Frequently Asked Questions Cryptsetup/LUKS

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. LUKS2 Questions
11. 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 volume 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

LUKS2 COMPATIBILITY: This FAQ was originally written for LUKS1, not LUKS2. Hence regarding LUKS2, some of the answers found here may not apply. Updates for LUKS2 have been done and anything not applying to LUKS2 should clearly say LUKS1. However, this is a Frequently Asked Questions, and questions for LUKS2 are limited at this time or at least those that have reached me are. In the following, "LUKS" refers to both LUKS1 and LUKS2.

The LUKS1 on-disk format specification is at
https://www.kernel.org/pub/linux/utils/cryptsetup/LUKS_docs/on-disk-format.pdf
The LUKS2 on-disk format specification is at
https://gitlab.com/cryptsetup/LUKS2-docs

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. The LUKS2 header should be a bit more resilient as critical data starts later and is stored twice, but you can decidedly still destroy it or a keyslot permanently by accident.

DEBUG COMMANDS: While the --debug and --debug-json options should not leak secret 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 assurances 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 volume key will stay the same! That means that if you distribute an image to several machines, the same volume 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 VOLUME KEY: The LUKS passphrase is not used in deriving the volume key. It is used in decrypting a volume 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 volume 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 https://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

The Ubuntu Natty uinstaller has a "won't fix" defect that may destroy LUKS containers. This is quite old an not relevant for most people. Reference: https://bugs.launchpad.net/ubuntu/+source/partman-crypto/+bug/420080

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 licensed under a Creative Commons CC-BY-SA-4.0 "Attribution-ShareAlike 4.0 International" 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 https://creativecommons.org/licenses/by-sa/4.0/ for more details.

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 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 https://subspace.kernel.org/lists.linux.dev.html. Go to the URL mentioned in the email and select "unsubscribe".

Alternatively, you can send an empty Email to cryptsetup+help@lists.linux.dev. Make sure to send it from your list address.

The mailing list archive is here: https://lore.kernel.org/cryptsetup/

The legacy dm-crypt mailing list archive is here: https://lore.kernel.org/dm-crypt/

1.8 Unsubscribe from the mailing-list

Send mail to cryptsetup+unsubscribe@lists.linux.dev 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 subscription to cryptsetup@lists.linux.dev

and are sent from cryptsetup+help@lists.linux.dev. 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.

1.9 What can I do if cryptsetup is running out of memory?

Memory issues are generally related to the key derivation function. You may be able to tune usage with the options --pbkdf-memory or --pbkdf pbkdf2.

1.10 Can cryptsetup be run without root access?

Elevated privileges are required to use cryptsetup and LUKS. Some operations require root access. There are a few features which will work without root access with the right switches but there are caveats.

1.11 What are the problems with running as non root?

The first issue is one of permissions to devices. Generally, root or a group such as disk has ownership of the storage devices. The non root user will need write access to the block device used for LUKS.

Next, file locking is managed in /run/cryptsetup. You may use --disable-locks but cryptsetup will no longer protect you from race conditions and problems with concurrent access to the same devices.

Also, device mapper requires root access. cryptsetup uses device mapper to manage the decrypted container.

1.12 How can I report an issue in the cryptsetup project?

Before reporting any issue, please be sure you are using the latest upstream version and that you read the documentation (and this FAQ).

If you think you have discovered an issue, please report it through the project issue tracker New issue. For a possible security issue, please use the confidential checkbox.

Please fill in all information requested in the report template (specifically add debug output with all run environment data). Do not trim the output; debug output does not include private data.

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 through when creating a new LUKS encrypted container, i.e. encrypted disk, partition or loop-file.

All data will be lost, if there is data on the target, make a backup.
Make very sure you use the right target disk, partition or loop-file.
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: https://www.tansi.org/tools/index.html) in the following fashion:

    cat /dev/zero | wcs > <target device>

Plain "dd" also gives you the progress on a SIGUSR1, see its man-page. The GNU "dd" command supports the "status=progress" operand that gives you the progress without having to send it any signal.

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

Create the LUKS container.

