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 this /etc/crypttab is translated
    into systemd-cryptsetup@.service units by
    systemd-cryptsetup-generator(8).