Linux Unified Key Setup (LUKS) Disk Encryption is commonly used by Linux to encrypt storage media volumes.
Author(s): |
Joachim Metz <joachim.metz@gmail.com> |
Abstract: |
This document contains information about Linux Unified Key Setup (LUKS) Disk Encryption format. |
Classification: |
Public |
Keywords: |
LUKS, Linux Unified Key Setup, Disk Encryption |
Copyright (C) 2013-2020, Joachim Metz <joachim.metz@gmail.com>. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".
| Version | Author | Date | Comments |
|---|---|---|---|
0.0.1 |
J.B. Metz |
July 2013 |
Initial version. |
0.0.2 |
J.B. Metz |
December 2014 |
Switched to asccidoc format. |
0.0.3 |
J.B. Metz |
April 2015 |
Small changes. |
0.0.4 |
J.B. Metz |
December 2019 |
Formatting changes. |
0.0.5 |
J.B. Metz |
January 2020 |
Additional information for encryption types. |
0.0.6 |
J.B. Metz |
January 2020 |
Additional information about format version 2. |
Linux Unified Key Setup (LUKS) Disk Encryption is commonly used by Linux to encrypt storage media volumes.
A LUKS version 1 encrypted volume consist of:
-
volume header
-
8 x key slots
-
split master key material
-
encrypted (volume) data
The total reserved size of the LUKS metadata (volume header and split master key material) seems to commonly be 2 MiB.
A LUKS version 2 encrypted volume consist of:
-
metadata area
-
volume header
-
JSON area
-
backup metadata area
-
backup volume header
-
backup JSON area
-
keyslots area
-
encrypted (volume) data
To make recovery easier the backup metadata area starts at a fixed offset:
| Offset | Maximum JSON area size |
|---|---|
16384 (0x004000) |
12 KiB |
32768 (0x008000) |
28 KiB |
65536 (0x010000) |
60 KiB |
131072 (0x020000) |
124 KiB |
262144 (0x040000) |
252 KiB |
524288 (0x080000) |
508 KiB |
1048576 (0x100000) |
1020 KiB |
2097152 (0x200000) |
2044 KiB |
4194304 (0x400000) |
4092 KiB |
| Characteristics | Description |
|---|---|
Byte order |
big-endian |
Date and time values |
|
Character strings |
ASCII string with an end-of-string character |
To encrypt storage media LUKS Disk Encryption uses different kind of keys.
The Master Key (MK) is derived from the Split Master Key (SMK). The size of the MK is dependent on the master key size value in the volume header. Commonly the MK is 128-bit or 256-bit of size. The MK is used to de/encrypt the encrypted (volume) data.
The Split Master Key (SMK) is stored encrypted with a specific user key (UK) in the split master key material. The size of the key material and hence the SMK is the size of the Master Key (MK) times the number of stripes.
The MK is determined from the SMK using the anti-forensic (AF) diffuser using the hashing method stored in the volume header. Also see [FRUHWIRTH05].
The resulting MK can be validated with the master key validation hash stored in the volume header. The validation hash can be calculated using the PBKDF2 algorithm with:
-
The hashing method stored in the volume header, as the as the pseudo-random function (PRF).
-
The number of iterations as stored in the volume header.
-
A salt, as stored in the volume header.
-
The master key as the input data.
The User Key (UK) is derived from the user password. The UK is used to de/encrypt the corresponding split master key material.
The user key is calculated as following:
-
Calculate the key using the PBKDF2 algorithm with:
-
The (password) hashing method stored in the volume header, as the as the pseudo-random function (PRF).
-
The number of iterations as stored in the corresponding key slot.
-
A salt, as stored in the corresponding key slot.
-
The password string as the input data.
-
A (output) key size that is the same as that of the Master Key (MK).
For now it is assumed that the user password contains ASCII characters only.
LUKS supports multiple encryption methods, different encryption chaining modes and initialization vector modes.
In the null initialization vector mode the initialization vector (IV) is filled with 0‑byte values.
In the plain and plain64 initialization vector mode the initialization vector (IV) is filled with respectively a 32-bit or 64-bit little-endian representation of the corresponding sector number padded with 0-byte values.
The sector number is relative to the start of the data not relative to the start of the volume header.
Int the encrypted sector-salt initialization vector (ESSIV) mode the initialization vector (IV) is determined by:
-
hashing the encryption key with hashing method defined in the initialization vector mode options.
-
encrypting the little-endian representation of the corresponding sector number padded with 0-byte values with the hash of the encryption key.
The sector number is relative to the start of the data not relative to the start of the volume header.
