Refuse to unlock LUKS devices with the NULL cipher

[travier]

Users of this pin will always want the data to be properly encrypted on the disk. Thus we should refuse to unlock a LUKS device if the NULL cipher is set for the content.

See: https://blog.trailofbits.com/2025/10/30/vulnerabilities-in-luks2-disk-encryption-for-confidential-vms/

We can always add an option later if we want to allow that use case but I doubt this will come up.

This logic might be useful for other Clevis PINs too.

[travier]

Another approach is to measure the LUKS header (from the blog post above):

Finally, one may measure the header data, with any random salts and digests removed, into the attestation state. This measurement is incorporated into any TPM sealing PCRs or attestations sent to a KMS. In this model, LUKS header configuration becomes part of the CVM identity and allows remote verifiers to set arbitrary policies with respect to what configurations are allowed to receive decryption keys.