Implementations of the SILC Authentication Payload and authentication
 routines.  The SILC Authentication Payload is used to deliver
 authentication data usually from client to server in purpose of
 gaining access to some service.  The Payload and the authentication
 routines supports both passphrase and public key (signature) based
 authentication.

 This interface defines also the SILC Key Agreement Payload that is
 used by client to agree on key material usually with another client
 in the network.