the flaw or to use a compatibility feature introduced in cryptsetup
1.6.4 and gcrypt 1.6.1 or later. Version 1.6.0 cannot be used.
Steps:
- Make at least a header backup or better, refresh your full backup.
(You have a full backup, right? See Item 6.1 and following.)
- Make sure you have cryptsetup 1.6.4 or later and check the gcrypt
version:
cryptsetup luksDump <your luks device> --debug | grep backend
If gcrypt is at version 1.5.x or before:
- Reencrypt the LUKS header with a different hash. (Requires entering
all keyslot passphrases. If you do not have all, remove the ones you
do not have before.):
cryptsetup-reencrypt --keep-key --hash sha256 <your luks device>
If gcrypt is at version 1.6.1 or later:
- Patch the hash name in the LUKS header from "whirlpool" to
"whirlpool_gcryptbug". This activates the broken implementation.
The detailed header layout is in Item 6.12 of this FAQ and in the
LUKS on-disk format specification. One way to change the hash is
with the following command:
echo -n -e 'whirlpool_gcryptbug\0' | dd of=<luks device> bs=1 seek=72 conv=notrunc
- You can now open the device again. It is highly advisable to change
the hash now with cryptsetup-reencrypt as described above. While you
can reencrypt to use the fixed whirlpool, that may not be a good idea
as almost nobody seems to use it and hence the long time until the
bug was discovered.
9. The Initrd question
* 9.1 My initrd is broken with cryptsetup or does now work as I want it to
That is not nice! However the initrd is supplied by your distribution, not by
the cryptsetup project and hence you should complain to them. We cannot
really do anything about it.
* 9.2 CVE-2016-4484 says cryptsetup is broken!
Not really. It says the initrd in some Debian versions have a behavior that
under some very special and unusual conditions may be considered
a vulnerability. Incidentally, at this time (1-Jan-17) CVE-2016-4484 still says
absolutely nothing, which means that the reporters could not be bothered
do actually describe the problem so far and hence it cannot be that bad.
If it were, you would expect that they would have a CVE description in
there more than 30 days (!) after reporting the problem to the press.
What happens is that you can trick the initrd to go to a rescue-shell
if you enter the LUKS password wrongly in a specific way. But falling
back to a rescue shell on initrd errors is a sensible default behavior
in the first place. It gives you about as much access as booting
a rescue system from CD or USB-Stick or as removing the disk would
give you. So this only applies when an attacker has physical access,
but cannot boot anything else or remove the disk. These will be rare
circumstances indeed, and if you rely on the default distribution
initrd to keep you safe under these circumstances, than you have
bigger problems than this somewhat expected behavior.
My take is this was much more driven by some big egos that wanted
to make a splash for self-aggrandizement, than by any actual
security concerns. Ignore it.
* 9.3 How do I do my own initrd with cryptsetup?
It depends on the distribution. Below, I give a very simple example
and step-by-step instructions for Debian. With a bit of work, it
should be possible to adapt this to other distributions. Note that
the description is pretty general, so if you want to do other things
with an initrd it provides an useful starting point for that too.
01) Unpacking an existing initrd to use as template
A Linux initrd is in gzip'ed cpio format. To unpack it, use something
like this:
md tmp; cd tmp; cat ../initrd | gunzip | cpio -id
After this, you have the full initrd content in tmp/
02) Inspecting the init-script
The init-script is the only thing the kernel cares about. All activity
starts there. Its traditional location is /sbin/init on disk, but /init
in an initrd. In an initrd unpacked as above it is tmp/init.
While init can be a binary despite usually being called "init script",
in Debian the main init on the root partition is a binary, but the
init in the initrd (and only that one is called by the kernel) is a script
and starts like this:
#!/bin/sh
....
The "sh" used here is in tmp/bin/sh as just unpacked, and in
Debian it currently is a busybox.
03) Creating your own initrd
The two examples below should give you most of what is needed.
Here is a really minimal example. It does nothing but set up some
things and then drop to an interactive shell. It is perfect to try
out things that you want to go into the init-script.
