SilcPublicKey
    silc_message_signed_get_public_key(SilcMessagePayload payload,
                                       const unsigned char **pk_data,
                                       SilcUInt32 *pk_data_len);

    Returns the decoded SilcPublicKey from the message payload or NULL
    if it does not include public key.  The caller must free the returned
    public key pointer.  This also returns the raw public key (before
    decoding) into `pk_data' and `pk_data_len' if they are provided.  The
    caller must not free these pointers.