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{{Short description|Disk encryption software}}
{{redirect|LUKS|the American painter|George Luks}}
The '''Linux Unified Key Setup''' ('''LUKS''') is a [[disk encryption]] specification created by Clemens Fruhwirth in 2004 and originally intended for [[Linux]].

LUKS implements a platform-independent standard on-disk format for use in various tools. This facilitates compatibility and interoperability among different programs and operating systems, and assures that they all implement [[password management]] in a secure and documented manner.<ref>{{cite web |url=https://gitlab.com/cryptsetup/cryptsetup/-/wikis/LUKS-standard/on-disk-format.pdf |title=LUKS On-Disk Format Specification Version 1.2.3 |first=Clemens |last=Fruhwirth |date=2018-01-20 |accessdate=2021-09-23 }}</ref>

== Description ==

LUKS is used to encrypt a [[block device]]. The contents of the encrypted device are arbitrary, and therefore any filesystem can be encrypted, including [[swap partition]]s.<ref>{{Cite web|url=https://docs.fedoraproject.org/en-US/quick-docs/encrypting-drives-using-LUKS/|title=Encrypting drives using LUKS|website=Fedora Docs|access-date=6 May 2022}}</ref> There is an unencrypted [[header (computing)|header]] at the beginning of an encrypted volume, which allows up to 8 (LUKS1) or 32 (LUKS2) [[encryption key]]s to be stored along with encryption parameters such as cipher type and key size.<ref name="redhat">{{Cite web|url=https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/security_hardening/encrypting-block-devices-using-luks_security-hardening|title=Chapter 12. Encrypting block devices using LUKS|website=Red Hat Customer Portal}}</ref><ref>{{Cite web|url=https://www.golinuxcloud.com/how-to-encrypt-hard-disk-partition-luks-linux/|title=How to Encrypt Hard Disk (partition) using LUKS in Linux|date=27 February 2019 }}</ref>

The presence of this header is a major difference between LUKS and [[dm-crypt]], since the header allows multiple different passphrases to be used, with the ability to change and remove them. If the header is lost or corrupted, the device will no longer be decryptable.<ref>{{Cite web|url=https://www.linode.com/docs/guides/encrypt-data-disk-with-dm-crypt/|title=How to Encrypt Your Data with dm-crypt|website=Linode|date=22 November 2022 }}</ref>

Encryption is done with a multi-layer approach. First, the block device is encrypted using a ''master key.'' This master key is encrypted with each active ''user key''.<ref>{{Cite journal|url=https://eprint.iacr.org/2016/274.pdf|title=What Users Should Know About Full Disk Encryption Based on LUKS|first1=Simone|last1=Bossi|first2=Andrea|last2=Visconti|date=2015}}</ref> User keys are derived from passphrases, [[FIDO Alliance|FIDO2]] security keys, [[Trusted Platform Module|TPMs]] or [[Smart card|smart cards]].<ref>{{Cite web |title=systemd-cryptenroll - ArchWiki |url=https://wiki.archlinux.org/title/Systemd-cryptenroll |access-date=2023-11-22 |website=wiki.archlinux.org}}</ref><ref>{{Cite web |date=20 April 2014 |title=How to encrypt a LUKS container using a smart card or token |url=https://blog.g3rt.nl/luks-smartcard-or-token.html}}</ref> The multi-layer approach allows users to change their passphrase without re-encrypting the whole block device. Key slots can contain information to verify user passphrases or other types of keys.

