cryptsetup-luksChangeKey - Man Page

cryptsetup luksChangeKey [<options>] <device> [<new key file>]

Changes an existing passphrase. The passphrase to be changed must be supplied interactively or via --key-file. The new passphrase can be supplied interactively or in a file given as the positional argument.

If a keyslot is specified (via --key-slot), the passphrase for that keyslot must be given, and the new passphrase will overwrite the specified keyslot. If no keyslot is specified and there is still a free keyslot, then the new passphrase will be put into a free keyslot before the keyslot containing the old passphrase is purged. If there is no free keyslot, then the keyslot with the old passphrase is overwritten directly.

WARNING: If a keyslot is overwritten, a media failure during this operation can cause the overwrite to fail after the old passphrase has been wiped, making the LUKS container inaccessible. LUKS2 mitigates that by never overwriting the existing keyslot area as long as there’s a free space in the keyslots area at least for one more LUKS2 keyslot.

If you need to use both luksChangeKey and reencrypt (e.g., to recover from a key leak), you need to use them in that order to avoid leaking the new volume key.

Some parameters are effective only if used with the LUKS2 format that supports per-keyslot parameters. For LUKS1, the PBKDF type and hash algorithm are always the same for all keyslots.

<options> can be [--key-file, --keyfile-offset, --keyfile-size, --new-keyfile-offset, --iter-time, --pbkdf, --pbkdf-force-iterations, --pbkdf-memory, --pbkdf-parallel, --new-keyfile-size, --key-slot, --force-password, --hash, --header, --disable-locks, --type, --keyslot-cipher, --keyslot-key-size, --timeout, --verify-passphrase].

--batch-mode,  -q

Suppresses all confirmation questions. Use with care!

If the --verify-passphrase option is not specified, this option also switches off the passphrase verification.

--debug or --debug-json

Run in debug mode with full diagnostic logs. Debug output lines are always prefixed by #.

If --debug-json is used, additional LUKS2 JSON data structures are printed.

--disable-locks

Disable lock protection for metadata on disk. This option is valid only for LUKS2 and is ignored for other formats.

WARNING: Do not use this option unless you run cryptsetup in a restricted environment where locking is impossible to perform (where /run directory cannot be used).

--force-password

Do not use password quality checking for new LUKS passwords.

This option is ignored if cryptsetup is built without password quality checking support.

For more info about password quality check, see the manual page for pwquality.conf(5) and passwdqc.conf(5).

--hash,  -h <hash-spec>

The specified hash is used for PBKDF2 and the AF splitter.

--header <device or file storing the LUKS header>

Use a detached (separated) metadata device or file where the LUKS header is stored. This option allows one to store the ciphertext and LUKS header on different devices.

For commands that change the LUKS header (e.g., luksAddKey), specify the device or file with the LUKS header directly as the LUKS device.

--help,  -?

Show help text and default parameters.

--iter-time,  -i <number of milliseconds>

The number of milliseconds to spend with PBKDF passphrase processing. Specifying 0 as a parameter selects the compiled-in default.

--key-file,  -d file

Read the passphrase from the file.

If the name given is "-", then the passphrase will be read from stdin. In this case, reading will not stop at newline characters.

The passphrase supplied via --key-file is always the passphrase for the existing keyslot requested by the command.

If you want to set a new passphrase via a key file, you have to use a positional argument.

See section NOTES ON PASSPHRASE PROCESSING in cryptsetup(8) for more information.

--keyfile-offset value

Skip value bytes at the beginning of the key file.

--keyfile-size,  -l value

Read a maximum of value bytes from the key file. The default is to read the whole file up to the compiled-in maximum that can be queried with --help. Supplying more data than the compiled-in maximum aborts the operation.

This option is useful to cut trailing newlines, for example. If --keyfile-offset is also given, the size count starts after the offset.

--key-slot,  -S <0-N>

For LUKS operations that add key material, this option allows you to specify which keyslot is selected for the new key.

The maximum number of keyslots depends on the LUKS version. LUKS1 can have up to 8 keyslots. LUKS2 can have up to 32 keyslots based on keyslot area size and key size, but a valid keyslot ID can always be between 0 and 31 for LUKS2.

--keyslot-cipher <cipher-spec>

This option can be used to set specific cipher encryption for the LUKS2 keyslot area.

--keyslot-key-size <bits>

This option can be used to set a specific key size for the LUKS2 keyslot area.

--new-keyfile-offset value

Skip value bytes at the start when adding a new passphrase from the key file.

--new-keyfile-size value

Read a maximum of value bytes when adding a new passphrase from the key file. The default is to read the whole file up to the compiled-in maximum length that can be queried with --help. Supplying more than the compiled-in maximum aborts the operation. When --new-keyfile-offset is also given, reading starts after the offset.