LUKS1:

    cryptsetup luksFormat --type luks1 <target device>

LUKS2:

    cryptsetup luksFormat --type luks2 <target device>

Just follow the on-screen instructions.

Note: Passphrase iteration count is based on time and hence security level depends on CPU power of the system the LUKS container is created on. For example on a Raspberry Pi and LUKS1, I found some time ago that the iteration count is 15 times lower than for a regular PC (well, for my old one). Depending on security requirements, this may need adjustment. For LUKS1, you can just look at the iteration count on different systems and select one you like. You can also change the benchmark time with the -i parameter to create a header for a slower system.

For LUKS2, the parameters are more complex. ARGON2 has iteration, parallelism and memory parameter. cryptsetup actually may adjust the memory parameter for time scaling. Hence to use -i is the easiest way to get slower or faster opening (default: 2000 = 2sec). Just make sure to not drop this too low or you may get a memory parameter that is to small to be secure. The luksDump command lists the memory parameter of a created LUKS2 keyslot in kB. That parameter should probably be not much lower than 100000, i.e. 100MB, but don't take my word for it.

Map the container. Here it will be mapped to /dev/mapper/c1:

    cryptsetup luksOpen <target device> c1

(Optionally) wipe the container (make sure you have the right target!):

    cat /dev/zero > /dev/mapper/c1

This will take a while. 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.

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

Mount your encrypted file system, here on /mnt:

    mount /dev/mapper/c1 /mnt

Make a LUKS header backup and plan for a container backup. See Section 6 for details.

Done. You can now use the encrypted file system to store data. Be sure to read through 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? What about RAID?

Also see Item 2.8.
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 add LVM 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 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.

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 to move that to additional encrypted partitions. If you really are concerned your root partition may be sabotaged by somebody with physical access (who 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).

That said, if you want an encrypted root partition, you have to store an initrd with cryptsetup somewhere else. The traditional approach is to have a separate partition under /boot for that. You can also put that initrd on a bootable memory stick, bootable CD or bootable external drive as well. The kernel and Grub typically go to the same location as that initrd. A minimal example what such an initrd can look like is given in Section 9.

(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 different RAID 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 as most do today.

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.

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, as 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-partition RAID1 or by using LVM), as that 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 however. 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.

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/cryptdisks 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 volume-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 volume key that is stored in the anti-forensic stripes. LUKS2 adds some more flexibility.

Advantages are a higher usability, automatic configuration of non-default crypto parameters, defenses against low-entropy passphrases like salting and iterated PBKDF2 or ARGON 2 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. Unless you have a specific need, use LUKS2.

2.5 Can I encrypt an 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 file to /dev/loop0
    cryptsetup luksFormat --type luks2 /dev/loop0   # create LUKS2 container

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?

Also see Item 2.2.
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 auto-detection 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 (LUKS)
  |
  RAID
  |
  Raw partitions (optional)
  |
  Raw disks      <- bottom

The big advantage of this is that you can manage the RAID container just like any other regular RAID container, it does not care that its content is encrypted. This strongly cuts down on complexity, something very valuable with storage encryption.

Try to avoid so-called fake RAID (RAID configured from BIOS but handled by proprietary drivers). Note that some fake RAID firmware automatically writes signature on disks if enabled. This causes corruption of LUKS metadata. Be sure to switch the RAID option off in BIOS if you do not use it.

Another data corruption can happen if you resize (enlarge) the underlying device and some remnant metadata appear near the end of the resized device (like a secondary copy of the GPT table). You can use wipefs command to detect and wipe such signatures.

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 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. Make sure no trailing newline (0x0a) is contained in the input 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 volume 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 volume 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 onward by dm-crypt (regardless of direct key management or LUKS key management). A bit more information on the process by which transactional guarantees are implemented can be found here:

https://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. This is likely still true with SSDs. 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 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). This 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.

2.15 Can I resize a dm-crypt or LUKS container?

Yes, you can, as neither dm-crypt nor LUKS1 stores partition size and LUKS2 uses a generic "whole device" size as default. Note that LUKS2 can use specified data-area sizes as a non-standard case and that these may cause issues when resizing a LUKS2 container if set to a specific value.

