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systemd-cryptsetup@.service, systemd-cryptsetup — Full disk decryption logic






systemd-cryptsetup@.service is a service responsible for setting up encrypted block devices. It is instantiated for each device that requires decryption for access.

systemd-cryptsetup@.service instances are part of the system-systemd\x2dcryptsetup.slice slice, which is destroyed only very late in the shutdown procedure. This allows the encrypted devices to remain up until filesystems have been unmounted.

systemd-cryptsetup@.service will ask for hard disk passwords via the password agent logic, in order to query the user for the password using the right mechanism at boot and during runtime.

At early boot and when the system manager configuration is reloaded, /etc/crypttab is translated into systemd-cryptsetup@.service units by systemd-cryptsetup-generator(8).

In order to unlock a volume a password or binary key is required. systemd-cryptsetup@.service tries to acquire a suitable password or binary key via the following mechanisms, tried in order:

  1. If a key file is explicitly configured (via the third column in /etc/crypttab), a key read from it is used. If a PKCS#11 token, FIDO2 token or TPM2 device is configured (using the pkcs11-uri=, fido2-device=, tpm2-device= options) the key is decrypted before use.

  2. If no key file is configured explicitly this way, a key file is automatically loaded from /etc/cryptsetup-keys.d/volume.key and /run/cryptsetup-keys.d/volume.key, if present. Here too, if a PKCS#11/FIDO2/TPM2 token/device is configured, any key found this way is decrypted before use.

  3. If the try-empty-password option is specified it is then attempted to unlock the volume with an empty password.

  4. The kernel keyring is then checked for a suitable cached password from previous attempts.

  5. Finally, the user is queried for a password, possibly multiple times, unless the headless option is set.

If no suitable key may be acquired via any of the mechanisms describes above, volume activation fails.

See Also

systemd(1), systemd-cryptsetup-generator(8), crypttab(5), systemd-cryptenroll(1), cryptsetup(8)