There are two versions of LUKS, with LUKS2 featuring resilience to header corruption, and using the [[Argon2]] [[key derivation function]] by default, whereas LUKS1 uses [[PBKDF2]].<ref>{{Cite web|url=https://infosecwriteups.com/how-luks-works-with-full-disk-encryption-in-linux-6452ad1a42e8|title=How LUKS works with Full Disk Encryption in Linux|date=25 September 2021 }}</ref> Conversion between both versions of LUKS is possible in certain situations, but some features may not be available with LUKS1 such as Argon2.<ref name="redhat" /> LUKS2 uses [[JSON]] as a metadata format.<ref name="redhat" /><ref name="luks2-format">{{Cite web|url=https://gitlab.com/cryptsetup/cryptsetup/blob/master/docs/on-disk-format-luks2.pdf|title=on-disk-format-luks2.pdf|date=7 March 2024 }}</ref>

Available cryptographic algorithms depend on individual kernel support of the host. [[Libgcrypt]] can be used as a backend for hashing, which supports all of its algorithms.<ref name="man8cryptsetup">{{man|8|cryptsetup|die.net}}</ref> It is up to the operating system vendor to choose the default algorithm.<ref>{{Cite web|url=https://eforensicsmag.com/breaking-luks-encryption-by-oleg-afonin/|title=Breaking LUKS Encryption|website=eForensics|date=21 August 2020 }}</ref> LUKS1 makes use of an [[anti-forensics]] technique called AFsplitter, allowing for secure [[data erasure]] and protection.<ref>{{Cite web|url=https://clemens.endorphin.org/AFsplitter|title=AFsplitter}}</ref>

=== LUKS with LVM ===

[[Logical Volume Manager (Linux)|Logical Volume Management]] can be used alongside LUKS.<ref name="archwiki">{{Cite web|url=https://wiki.archlinux.org/title/Dm-crypt/Encrypting_an_entire_system|title=dm-crypt/Encrypting an entire system|access-date=6 May 2022}}</ref>

; LVM on LUKS : When LVM is used on an unlocked LUKS container, all underlying partitions (which are LVM logical volumes) can be encrypted with a single key. This is akin to splitting a LUKS container into multiple partitions. The LVM structure is not visible until the disk is decrypted.<ref>{{Cite web|url=https://tonisagrista.com/blog/2020/arch-encryption/|title= Arch with LVM on LUKS}}</ref>
; LUKS on LVM : When LUKS is used to encrypt LVM logical volumes, an encrypted volume can span multiple devices. The underlying LVM volume group is visible without decrypting the encrypted volumes.<ref>{{Cite web|url=https://amkeisntall.wordpress.com/2014/09/12/luks-on-lvm-encrypted-logical-volumes-and-secure-backups/|title=LUKS on LVM: encrypted logical volumes and secure backups|date=12 September 2014 }}</ref>

=== Full disk encryption ===
[[File:Debian Graphical Installer Partman-auto init automatically partition 1.png|thumb|[[Debian-Installer]] showing an option for automated partitioning with [[Logical Volume Manager (Linux)|LVM]] on LUKS]]
A common usage of LUKS is to provide [[full disk encryption]], which involves encrypting the [[root directory|root partition]] of an operating system installation, which protects the operating system files from being [[Evil maid attack|tampered with]] or read by [[adversary (cryptography)|unauthorized parties]].<ref name="archwiki" />

On a Linux system, the [[boot partition]] (<code>/boot</code>) may be encrypted if the [[bootloader]] itself supports LUKS (e.g. [[GRUB]]). This is undertaken to prevent tampering with the [[Linux kernel]]. However, the [[Bootloader#first stage bootloader|first stage bootloader]] or an [[EFI system partition]] cannot be encrypted (see [[Full disk encryption#The boot key problem]]).<ref name="archwiki" />

On mobile Linux systems, [[postmarketOS]] has developed [https://wiki.postmarketos.org/wiki/Osk-sdl osk-sdl] to allow a full disk encrypted system to be unlocked using a touch screen.

For systems running [[systemd]], the <code>systemd-homed</code> component can be used to encrypt individual [[home directory|home directories]].<ref>{{Cite web|url=https://systemd.io/HOME_DIRECTORY/|title=Home Directories|website=systemd}}</ref>