Whether you should do this is a different question. Personally I recommend backup, recreation of the dm-crypt or LUKS container 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. A backup is really non-optional here, as a lot can go wrong, resulting in partial or complete data loss. But if you have that backup, you can also just recreate everything.

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

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 and 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 volume-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 volume-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. Also note that SSDs typically have much larger blocks internally (e.g. 128kB or even larger).

2.19 How can I wipe a device with crypto-grade randomness?

The conventional recommendation if you want to do more than just a zero-wipe is to use something like

    cat /dev/urandom >  <target-device>

That used to very slow and painful at 10-20MB/s on a fast computer, but newer kernels can give you > 200MB/s (depending on hardware). An alternative is using cryptsetup and a plain dm-crypt device with a random key, which is fast and on 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/<device> target

This maps the container as plain under /dev/mapper/target with a random password. For the actual wipe you have several options. Basically, you pipe zeroes into the opened container that then get encrypted. 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 https://www.tansi.org/tools/index.html ):

    cat /dev/zero | wcs > /dev/mapper/to_be_wiped

Or use plain "dd", which gives you the progress when sent a SIGUSR1, see the dd man page. The GNU "dd" command supports the "status=progress" operand that gives you the progress without having to send it any signal.

Remove the mapping at the end and you are done.

2.20 How do I wipe only the LUKS header?

This does not describe an emergency wipe procedure, see Item 5.4 for that. This procedure here 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. Please only do this if you have a current backup.

LUKS1:

Determine header size in 512 Byte sectors with luksDump:

     cryptsetup luksDump <device with LUKS container>

->   ...
     Payload offset: <number> [of 512 byte sectors]
     ...

Take the result number, multiply by 512 zeros and write to the start of the device, e.g. using one of the following alternatives:

     dd bs=512 count=<number> if=/dev/zero of=<device>

     head -c <number * 512> /dev/zero > /dev/<device>

LUKS2:
(warning, untested! Remember that backup?) This assumes the LUKS2 container uses the defaults, in particular there is only one data segment.

Determine the data-segment offset using luksDump, same as above for LUKS1:

     cryptsetup luksDump <device with LUKS container>
->   ...  
     Data segments:
        0: crypt
           offset: <number> [bytes]
     ...

Overwrite the stated number of bytes from the start of the device. Just to give yet another way to get a defined number of zeros:

     head -c <number> /dev/zero > /dev/<device>

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 unlock 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 (2 seconds for LUKS2). 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 (LUKS2: up to 32 key-slots) have to be tried in order to find the right one.

If this is the 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 the unlock time adjusted to the slow machine. Use luksKeyAdd and then luksKillSlot or luksRemoveKey. You can also use the -i option to reduce iteration time (and security level) when setting a passphrase. Default is 1000 (1 sec) for LUKS1 and 2000 (2sec) for LUKS2.

However, this operation will not change volume key iteration count ("MK iterations" for LUKS1, "Iterations" under "Digests" for LUKS2). 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 MK iterations are not very security relevant.

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 or later:

    cryptsetup luksHeaderBackup --header-backup-file <file> <device>
    cryptsetup luksHeaderRestore --header-backup-file <file> <device>

3.6 I see a data corruption with the Intel QAT kernel driver; why?

Intel QAT crypto API drivers have severe bugs that are not fixed for years.

If you see data corruption, please disable the QAT in the BIOS or avoid loading kernel Intel QAT drivers (switch to software crypto implementation or AES-NI).

For more info, see posts in dm-devel list https://lore.kernel.org/dm-devel/?q=intel+qat

4. Troubleshooting

4.1 I get the error "LUKS keyslot x is invalid." What does that mean?

For LUKS1, this means that the given keyslot has an offset that points outside the valid keyslot area. Typically, the reason is a corrupted LUKS1 header because something was written to the start of the device the LUKS1 container is on. For LUKS2, I do not know when this error can happen, but I expect it will be something similar. 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 with that correct passphrase and a respective key-slot that 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 for LUKS1 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 + LUKS1 + XFS seems to uncover RAM problems that do not cause obvious problems otherwise. Symptoms vary, but often the problem manifests itself when copying large amounts of data, typically several times larger than your main memory.