In the benbi initialization vector mode the initialization vector (IV) is filled with a 64-bit big-endian representation of the corresponding cipher block (or narrow block)-count (starting at 1) padded with 0-byte values.
The sector number is relative to the start of the data not relative to the start of the volume header.
The cipher block-count is calculated as:
cipher block-count = ( sector number << ( log2( bytes per sectory ) - log2( IV size ) ) ) + 1
|
Note
|
Benbi is presumably the abbreviation of big-endian numeric block index, or equivalent. |
Decryption uses: AES-CBC with Master Key (MK) decryption of sector data
The initialization vector of the AES-CBC is dependent on the initialization vector mode defined in the volume header. In recent versions of Linux, AES-CBC is combined with the ESSIV initialization vector mode by default.
The initialization vector is 16 bytes of size.
Decryption uses: AES-ECB with Master Key (MK) decryption of sector data
No initialization vector is used.
The initialization vector is 16 bytes of size.
The volume header is 592 bytes in size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
6 |
"LUKS\xba\xbe" |
Signature |
6 |
2 |
1 |
Format version |
8 |
32 |
Encryption method (Cipher name) |
|
40 |
32 |
Encryption mode (Cipher mode) |
|
72 |
32 |
Hashing method |
|
104 |
4 |
Encrypted volume start sector |
|
108 |
4 |
Master key size |
|
112 |
20 |
Master key validation hash |
|
132 |
32 |
Master key salt |
|
164 |
4 |
Master key number of iterations |
|
168 |
40 |
Volume identifier |
|
208 |
8 x 48 |
Key slots |
Is the volume header padded with 0-byte values up to 4096 bytes?
The volume header (or binary header) is 4096 bytes in size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
6 |
"LUKS\xba\xbe" |
Signature |
6 |
2 |
2 |
Format version |
8 |
8 |
Metadata area size |
|
16 |
8 |
Epoch (or sequence identifier) |
|
24 |
48 |
Label |
|
72 |
32 |
Metadata area checksum method (or algorithm) |
|
104 |
64 |
Salt |
|
168 |
40 |
Volume identifier |
|
208 |
48 |
Unknown (subsystem) |
|
256 |
8 |
Header offset |
|
264 |
184 |
Unknown (padding) |
|
448 |
64 |
Metadata area checksum |
|
512 |
7 x 512 = 3584 |
Unknown (padding) |
LUKS header information Keyslots area: 16744448 [bytes] UUID: a3836cf7-0abe-4739-85e5-2bc728fe9cbc Label: (no label) Subsystem: (no subsystem) Flags: (no flags) Data segments: 0: crypt offset: 16777216 [bytes] length: (whole device) cipher: aes-ecb sector: 512 [bytes] Keyslots: 0: luks2 Key: 256 bits Priority: normal Cipher: aes-ecb Cipher key: 256 bits PBKDF: argon2i Time cost: 6 Memory: 1048576 Threads: 4 Salt: 5f 73 a0 84 1a 94 3c b3 e4 62 e6 85 95 2b b8 c3 fe 15 b1 19 51 34 30 73 37 e2 16 e5 8e 49 41 25 AF stripes: 4000 AF hash: sha1 Area offset:32768 [bytes] Area length:131072 [bytes] Digest ID: 0 Tokens: Digests: 0: pbkdf2 Hash: sha1 Iterations: 154931 Salt: c3 14 fd ef e8 d8 1c a8 40 6a dd eb 65 be 97 5e ec 17 55 19 f7 0c ce ed bc 6c bc f9 9b a4 33 7f Digest: 58 74 f5 4a 83 8b 3f 7b 6e 9a 62 03 86 23 53 c4 fd fd 75 fc
The JSON area is stored directly after the volume header and must be 4096-byte aligned. The JSON area is variable of size and constists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
… |
JSON string |
|
… |
… |
Unknown (padding) |
{
"keyslots": {
"0": {
"type": "luks2",
"key_size": 32,
"af": {
"type": "luks1",
"stripes": 4000,
"hash": "sha1"
},
"area": {
"type": "raw",
"offset": "32768",
"size": "131072",
"encryption": "aes-ecb",
"key_size": 32
},
"kdf": {
"type": "argon2i",
"time": 6,
"memory": 1048576,
"cpus": 4,
"salt": "X3OghBqUPLPkYuaFlSu4w/4VsRlRNDBzN+IW5Y5JQSU="
}
}
},
"tokens": {},
"segments": {
"0": {
"type": "crypt",
"offset": "16777216",
"size": "dynamic",
"iv_tweak": "0",
"encryption": "aes-ecb",
"sector_size": 512
}
},
"digests": {
"0": {
"type": "pbkdf2",
"keyslots": [
"0"
],
"segments": [
"0"
],
"hash": "sha1",
"iterations": 154931,
"salt": "wxT97+jYHKhAat3rZb6XXuwXVRn3DM7tvGy8+ZukM38=",
"digest": "WHT1SoOLP3tummIDhiNTxP39dfw="
}
},
"config": {
"json_size": "12288",
"keyslots_size": "16744448"
}
}
The backup (or secondary) volume header - format version 2 is the same as the Volume header - format version 2 with a different signature: "SKUL\xba\xbe".
