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FAQ
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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
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
http://code.google.com/p/cryptsetup/wiki/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.
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.
NO WARNING ON NON-INERACTIVE FORMAT: If you feed cryptsetup from
STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
warning that you are about to format (and e.g. will lose any
pre-existing LUKS container on the target), as it assumes it is
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 94 printable characters from the first 128 characters of
the ASCII table, as they will always have the same binary
representation. Other characters may have different encoding
depending on system configuration and your passphrase will not
work with a different encoding. A table of the standardized first
128 ASCII caracters can, e.g. be found on
http://en.wikipedia.org/wiki/ASCII
* 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
* 1.4 Who wrote this?
Current FAQ maintainer is Arno Wagner <[email protected]>. 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.
* 1.5 Where is the project website?
There is the project website at http://code.google.com/p/cryptsetup/
Please do not post questions there, nobody will read them. Use
the mailing-list instead.
* 1.6 Is there a mailing-list?
Instructions on how to subscribe to the mailing-list are at on the
project website. People are generally helpful and friendly on the
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 [email protected]
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:
http://dir.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt
2. Setup
* 2.1 What is the difference between "plain" and LUKS format?
Plain format is just that: It has no metadata on disk, reads all
paramters from the commandline (or the defaults), derives a
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 the US, the UK and dictatorships around the world,
they can force you to give up the keys (using imprisonment or worse
to pressure you), and in the worst case, they only need a
nebulous "suspicion" about the presence of encrypted data. 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.
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 ath the begining of the disk, see below under "What
does the LUKS on-disk format looks like?". The passphrases are used
to decryt a single master key that is stored in the anti-forensic
stripes.
Advantages are a higher usability, automatic configuration of
non-default crypto parameters, defenses against low-entropy
passphrases like salting and iterated PBKDF2 passphrase hashing,
the ability to change passhrases, and others.
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).
* 2.2 Can I encrypt an already existing, non-empty partition to use
LUKS?
There is no converter, and it is not really needed. The way to do
this is to make a backup of the device in question, securely wipe
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.
* 2.3 How do I use LUKS with a loop-device?
This can be very handy for experiments. Setup is just the same as
with any block device. If you want, for example, to use a 100MiB
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".
* 2.4 When I add a new key-slot to LUKS, it asks for a passphrase but
then complains about there not being a key-slot with that
passphrase?
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.
* 2.5 Encrytion on top of RAID or the other way round?
Unless you have special needs, place encryption between RAID and
filesystem, i.e. encryption on top of RAID. You can do it the other
way round, but you have to be aware that you then need to give the
pasphrase for each individual disk and RAID autotetection will not
work anymore. Therefore it is better to encrypt the RAID device,
e.g. /dev/dm0 .
* 2.6 How do I read a dm-crypt key from file?
Note that the file will still be hashed first, just like keyboard
input. Use the --key-file option, like this:
cryptsetup create --key-file keyfile e1 /dev/loop0
* 2.7 How do I read a LUKS slot key from file?
What you really do here is to read a passphrase from file, just as
you would with manual entry of a passphrase for a key-slot. You can
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
* 2.8 How do I read the LUKS master key from file?
The question you should ask yourself first is why you would want to
do this. The only legitimate reason I can think of is if you want
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.
* 2.9 What are the security requirements for a key read from file?
A file-stored key or passphrase has the same security requirements
as one entered interactively, however you can use random bytes and
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
* 2.10 If I map a journaled file system using dm-crypt/LUKS, does it
still provide its usual transactional guarantees?
As far as I know it does (but I may be wrong), but please note that
these "guarantees" are far weaker than they appear to be. For
example, you may not get a hard flush to disk surface even on a
call to fsync. In addition, the HDD itself may do independent
write reordering. Some other things can go wrong as well. The
filesystem developers are aware of these problems and typically
can make it work anyways. That said, dm-crypt/LUKS should not make
things worse.
Personally, I have several instances of ext3 on dm-crypt and have
not noticed any specific problems.
Update: I did run into frequent small freezes (1-2 sec) when putting
a vmware image on ext3 over dm-crypt. This does indicate that the
transactional guarantees are in place, but at a cost. When I went
back to ext2, the problem went away. This also seems to have gotten
better with kernel 2.6.36 and the reworking of filesystem flush
locking. Kernel 2.6.38 is expected to have more improvements here.
* 2.11 Can I use LUKS or cryptsetup with a more secure (external)
medium for key storage, e.g. TPM or a smartcard?
Yes, see the answers on using a file-supplied key. You do have to
write the glue-logic yourself though. Basically you can have
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.
* 2.12 Can I resize a dm-crypt or LUKS partition?
Yes, you can, as neither dm-crypt nor LUKS stores partition size.
Whether you should is a different question. Personally I recommend
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 typicaly 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.
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 inteference between the hardening and and
the way cryptsetup benchmarks PBKDF2. The solution to this is
currently not quite clear for an encrypted root filesystem. For
other uses, you can apparently specify USE="dynamic" as compile
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 contaner is on. Refer to Section "Backup and
Data Recovery" and ask on the mailing list if you have trouble
diagnosing and (if still possible) repairing this.