Note: One thing you should always do on large data copying or 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 data 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 a 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: If you flip one bit in an encrypted data block, the decrypted version has half of its bits flipped. This is actually an important security property for modern ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you can get a completely changed 512 byte block for a 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 any 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 either 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 and use the largest memory area you can get)
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 in the past 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. A 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. If your passphrase is 200 times the letter "a", it is long but has very low entropy and is pretty insecure.

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 https://gist.github.com/epixoip/a83d38f412b4737e99bbef804a270c40 This thing costs 23kUSD and does 68Ghashes/sec for SHA1. This is in 2017.

Incidentally, my older calculation for a machine around 1000 times slower was off by a factor of about 1000, but in the right direction, i.e. I estimated the attack to be too easy. Nobody noticed ;-) On the plus side, the tables are now (2017) pretty much accurate.

More references can be found at the end of this document. Note that these are estimates from the defender side, so assuming something is easier than it actually is fine. An attacker may still have significantly higher cost than estimated here.

LUKS1 used SHA1 (since version 1.7.0 it uses SHA256) for hashing per default. We will leave aside the check whether a try actually decrypts a key-slot. I will assume a useful lifetime of the hardware of 2 years. (This is on the low side.) Disregarding downtime, the machine can then break

     N = 68*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18

passphrases for EUR/USD 23k. That is one 62 bit passphrase hashed once with SHA1 for EUR/USD 23k. This can be parallelized, it can be done faster than 2 years with several of these machines.

For LUKS2, things look a bit better, as the advantage of using graphics cards is massively reduced. Using the recommendations below should hence be fine for LUKS2 as well and give a better security margin.

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 LUKS1, 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.

LUKS1 and LUKS2: 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. 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, if true, that is impossible and hence the whole idea is not compatible with fair laws. Note that in this context, countries like the US or the UK are not civilized and do not have fair laws.

As a side-note, standards for biometrics (fingerprint, retina, vein-pattern, etc.) are often different and much lower. If you put your LUKS passphrase into a device that can be unlocked using biometrics, they may force a biometric sample in many countries where they could not force you to give them a passphrase you solely have in your memory and can claim to have forgotten if needed (it happens). If you need protection on this level, make sure you know what the respective legal situation is, also while traveling, and make sure you decide beforehand what you will do if push comes to shove as they will definitely put you under as much pressure as they can legally apply.

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? Tough 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 container?

For LUKS, if you are in a desperate hurry, overwrite the LUKS header and key-slot area. For LUKS1 and LUKS2, just be generous and overwrite the first 100MB. 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 100000000 /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.

Also note that SSDs and also some HDDs (SMR and hybrid HDDs, for example) may not actually overwrite the header and only do that an unspecified and possibly very long time later. The only way to be sure there is physical destruction. If the situation permits, do both overwrite and physical destruction.

If you have time, overwrite the whole drive with a single pass of random data. This is enough for most HDDs. For SSDs or FLASH (USB sticks) or SMR or hybrid drives, you may want to overwrite the whole drive several times to be sure data is not retained. This is possibly still insecure as the respective technologies are not fully understood in this regard. Still, due to the anti-forensic properties of the LUKS key-slots, a single overwrite 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: https://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf

Keep in mind to also erase all backups.

Example for a random-overwrite erase of partition sde1 done with dd_rescue:

    dd_rescue -w /dev/urandom /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 random data. For an SSD, FLASH drive (USB stick) hybrid HDD or SMR HDD, you may want to overwrite the complete drive several times and use physical destruction in addition, see last item. For re-writable CD/DVD, a single overwrite should 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 during overwrites.

The best option is to actually encrypt the backup, for example with PGP/GnuPG and then just destroy all copies of the encryption key if needed. Best keep them on paper, as that has excellent durability and secure destruction is easy, for example by burning and then crushing the ashes to a fine powder. A blender and water also works nicely.

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 LUKS1 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 LUKS1 container is still open, then it may be possible to extract the volume key from the running system. See Item "How do I recover the volume key from a mapped LUKS1 container?" in Section "Backup and Data Recovery".

