SilcPublicKey
    silc_message_signed_get_public_key(SilcMessageSignedPayload sig,
                                       unsigned char **pk_data,
                                       SilcUInt32 *pk_data_len);

    Returns the decoded SilcPublicKey from the SilcMessageSignedPayload
    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.