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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 will ask for hard disk passwords via the password agent logic[1], 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 is configured (using the pkcs11-uri= option) 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 token 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.

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

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

password agent logic
systemd 246