For LUKS2, things are both better and worse. First, the salts are in a less vulnerable position now. But, on the other hand, the keys of a mapped (open) container are now stored in the kernel key-store, and while there probably is some way to get them out of there, I am not sure how much effort that needs.

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?

Short answer: yes. Do not use a low-entropy passphrase.

Note: For LUKS2, protection for bad passphrases is a bit better due to the use of Argon2, but that is only a gradual improvement.

Longer answer:
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 too 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 and switch some words around, then this is good passphrase material.

5.10 What is "iteration count" and why is decreasing it a bad idea?

LUKS1:
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 LUKS1, 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 low entropy passphrases are vulnerable. Lowering it even further increases this danger significantly.

LUKS2: Pretty much the same reasoning applies. The advantages of using GPUs or FPGAs in an attack have been significantly reduced, but that is the only main difference.

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.

That said, LUKS2 defaults to Argon2, which has a large-memory property and massively reduces the advantages of GPUs and FPGAs.

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 volume-key. All other security aspects are independent of CPU speed.

The volume key is less critical, as you really have to work at it to give it low entropy. One possibility to mess this up is to supply the volume key yourself. If that key is low-entropy, then you get what you deserve. The other known possibility to create a LUKS container with a bad volume key 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 or installation in a VM under certain circumstances).

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 volume 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 about this idea, so let me make it clear that this refers to the situation where the attackers can prove that there is data that either 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 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 a first glance it seems possible to do it. And from a cryptographic point of view, it actually is possible.

So, does the idea 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 your 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: https://www.schneier.com/academic/paperfiles/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, Hybrid and SMR Drives?

The problem is that you cannot reliably erase parts of these devices, mainly due to wear-leveling and possibly defect management and delayed writes to the main data area.

For example for SSDs, when overwriting a sector, 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. That is not present in USB keys though and may or may not be secure for a hybrid drive.

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.

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 main disk, but otherwise you get LUKS with the same security as on a traditional magnetic disk. Note however that storage vendors are prone to lying to their customers. For example, it recently came out that HDDs sold without any warning or mentioning in the data-sheets were actually using SMR and that will write data first to a faster area and only overwrite the original data area some time later when things are quiet.

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 LUKS1 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 same slot-key. But if you have the slot-key, you can already unlock the key-slot and get the volume key, breaking everything. So basically, this SHA-1 vulnerability allows you to open a LUKS1 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.

Since version 1.7.0, cryptsetup uses SHA-256 as default to ensure that it will be compatible in the future. There are already some systems where SHA-1 is completely phased out or disabled by a security policy.

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 actual 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 no 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.

5.23 What is cryptsetup CVE-2021-4122?

CVE-2021-4122 describes a possible attack against data confidentiality through LUKS2 online reencryption extension crash recovery.

An attacker can modify on-disk metadata to simulate decryption in progress with crashed (unfinished) reencryption step and persistently decrypt part of the LUKS device.

This attack requires repeated physical access to the LUKS device but no knowledge of user passphrases.

The decryption step is performed after a valid user activates the device with a correct passphrase and modified metadata. There are no visible warnings for the user that such recovery happened (except using the luksDump command). The attack can also be reversed afterward (simulating crashed encryption from a plaintext) with possible modification of revealed plaintext.

The problem was fixed in cryptsetup version 2.4.3 and 2.3.7.

For more info, please see the report here: https://seclists.org/oss-sec/2022/q1/34

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 LUKS1 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 key-slot, you are good. Do not forget to close the device again.

Under some circumstances (damaged header), this fails. Then use the following steps in case it is LUKS1:

First determine the volume (volume) 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 for 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". Examples:

    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 non-mapped LUKS container, 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 volume 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 volume 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 volume 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 volume 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. For LUKS2 they are a bit better protected. but for LUKS1, these are right in the first sector. 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 completely unrecoverable, 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. For LUKS2 this may still be recoverable without that header backup, for LUKS1 it is not. You may also damage the key-slots in part or in full. See also last item.

