Iso9796d2Signer
ISO9796-2 - mechanism using a hash function with recovery (scheme 1)
                
Generate a signer with either implicit or explicit trailers for ISO9796-2.
            
Constructor for a signer with an explicit digest trailer.
            
             
Generate a signature for the loaded message using the key we were
            initialised with.
            
Return a reference to the recoveredMessage message.
            
Return true if the full message was recoveredMessage.
            
reset the internal state
return true if the signature represents a ISO9796-2 signature
            for the passed in message.