int m_authenticate(struct Client* cptr, struct Client* sptr, int parc, char* parv[])
{
  struct Client* acptr;
  int first = 0;
  char realhost[HOSTLEN + 3];
  char *hoststr = (cli_sockhost(cptr) ? cli_sockhost(cptr) : cli_sock_ip(cptr));

  if (!CapActive(cptr, CAP_SASL))
    return 0;

  if (parc < 2) /* have enough parameters? */
    return need_more_params(cptr, "AUTHENTICATE");

  if (strlen(parv[1]) > 400)
    return send_reply(cptr, ERR_SASLTOOLONG);

  if (IsSASLComplete(cptr))
    return send_reply(cptr, ERR_SASLALREADY);

  /* Look up the target server */
  if (!(acptr = cli_saslagent(cptr))) {
    if (strcmp(feature_str(FEAT_SASL_SERVER), "*"))
      acptr = find_match_server((char *)feature_str(FEAT_SASL_SERVER));
    else
      acptr = NULL;
  }

  if (!acptr && strcmp(feature_str(FEAT_SASL_SERVER), "*"))
    return send_reply(cptr, ERR_SASLFAIL, ": service unavailable");

  /* If it's to us, do nothing; otherwise, forward the query */
  if (acptr && IsMe(acptr))
    return 0;

  /* Generate an SASL session cookie if not already generated */
  if (!cli_saslcookie(cptr)) {
    do {
      cli_saslcookie(cptr) = ircrandom() & 0x7fffffff;
    } while (!cli_saslcookie(cptr));
    first = 1;
  }

  if (strchr(hoststr, ':') != NULL)
    ircd_snprintf(0, realhost, sizeof(realhost), "[%s]", hoststr);
  else
    ircd_strncpy(realhost, hoststr, sizeof(realhost));
 
   if (acptr) {
     if (first) {
       if (!EmptyString(cli_sslclifp(cptr)))
         sendcmdto_one(&me, CMD_SASL, acptr, "%C %C!%u.%u S %s :%s", acptr, &me,
                       cli_fd(cptr), cli_saslcookie(cptr),
                      parv[1], cli_sslclifp(cptr));
      else
        sendcmdto_one(&me, CMD_SASL, acptr, "%C %C!%u.%u S :%s", acptr, &me,
                      cli_fd(cptr), cli_saslcookie(cptr), parv[1]);
      if (feature_bool(FEAT_SASL_SENDHOST))
        sendcmdto_one(&me, CMD_SASL, acptr, "%C %C!%u.%u H :%s@%s:%s", acptr, &me,
                      cli_fd(cptr), cli_saslcookie(cptr), cli_username(cptr),
                      realhost, cli_sock_ip(cptr));
    } else {
      sendcmdto_one(&me, CMD_SASL, acptr, "%C %C!%u.%u C :%s", acptr, &me,
                    cli_fd(cptr), cli_saslcookie(cptr), parv[1]);
     }
   } else {
     if (first) {
       if (!EmptyString(cli_sslclifp(cptr)))
         sendcmdto_serv_butone(&me, CMD_SASL, cptr, "* %C!%u.%u S %s :%s", &me,
                               cli_fd(cptr), cli_saslcookie(cptr),
                              parv[1], cli_sslclifp(cptr));
      else
        sendcmdto_serv_butone(&me, CMD_SASL, cptr, "* %C!%u.%u S :%s", &me,
                              cli_fd(cptr), cli_saslcookie(cptr), parv[1]);
      if (feature_bool(FEAT_SASL_SENDHOST))
        sendcmdto_serv_butone(&me, CMD_SASL, cptr, "* %C!%u.%u H :%s@%s:%s", &me,
                              cli_fd(cptr), cli_saslcookie(cptr), cli_username(cptr),
                              realhost, cli_sock_ip(cptr));
    } else {
      sendcmdto_serv_butone(&me, CMD_SASL, cptr, "* %C!%u.%u C :%s", &me,
                            cli_fd(cptr), cli_saslcookie(cptr), parv[1]);
    }
  }

  if (!t_active(&cli_sasltimeout(cptr)))
    timer_add(timer_init(&cli_sasltimeout(cptr)), sasl_timeout_callback, (void*) cptr,
              TT_RELATIVE, feature_int(FEAT_SASL_TIMEOUT));

  return 0;
}

When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.