6.10 How do I recover the volume key from a mapped LUKS1 container?

Note: LUKS2 uses the kernel keyring to store keys and hence this procedure does not work unless you have explicitly disabled the use of the keyring with "--disable-keyring" on opening.

This is typically only needed if you managed to damage your LUKS1 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 volume key of the LUKS1 container. Note that both ways to recreate a LUKS header with the old volume key described below will write the volume 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 volume key afterwards. Changing the volume key requires a full data backup, luksFormat and then restore of the backup. Alternatively the tool cryptsetup-reencrypt from the cryptsetup package can be used to change the volume key (see its man-page), but a full backup is still highly recommended.

First, there is a script by Milan that automates the whole process, except generating a new LUKS1 header with the old volume key (it prints the command for that though):

https://gitlab.com/cryptsetup/cryptsetup/blob/main/misc/luks-header-from-active

You can also do this manually. Here is how:

Get the volume 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 volume key.

Convert the volume 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 > <volume-key-file>

Do a luksFormat to create a new LUKS1 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 --volume-key-file=<volume-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 volume key. It just omits the conversion and hashes the volume key string.

If the header is intact and you just forgot the passphrase, just set a new passphrase like this:

      cryptsetup luksAddKey --volume-key-file=<volume-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 LUKS1 look like?

Note: For LUKS2, refer to the LUKS2 document referenced in Item 1.2

A LUKS1 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 volume 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 LUKS1 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 LUKS1 On-Disk Format Specification Version 1.2.3

LUKS1 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[]     volume 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 LUKS1 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 LUKS1 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 full sectors).

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 volume key. Even if you change the passphrase(s), the volume key stays the same. That means whoever has access to one of the clones can decrypt them all, completely bypassing the passphrases.

While you can use cryptsetup-reencrypt to change the volume key, this is probably more effort than to create separate LUKS containers in the first place.

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 volume key). In this case, changing the default passphrase will make it secure again.

6.16 How to convert the printed volume key to a raw one? A volume key printed via something like:

      cryptsetup --dump-volume-key luksDump /dev/<device> >volume-key

(i.e. without using --volume-key-file), which gives something like:

LUKS header information for /dev/<device>
Cipher name:   	aes
Cipher mode:   	xts-plain64
Payload offset:	32768
UUID:          	6e914442-e8b5-4eb5-98c4-5bf0cf17ecad
MK bits:       	512
MK dump:	e0 3f 15 c2 0f e5 80 ab 35 b4 10 03 ae 30 b9 5d 
		4c 0d 28 9e 1b 0f e3 b0 50 57 ef d4 4d 53 a0 12 
		b7 4e 43 a1 20 7e c5 02 1f f1 f5 08 04 3c f5 20 
		a6 0b 23 f6 7b 53 55 aa 22 d8 aa 02 e0 2f d5 04

can be converted to the raw volume key for example via:

      sed -E -n '/^MK dump:\t/,/^[^\t]/{0,/^MK dump:\t/s/^MK dump://; /^([^\t].*)?$/q; s/\t+//p;};' volume-key  |  xxd -r -p

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

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.

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, then you have bigger problems than this somewhat expected behavior.

The CVE was exaggerated and should not be assigned to upstream cryptsetup in the first place (it is a distro specific initrd issue). It was driven more by a try 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?

Note: The instructions here apply to an initrd in initramfs format, not to an initrd in initrd format. The latter is a filesystem image, not a cpio-archive, and seems to not be widely used anymore.

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 a useful starting point for that too.

Unpacking an existing initrd to use as template

A Linux initrd is in gzip'ed cpio format. To unpack it, use something like this:

     mkdir tmp; cd tmp; cat ../initrd | gunzip | cpio -id

After this, you have the full initrd content in tmp/

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.

Creating your own initrd

The two examples below should give you most of what is needed. This is tested with LUKS1 and should work with LUKS2 as well. If not, please let me know.

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 *** 
           echo "    (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

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.

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. LUKS2 Questions

10.1 Is the cryptography of LUKS2 different?

Mostly not. The header has changed in its structure, but the cryptography is the same. The one exception is that PBKDF2 has been replaced by Argon2 to give better resilience against attacks by graphics cards and other hardware with lots of computing power but limited local memory per computing element.