== Operating system support ==

The [[reference implementation (computing)|reference implementation]] for LUKS operates on Linux and is based on an enhanced version of [[cryptsetup]], using [[dm-crypt]] as the disk encryption backend. Under [[Microsoft]] [[Windows]], LUKS-encrypted disks can be used via the [[Windows Subsystem for Linux]].<ref>{{Cite web|url=https://devblogs.microsoft.com/commandline/servicing-the-windows-subsystem-for-linux-wsl-2-linux-kernel/|title=Servicing the Windows Subsystem for Linux (WSL) 2 Linux Kernel|website=Microsoft Developer Blogs|date=16 April 2021 }}</ref> (Formerly, this was possible with LibreCrypt,<ref>{{Cite web|url=https://github.com/t-d-k/LibreCrypt|title=LibreCrypt|website=GitHub|date=27 July 2022 }}</ref> which currently has fundamental security holes,<ref>{{Cite web|url=https://github.com/t-d-k/LibreCrypt/issues/38|title=Flaw in driver allows privilege escalation. Feedback wanted · Issue #38 · t-d-k/LibreCrypt|website=GitHub|date=30 September 2015}}</ref><ref>{{Cite web|url=https://github.com/t-d-k/LibreCrypt/issues/39|title=Driver allows writing to arbitrary devices · Issue #39 · t-d-k/LibreCrypt|website=[[GitHub]] |date=7 October 2015}}</ref> and which succeeded [[FreeOTFE]], formerly DoxBox.)

[[DragonFly BSD]] supports LUKS.<ref>{{Cite web|url=https://www.dragonflybsd.org/features/#index6h2|title=DragonFly's Major Features List|access-date=6 May 2022}}</ref>

=== Installer support ===

Several Linux distributions allow the root device to be encrypted upon OS installation. These installers include [[Calamares (software)|Calamares]],<ref>{{Cite web|url=https://www.phoronix.com/scan.php?page=news_item&px=Calamares-LUKS-Support|title=Calamares Installer Adds LUKS Encryption Support|author=Michael Larabel|date=8 May 2016|website=Phoronix}}</ref> [[Ubiquity (software)|Ubiquity]],<ref>{{Cite web|url=https://www.maketecheasier.com/encrypt-hard-disk-in-ubuntu/|title=How to Encrypt Your Hard Disk in Ubuntu|website=Make Tech Easier|date=13 January 2017 }}</ref> [[Debian-Installer]],<ref>{{Cite web|url=https://wiki.debian.org/DebianInstaller/PartmanCrypto|title=PartmanCrypto|website=Debian Wiki|access-date=6 May 2022}}</ref> and more.

== On-disk format ==

LUKS headers are backward compatible; newer versions of LUKS are able to read headers of previous versions.<ref name="luks1-standard">{{Cite web|url=https://gitlab.com/cryptsetup/cryptsetup/wikis/LUKS-standard/on-disk-format.pdf|title=LUKS On-Disk Format Specification}}</ref>

=== LUKS1 ===
{| class="wikitable"
|+ LUKS1 Header<ref name="luks1-standard" />
|-
!colspan="2"| Offset !! Data type !! Description
|-
| {{0}}{{0}}0 || {{0}}{{0}}0hex || char[6] || [[Magic number (programming)|Magic number]] {'L', 'U', 'K', 'S', 0xBA, 0xBE }
|-
| {{0}}{{0}}6 || {{0}}{{0}}6hex || uint16_t || LUKS Version (0x0001 for LUKS1)
|-
| {{0}}{{0}}8 || {{0}}{{0}}8hex || char[32] || Cipher Algorithm (e.g. "twofish", "aes")
|-
| {{0}}40 || {{0}}28hex || char[32] || Cipher mode (e.g. "cbc-essiv:sha256")
|-
| {{0}}72 || {{0}}48hex || char[32] || [[Cryptographic hash function]] (e.g. "sha1", "ripemd160")
|-
| 104 || {{0}}68hex || uint32_t || Payload offset (position of encrypted data) in 512 byte offsets
|-
| 108 || {{0}}6Chex || uint32_t || Number of key bytes
|-
| 112 || {{0}}70hex || char[20] || [[PBKDF2]] master key [[checksum]]
|-
| 132 || {{0}}84hex || char[32] || PBKDF2 master key salt parameter
|-
| 164 || {{0}}A4hex || uint32_t || PBKDF2 master key iterations (Default: 10)
|-
| 168 || {{0}}A8hex || char[40] || [[UUID]] of the partition (e.g. "504c9fa7-d080-4acf-a829-73227b48fb89")
|-
| 208 || {{0}}D0hex || (48 Bytes) || Keyslot 1
|-
|colspan="4"| …
|-
| 544 || 220hex || (48 Bytes) || Keyslot 8
|-
|colspan="4"| ''592 Bytes total''
|}