* 4.2 Can a bad RAM module cause problems?
LUKS and dm-crypt can give the RAM quite a workout, especially when
combined with software RAID. In particular the combination RAID5 +
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.
* 4.3 How do I test RAM?
First you should know that overclocking often makes memory
problems worse. So if you overclock (which I strongly recommend
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 memterster 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.
5. Security Aspects
* 5.1 Is LUKS insecure? Everybody can see I have encrypted data!
In practice it does not really matter. In most civilized countries
you can just refuse to hand over the keys, no harm done. In some
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?"
* 5.2 Should I initialize (overwrite) a new LUKS/dm-crypt partition?
If you just create a filesystem on it, most of the old data will
still be there. If the old data is sensitive, you should overwrite
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
* 5.3 How do I securely erase a LUKS (or other) partition?
For LUKS, if you are in a desperate hurry, overwrite the LUKS
header and key-slot area. This means overwriting the first
(keyslots x stripes x keysize) + offset bytes. For the default
parameters, this is the 1'052'672 bytes, i.e. 1MiB + 4096 of the
LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
512 bit key) this is 2MiB. (The diferent offset stems from
differences in the sector alignment of the key-slots.) If in doubt,
just be generous and overwrite the first 10MB or so, it will likely
still be fast enough. A single overwrite with zeros should be
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 reseach results on erasing SSDs and FLASH
drives:
http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf
Keep in mind to also erase all backups.
Example for a zero-overwrite erase of partition sde1 done with
dd_rescue:
dd_rescue -w /dev/zero /dev/sde1
* 5.4 How do I securely erase a backup of a LUKS partition or header?
That depends on the medium it is stored on. For HDD and SSD, use
overwrite with zeros. For an SSD or FLASH drive (USB stick), you
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.
* 5.5 What about backup? Does it compromise security?
That depends. See item 6.7.
* 5.6 Why is all my data permanently gone if I overwrite the LUKS
header?
Overwriting the LUKS header in part or in full is the most common
reason why access to LUKS containers is lost permanently.
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" value and without that no
decryption is possible. While the salt is not secret, it is
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 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".
* 5.7 What is a "salt"?
A salt is a random key-grade value added to the passphrase before
it is processed. It is not kept secret. The reason for using salts
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.
* 5.8 Is LUKS secure with a low-entropy (bad) passphrase?
Note: You should only use the 94 printable characters from 7 bit
ASCII code to prevent your passphrase from failing when the
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.
* 5.9 What is "iteration count" and why is decreasing it a bad idea?
Iteration count is the number of PBKDF2 iterations a passphrase is
put through before it is used to unlock a key-slot. Iterations are
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.
* 5.10 Some people say PBKDF2 is insecure?
There is some discussion that a hash-function should have a "large
memory" property, i.e. that it should require a lot of memory to be
computed. This serves to prevent attacks using special programmable
circuits, like FPGAs, and attacks using graphics cards. PBKDF2
does not need a lot of memory and is vulnerable to these attacks.
However, the publication usually refered in these discussions is
not very convincing in proving that the presented hash really is
"large memory" (that may change, email the FAQ maintainer when it
does) and it is of limited usefulness anyways. Attackers that use
clusters of normal PCs will not be affected at all by a "large
memory" property. For example the US Secret Service is known to
use the off-hour time of all the office PCs of the Treasury for
password breaking. The Treasury has about 110'000 employees.
Asuming every one has an office PC, that is significant computing
power, all of it with plenty of memory for computing "large
memory" hashes. Bot-net operators also have all the memory they
want. The only protection against a resouceful attacker is a
high-entropy passphrase, see items 5.8 and 5.9.
* 5.11 What about iteration count with plain dm-crypt?
Simple: There is none. There is also no salting. If you use plain
dm-crypt, the only way to be secure is to use a high entropy
passphrase. If in doubt, use LUKS instead.
* 5.12 Is LUKS with default parameters less secure on a slow CPU?
Unfortunately, yes. However the only aspect affected is the
protection for low-entropy passphrase or master-key. All other
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-startved 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=10" 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.
* 5.13 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?
The problem is that cbc-plain has a fingerprint vulnerability, where
a specially crafted file placed into the crypto-container can be
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.
* 5.14 Are there any problems with "plain" IV? What is "plain64"?
First, "plain" and "plain64" are both not secure to use with CBC,
see previous FAQ item.
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. Also note that "plain64" is backwards compatible for
volume sizes <= 2TiB, but not for those > 2TiB. Finally, "plain64"
does not cause any performance penalty compared to "plain".
* 5.15 What about XTS mode?
XTS mode is potentially even more secure than cbc-essiv (but only if
cbc-essiv is insecure in your scenario). It is a NIST standard and
used, e.g. in Truecrypt. At the moment, if you want to use it, 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.
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 occuurence. For example an
accidental format as FAT or some software overwriting the first
sector where it suspects a partition boot sector typically makes a
LUKS partition permanently inacessible. 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 h <device>
To restore, use the inverse command, i.e.
cryptsetup luksHeaderRestore --header-backup-file h <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 alos use the more general test
blkid -p <device>