10.2 What new features does LUKS2 have?

There are quite a few. I recommend reading the man-page and the on-disk format specification, see Item 1.2.

To list just some:

A lot of the metadata is JSON, allowing for easier extension
Max 32 key-slots per default
Better protection for bad passphrases now available with Argon2
Authenticated encryption
The LUKS2 header is less vulnerable to corruption and has a 2nd copy

10.3 Why does LUKS2 need so much memory?

LUKS2 uses Argon2 instead of PBKDF2. That causes the increase in memory. See next item.

10.4 Why use Argon2 in LUKS 2 instead of PBKDF2?

LUKS tries to be secure with not-so-good passwords. Bad passwords need to be protected in some way against an attacker that just tries all possible combinations. (For good passwords, you can just wait for the attacker to die of old age...) The situation with LUKS is not quite the same as with a password stored in a database, but there are similarities.

LUKS does not store passwords on disk. Instead, the passwords are used to decrypt the volume-key with it and that one is stored on disk in encrypted form. If you have a good password, with, say, more than 80 bits of entropy, you could just put the password through a single crypto-hash (to turn it into something that can be used as a key) and that would be secure. This is what plain dm-crypt does.

If the password has lower entropy, you want to make this process cost some effort, so that each try takes time and resources and slows the attacker down. LUKS1 uses PBKDF2 for that, adding an iteration count and a salt. The iteration count is per default set to that it takes 1 second per try on the CPU of the device where the respective passphrase was set. The salt is there to prevent precomputation.

The problem with that is that if you use a graphics card, you can massively speed up these computations as PBKDF2 needs very little memory to compute it. A graphics card is (grossly simplified) a mass of small CPUs with some small very fast local memory per CPU and a large slow memory (the 4/6/8 GB a current card may have). If you can keep a computation in the small, CPU-local memory, you can gain a speed factor of 1000 or more when trying passwords with PBKDF2.

Argon2 was created to address this problem. It adds a "large memory property" where computing the result with less memory than the memory parameter requires is massively (exponentially) slowed down. That means, if you set, for example, 4GB of memory, computing Argon2 on a graphics card with around 100kB of memory per "CPU" makes no sense at all because it is far too slow. An attacker has hence to use real CPUs and furthermore is limited by main memory bandwidth.

Hence the large amount of memory used is a security feature and should not be turned off or reduced. If you really (!) understand what you are doing and can assure good passwords, you can either go back to PBKDF2 or set a low amount of memory used for Argon2 when creating the header.

10.5 LUKS2 is insecure! It uses less memory than the Argon2 RFC say!

Well, not really. The RFC recommends 6GiB of memory for use with disk encryption. That is a bit insane and something clearly went wrong in the standardization process here. First, that makes Argon2 unusable on any 32 bit Linux and that is clearly a bad thing. Second, there are many small Linux devices around that do not have 6GiB of RAM in the first place. For example, the current Raspberry Pi has 1GB, 2GB or 4GB of RAM, and with the RFC recommendations, none of these could compute Argon2 hashes.

Hence LUKS2 uses a more real-world approach. Iteration is set to a minimum of 4 because there are some theoretical attacks that work up to an iteration count of 3. The thread parameter is set to 4. To achieve 2 second/slot unlock time, LUKS2 adjusts the memory parameter down if needed. In the other direction, it will respect available memory and not exceed it. On a current PC, the memory parameter will be somewhere around 1GB, which should be quite generous. The minimum I was able to set in an experiment with "-i 1" was 400kB of memory and that is too low to be secure. A Raspberry Pi would probably end up somewhere around 50MB (have not tried it) and that should still be plenty.

That said, if you have a good, high-entropy passphrase, LUKS2 is secure with any memory parameter.

10.6 How does re-encryption store data while it is running?

All metadata necessary to perform a recovery of said segment (in case of crash) is stored in the LUKS2 metadata area. No matter if the LUKS2 reencryption was run in online or offline mode.

