I don't see that a PGP-signed document would not be acceptable under Part 97
as a mechanism for facilitating communication. After all, anyone with the
public key can verify the identity of the sender and determine that the
message does not contain illicit information. By extension, a similar
method could be devised to grant access to a resource or facility.
There are other methods for authetication involving challenge and response
which should also be acceptable under Part 97.
-- KD6PAG (Networking Old-Timer, Satellite QRPer)