SilcMessageSignedPayload
    silc_message_signed_payload_parse(const unsigned char *data,
                                      SilcUInt32 data_len);

    Parses the SilcMessageSignedPayload Payload from the `data' of
    length of `data_len' bytes.  The `data' must be payload without
    the actual message payload.  Returns the parsed payload or NULL
    on error.  Caller must free the returned payload.  Application
    usually does not need to call this since the function
    silc_message_payload_parse calls this automatically for signed
    messages.