10.7 What do I do if re-encryption crashes?

In case of a reencryption application crash, try to close the original device via following command first:

    cryptsetup close <my_crypt_device>.

Cryptsetup assesses if it's safe to teardown the reencryption device stack or not. It will also cut off I/O (via dm-error mapping) to current hotzone segment (to make later recovery possible). If it can't be torn down, i.e. due to a mounted fs, you must unmount the filesystem first. Never try to tear down reencryption dm devices manually using e.g. dmsetup tool, at least not unless cryptsetup says it's safe to do so. It could damage the data beyond repair.

10.8 Do I need to enter two passphrases to recover a crashed re-encryption?

Cryptsetup (command line utility) expects the passphrases to be identical for the keyslot containing old volume key and for the keyslot containing new one. So the recovery happens during normal the "cryptsetup open" operation or the equivalent during boot.

Re-encryption recovery can be also performed in offline mode by the "cryptsetup repair" command.

10.9 What is an unbound keyslot and what is it used for?

Quite simply, an 'unbound key' is an independent 'key' stored in a luks2 keyslot that cannot be used to unlock a LUKS2 data device. More specifically, an 'unbound key' or 'unbound luks2 keyslot' contains a secret that is not currently associated with any data/crypt segment (encrypted area) in the LUKS2 'Segments' section (displayed by luksDump).

This is a bit of a more general idea. It basically allows one to use a keyslot as a container for a key to be used in other things than decrypting a data segment.

As of April 2020, the following uses are defined:

LUKS2 re-encryption. The new volume key is stored in an unbound keyslot which becomes a regular LUKS2 keyslot later when re-encryption is finished.
Somewhat similar is the use with a wrapped key scheme (e.g. with the paes cipher). In this case, the VK (Volume Key) stored in a keyslot is an encrypted binary binary blob. The KEK (Key Encryption Key) for that blob may be refreshed (Note that this KEK is not managed by cryptsetup!) and the binary blob gets changed. The KEK refresh process uses an 'unbound keyslot'. First the future effective VK is placed in the unbound keyslot and later it gets turned into the new real VK (and bound to the respective crypt segment).

10.10 What about the size of the LUKS2 header?

While the LUKS1 header has a fixed size that is determined by the cipher spec (see Item 6.12), LUKS2 is more variable. The default size is 16MB, but it can be adjusted on creation by using the --luks2-metadata-size and --luks2-keyslots-size options. Refer to the man-page for details. While adjusting the size in an existing LUKS2 container is possible, it is somewhat complicated and risky. My advice is to do a backup, recreate the container with changed parameters and restore that backup.

10.11 Does LUKS2 store metadata anywhere except in the header?

It does not. But note that if you use the experimental integrity support, there will be an integrity header as well at the start of the data area and things get a bit more complicated. All metadata will still be at the start of the device, nothing gets stored somewhere in the middle or at the end.

10.12 What is a LUKS2 Token?

A LUKS2 token is an object that describes "how to get a passphrase or key" to unlock particular keyslot. A LUKS2 token is stored as json data in the LUKS2 header. The token can be related to all keyslots or a specific one. As the token is stored in JSON formay it is text by default but binary data can be encoded into it according to the JSON conventions.

Documentation on the last changes to LUKS2 tokens can be found in the release notes. As of version 2.4 of cryptsetup, there are significant features. The standard documentation for working with tokens is in the luks2 reference available as PDF on the project page.

11. 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: See Item 1.2

Other Documentation

Arch Linux on LUKS, LVM and full-disk encryption: https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_entire_system

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
https://it.slashdot.org/story/12/12/05/0623215/new-25-gpu-monster-devours-strong-passwords-in-minutes

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SSD and Flash Disk Related
Disk Encryption
Attacks Against Disk Encryption
Risk Management as Relevant for Disk Encryption
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In no particular order:

Arno Wagner
Milan Broz

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Frequently Asked Questions Cryptsetup/LUKS

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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. LUKS2 Questions

11. References and Further Reading

A. Contributors

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Frequently Asked Questions Cryptsetup/LUKS

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. LUKS2 Questions

11. References and Further Reading

A. Contributors