int _gsasl_plain_client_step(Gsasl_session *sctx, void *mech_data, const char *input, size_t input_len, char **output, size_t *output_len)
int _gsasl_plain_server_step(Gsasl_session *sctx, void *mech_data, const char *input, size_t input_len, char **output, size_t *output_len)