Considering adding support for CKM_CONCATENATE_DATA_AND_BASE

[nwalfield]

In order to implement OpenPGP's X25519 decryption completely on the token, we need CKM_CONCATENATE_DATA_AND_BASE. RFC 9580 says:

See Section 6.1 of [RFC7748] for more details on the computation of the ephemeral public key and the shared secret. The HMAC-based Key Derivation Function (HKDF) [RFC5869] is then used with SHA256 [RFC6234] and an info parameter of "OpenPGP X25519" and no salt. The input of HKDF is the concatenation of the following three values:

• 32 octets of the ephemeral X25519 public key from this packet.

• 32 octets of the recipient public key material.

• 32 octets of the shared secret.

Right now, we extract the shared secret, prepend the public keys and load the result back onto the token. Using CKM_CONCATENATE_DATA_AND_BASE, this could be done entirely on token.

[Jakuje]

These are very trivial KDFs so it should not be large amount of work to add wrappers for them. If nobody will take that, I will try to have a look into that in coming weeks.