--pbkdf <PBKDF spec>

Set Password-Based Key Derivation Function (PBKDF) algorithm for LUKS keyslot. The PBKDF can be: pbkdf2 (for PBKDF2 according to RFC2898), argon2i for Argon2i or argon2id for Argon2id (see Argon2 for more info).

For LUKS1, only PBKDF2 is accepted (no need to use this option). The default PBKDF for LUKS2 is set during compilation time and is available in the cryptsetup --help output.

A PBKDF is used for increasing the dictionary and brute-force attack cost for keyslot passwords. The parameters can be time, memory and parallel cost.

For PBKDF2, only the time cost (number of iterations) applies. For Argon2i/id, there is also memory cost (memory required during the process of key derivation) and parallel cost (number of threads that run in parallel during the key derivation.

Note that increasing memory cost also increases time, so the final parameter values are measured by a benchmark. The benchmark tries to find iteration time (--iter-time) with required memory cost --pbkdf-memory. If it is not possible, the memory cost is decreased as well. The parallel cost --pbkdf-parallel is constant and is checked against available CPU cores.

You can see all PBKDF parameters for a particular LUKS2 keyslot with the cryptsetup-luksDump(8) command.

If you do not want to use benchmark and want to specify all parameters directly, use --pbkdf-force-iterations with --pbkdf-memory and --pbkdf-parallel. This will override the values without benchmarking. Note it can cause extremely long unlocking time or cause out-of-memory conditions with unconditional process termination. Use only in specific cases, for example, if you know that the formatted device will be used on some small embedded system.

MINIMAL AND MAXIMAL PBKDF COSTS: For PBKDF2, the minimum iteration count is 1000 and the maximum is 4294967295 (maximum for 32-bit unsigned integer). Memory and parallel costs are not supported for PBKDF2. For Argon2i and Argon2id, the minimum iteration count (CPU cost) is 4, and the maximum is 4294967295 (maximum for a 32-bit unsigned integer). Minimum memory cost is 32 KiB and maximum is 4 GiB. If the memory cost parameter is benchmarked (not specified by a parameter), it is always in the range from 64 MiB to 1 GiB. Memory cost above 1GiB (up to the 4GiB maximum) can be setup only by the --pbkdf-memory parameter. The parallel cost minimum is 1 and maximum 4 (if enough CPU cores are available, otherwise it is decreased by the available CPU cores).

WARNING: Increasing PBKDF computational costs above the mentioned limits provides negligible additional security improvement. While elevated costs significantly increase brute-force overhead, they offer negligible protection against dictionary attacks. The marginal cost increase for processing an entire dictionary remains fundamentally insufficient.

The hardcoded PBKDF limits represent engineered trade-offs between cryptographic security and operational usability. LUKS maintains portability and must be used within a reasonable time on resource-constrained systems.

Cryptsetup deliberately restricts maximum memory cost (4 GiB) and parallel cost (4) parameters due to architectural limitations (like embedded and legacy systems).

PBKDF memory cost mandates actual physical RAM allocation with intensive write operations that must remain in physical RAM. Any swap usage results in unacceptable performance degradation. Memory management often overcommits allocations beyond available physical memory, expecting most allocated memory to remain unused. In such situations, as PBKDF always uses all allocated memory, it frequently causes out-of-memory failures that abort cryptsetup operations.

--pbkdf-force-iterations number

Avoid the PBKDF benchmark and set the time cost (iterations) directly. It can be used only for a LUKS/LUKS2 device. See --pbkdf option for more info.

--pbkdf-memory number

Set the memory cost for PBKDF (for Argon2i/id, the number represents kilobytes). Note that it is the maximal value; PBKDF benchmark or available physical memory can decrease it. This option is not available for PBKDF2.

--pbkdf-parallel number

Set the parallel cost for PBKDF (number of threads, up to 4). Note that it is the maximal value; it is decreased automatically if the CPU online count is lower. This option is not available for PBKDF2.

--timeout,  -t seconds

The number of seconds to wait before a timeout on passphrase input via terminal. It is relevant every time a passphrase is asked. It has no effect if used in conjunction with --key-file.

This option is useful when the system should not stall if the user does not input a passphrase, e.g., during boot. The default is a value of 0 seconds, which means to wait forever.

--type type

Specifies required device type, for more info, read the BASIC ACTIONS section in cryptsetup(8).

--usage

Show short option help.

--verify-passphrase,  -y

When interactively asking for a passphrase, ask for it twice and complain if both inputs do not match. Ignored on input from file or stdin.

--version,  -V

Show the program version.

Report bugs at cryptsetup mailing list or in Issues project section.

Please attach the output of the failed command with --debug option added.

Cryptsetup FAQ

cryptsetup(8), integritysetup(8) and veritysetup(8)

Part of cryptsetup project.

cryptsetup(8).