!/bin/sh
export PATH=/sbin:/bin
[ -d /sys ] || mkdir /sys
[ -d /proc ] || mkdir /proc
[ -d /tmp ] || mkdir /tmp
mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
mount -t proc -o nodev,noexec,nosuid proc /proc
echo "initrd is running, starting BusyBox..."
exec /bin/sh --login
Here is an example that opens the first LUKS-partition it
finds with the hard-coded password "test2" and then
mounts it as root-filesystem. This is intended to be
used on an USB-stick that after boot goes into a safe,
as it contains the LUKS-passphrase in plain text and is
not secure to be left in the system. The script contains
debug-output that should make it easier to see what
is going on. Note that the final hand-over to the
init on the encrypted root-partition is done
by "exec switch_root /mnt/root /sbin/init", after
mounting the decrypted LUKS container
with "mount /dev/mapper/c1 /mnt/root".
The second argument of switch_root is relative to the
first argument, i.e. the init started with this command
is really /mnt/sbin/init before switch_root runs.
!/bin/sh
export PATH=/sbin:/bin
[ -d /sys ] || mkdir /sys
[ -d /proc ] || mkdir /proc
[ -d /tmp ] || mkdir /tmp
mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
mount -t proc -o nodev,noexec,nosuid proc /proc
echo "detecting LUKS containers in sda1-10, sdb1-10"; sleep 1
for i in a b
do
for j in 1 2 3 4 5 6 7 8 9 10
do
sleep 0.5
d="/dev/sd"$i""$j
echo -n $d
cryptsetup isLuks $d >/dev/null 2>&1
r=$?
echo -n " result: "$r""
# 0 = is LUKS, 1 = is not LUKS, 4 = other error
if expr $r = 0 > /dev/null
then
echo " is LUKS, attempting unlock"
echo -n "test2" | cryptsetup luksOpen --key-file=- $d c1
r=$?
echo " result of unlock attempt: "$r""
sleep 2
if expr $r = 0 > /dev/null
then
echo "*** LUKS partition unlocked, switching root *** (waiting 30 seconds before doing that)"
mount /dev/mapper/c1 /mnt/root
sleep 30
exec switch_root /mnt/root /sbin/init
fi
else
echo " is not LUKS"
fi
done
done
echo "FAIL finding root on LUKS, loading BusyBox..."; sleep 5
exec /bin/sh --login
04) What if I want a binary in the initrd, but libraries are missing?
That is a bit tricky. One option is to compile statically, but that
does not work for everything. Debian puts some libraries into
lib/ and lib64/ which are usually enough. If you need more, you
can add the libraries you need there. That may or may not need a
configuration change for the dynamic linker "ld" as well.
Refer to standard Linux documentation
on how to add a library to a Linux system. A running initrd is
just a running Linux system after all, it is not special in any way.
05) How do I repack the initrd?
Simply repack the changed directory. While in tmp/, do
the following:
find . | cpio --create --format='newc' | gzip > ../new_initrd
Rename "new_initrd" to however you want it called (the name of
the initrd is a kernel-parameter) and move to /boot. That is it.
10. References and Further Reading
* Purpose of this Section
The purpose of this section is to collect references to all materials
that do not fit the FAQ but are relevant in some fashion. This can
be core topics like the LUKS spec or disk encryption, but it can also
be more tangential, like secure storage management or cryptography
used in LUKS. It should still have relevance to cryptsetup and its
applications.
If you want to see something added here, send email to the maintainer
(or the cryptsetup mailing list) giving an URL, a description (1-3
lines preferred) and a section to put it in. You can also propose
new sections.
At this time I would like to limit the references to things that are
available on the web.
* Specifications
- LUKS on-disk format spec:
https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification
* Code Examples
- Some code examples are in the source package under docs/examples
- LUKS AF Splitter in Ruby by John Lane: https://rubygems.org/gems/afsplitter
* Brute-forcing passphrases
- http://news.electricalchemy.net/2009/10/password-cracking-in-cloud-part-5.html
- http://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes
* Tools
* SSD and Flash Disk Related
* Disk Encryption
* Attacks Against Disk Encryption
* Risk Management as Relevant for Disk Encryption
* Cryptography
* Secure Storage
A. Contributors
In no particular order:
- Arno Wagner
- Milan Broz
___
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