systemd-cryptsetup@.service, systemd-cryptsetup — Full disk decryption logic
systemd-cryptsetup@.service
/usr/lib/systemd/systemd-cryptsetup
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, 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:
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.
If no key file is configured explicitly this way, a key file is automatically loaded
from /etc/cryptsetup-keys.d/ and
volume.key/run/cryptsetup-keys.d/, if present. Here
too, if a PKCS#11 token is configured, any key found this way is decrypted before
use.volume.key
If the try-empty-password option is specified it is then attempted
to unlock the volume with an empty password.
The kernel keyring is then checked for a suitable cached password from previous attempts.
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.