{| class="wikitable"
|+ Format of each keyslot
|-
! Offset !! Data type !! Description
|-
| {{0}}0 || uint32_t || State of keyslot: Active=0x00AC71F3; Disabled=0x0000DEAD
|-
| {{0}}4 || uint32_t || PBKDF2 iteration parameter
|-
| {{0}}8 || char[32] || PBKDF2 salt parameter
|-
| 40 || uint32_t || Start sector of key
|-
| 44 || uint32_t || Number of anti-forensic stripes (Default: 4000)
|-
|colspan="3"| ''48 Bytes total''
|}

=== LUKS2 ===

LUKS2 devices begin with a binary header intended to allow recognition and fast detection by [[blkid]], which also contains information such as [[Checksum|checksums]]. All strings used in a LUKS2 header are [[null-terminated string]]s. Directly after the binary header comes the JSON area, containing the objects <code>config</code> (configuration), <code>keyslots</code>, <code>digests</code>, <code>segments</code> (describes encrypted areas on the disk), and <code>tokens</code> containing extra metadata.<ref name="luks2-format" />

The binary format for regular <code>luks2</code> keyslots are mostly similar to their predecessor, with the addition of different per-keyslot algorithms. Another type of key exists to allow redundancy in the case that a re-encryption process is interrupted.<ref name="luks2-format" />

== Examples ==

[[Cryptsetup]] is the reference implementation of the LUKS frontend.

To encrypt a device with the path <code>/dev/sda1</code>:
<syntaxhighlight lang="console">
# cryptsetup luksFormat /dev/sda1
</syntaxhighlight>

To unlock an encrypted device, where <code>name</code> is the [[dm-crypt|mapped]] device name:
<syntaxhighlight lang="console">
# cryptsetup open /dev/sda1 name
</syntaxhighlight>

=== Re-encrypting ===

Re-encrypting a LUKS container can be done either with the <code>cryptsetup</code> tool itself, or with a legacy tool called <code>cryptsetup-reencrypt</code>. These tools can also be used to add encryption to an existing unencrypted filesystem, or remove encryption from a block device.<ref name="man8cryptsetup" /><ref>{{Cite web|url=https://man7.org/linux/man-pages/man8/cryptsetup-reencrypt.8.html|title=CRYPTSETUP-REENCRYPT(8) Man page|website=man7.org}}</ref>

Both methods have similar syntax:

<syntaxhighlight lang="console">
# cryptsetup reencrypt /dev/sda1
</syntaxhighlight>

<syntaxhighlight lang="console">
# cryptsetup-reencrypt /dev/sda1
</syntaxhighlight>

== See also ==

* [[Comparison of disk encryption software]]

== References ==
{{reflist}}

==External links==
*{{Official website|https://gitlab.com/cryptsetup/cryptsetup/blob/master/README.md}}
*[https://gitlab.com/cryptsetup/cryptsetup/wikis/FrequentlyAskedQuestions Frequently Asked Questions (FAQ)]
*[https://github.com/t-d-k/librecrypt LibreCrypt: Implementation for Windows]
*[https://gitlab.com/cryptsetup/cryptsetup/wikis/Specification LUKS1 Specification]
*[https://gitlab.com/cryptsetup/cryptsetup/blob/master/docs/on-disk-format-luks2.pdf LUKS2 Specification]

{{Linux}}
{{Cryptographic software}}

[[Category:Cryptographic software]]
[[Category:Disk encryption]]
[[Category:Linux security software]]

[[de:Dm-crypt#Erweiterung mit LUKS]]

Linux Unified Key Setup - Revision history

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