The encryption mode consists of a string in the form:
cipher
Where known values of cipher are:
| Value | Description |
|---|---|
arc4 |
Alleged RC4 (ARC4) |
aes |
Advanced Encryption Standard (AES) |
anubis |
Anubis |
blowfish |
Blowfish |
cast5 |
Cast5 (RFC 2144) |
cast6 |
Cast6 (RFC 2612) |
serpent |
Serpent |
tnepres |
Reversed variant of Serpent |
twofish |
Twofish |
|
Note
|
It is currently assumed that these identifiers are case insensitive. |
The encryption mode consists of a string in the form:
chaining_mode[-initialization_vector_mode[:initialization_vector_options]]
Where known values of chaining mode are:
| Value | Description |
|---|---|
cbc |
Cipher-block chaining (CBC) |
ecb |
Electronic codebook (ECB) |
xts |
XEX-based tweaked-codebook mode with ciphertext stealing (XTS) |
What about ctr and lrw?
Note that currently it is assumed that these identifiers are case insensitive.
And known values of initialization vector mode are:
| Value | Description |
|---|---|
benbi |
The initialization vector is the 64-bit big-endian cipher block (or narrow block)-count (starting at 1). |
essiv |
Encrypted sector-salt initialization vector (ESSIV) |
lmk |
Compatible implementation of the block chaining mode used by the Loop-AES block device encryption system. |
null |
The initialization vector is always zero. |
plain |
The initialization vector is the 32-bit little-endian version of the sector number, padded with zeros if necessary. |
plain64 |
The initialization vector is the 64-bit little-endian version of the sector number, padded with zeros if necessary. |
plumb |
Unimplemented? |
|
Note
|
It is currently assumed that these identifiers are case insensitive. |
| Value | Description |
|---|---|
ripemd160 |
RIPEMD-160 |
sha1 |
SHA-1 |
sha224 |
SHA-224 |
sha256 |
SHA-256 |
sha512 |
SHA-512 |
wd256 |
Unknown |
|
Note
|
It is currently assumed that these identifiers are case insensitive. |
The hashing method must at least produce 20 bytes of hash data. Therefore hashing methods like: ghash, MD5 are unsupported.
The key slot is 48 bytes in size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
4 |
State (of key slot) |
|
4 |
4 |
Key material number of iterations |
|
8 |
32 |
Key material salt |
|
40 |
4 |
Key material start sector |
|
44 |
4 |
Key material number of (anti-forensic) stripes |
Note that cryptsetup 1.3.1 requires at +2 MiB and it will not complain before hand if the volume is too small.
Running "cryptsetup luksFormat" will not initialize the encrypted volume data, the data is initialized on write. The uninitialized encrypted data is treated as-is on decryption.
More encryption methods can be found by running "cat /proc/crypto".
[FRUHWIRTH05]
| Title: | New Methods in Hard Disk Encryption |
|---|---|
Author(s): |
Clemens Fruhwirth <clemens@endorphin.org> |
Date: |
July 18, 2005 |
URL: |
[BROZ18]
| Title: | LUKS2 On-Disk Format Specification - Version 1.0.0 |
|---|---|
Author(s): |
Milan Broz <gmazyland@gmail.com> |
Date: |
October 23, 2018 |
URL: |
https://gitlab.com/cryptsetup/cryptsetup/blob/master/docs/on-disk-format-luks2.pdf |
[FRUHWIRTH18]
| Title: | LUKS On-Disk Format Specification - Version 1.2.3 |
|---|---|
Author(s): |
Clemens Fruhwirth <clemens@endorphin.org> |
Date: |
January 20, 2018 |
URL: |
https://gitlab.com/cryptsetup/cryptsetup/-/wikis/LUKS-standard/on-disk-format.pdf |
[CRYPTSETUP]
| Title: | Cryptsetup and LUKS - open-source disk encryption |
|---|---|
URL: |
[WIKIPEDIA-PBKDF2]
| Title: | PBKDF2 |
|---|---|
URL: |
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