unsigned char *
    silc_attribute_get_verify_data(SilcDList attrs,
                                   SilcBool server_verification,
                                   SilcUInt32 *data_len);

    Constructs the data to be verified with the sender's digital
    signature and sender's public key.  This allocates the data from
    the list of attribute payloads and returns the buffer.  The caller
    must free it.  If `server_verification' is FALSE then data is
    constructed for user's digital signature verification, if it is
    TRUE then it is constructed for server's digital signature
    verification.