Debian's cryptsetup packaging provides decrypt_keyctl, a key management script that allows the user to specify that several LUKS devices use the same passphrase that should only be requested once during boot. To this end, a keyphrase successfully used to unlock a device is stored in the secure kernel keyring for 60 seconds, and then automatically used to unlock any other devices that come up in that time. Please provide similar functionality in systemd's cryptsetup handling.
systemd-cryptsetup has been storing the collected key for a while now in the kernel keyring. Closing.
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