In the public-key case, the receiver gets the signed document timestamped by the arbitrator. In the private-key case, the sender sends the message to the arbitrator, generates a certification of receipt, and then forwards both the message and certification to the receiver. If there are any disputes, they can be resolved by the arbitrator. Authentication Authentication is the process of verifying the identity of a user. It should not be confused with authorization, which is concerned with what privileges a user has.
Authorization is easy once the user's has been authenticated.
One simple protocol for authentication is to perform a challenge-response . Send second random number, encrypted with the secret key. Designing an authentication protocol is harder than it looks, since there are several subtle possible attacks. Do not give away any potentially valuable information. Have the initiator and responder use different keys for proof, even if this means have two shared keys.