PasswordMessage containing the
        password in clear-text form.  If
        this is the correct password, the server responds with an
        AuthenticationOk, otherwise it responds with an ErrorResponse.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationMD5Password</term>
      <listitem>
       <para>
        The frontend must now send a PasswordMessage containing the
        password (with user name) encrypted via MD5, then encrypted
        again using the 4-byte random salt specified in the
        AuthenticationMD5Password message.  If this is the correct
        password, the server responds with an AuthenticationOk,
        otherwise it responds with an ErrorResponse.  The actual
        PasswordMessage can be computed in SQL as <literal>concat('md5',
        md5(concat(md5(concat(password, username)), random-salt)))</literal>.
        (Keep in mind the <function>md5()</function> function returns its
        result as a hex string.)
       </para>
        <warning>
         <para>
          Support for MD5-encrypted passwords is deprecated and will be removed
          in a future release of <productname>PostgreSQL</productname>.  Refer
          to <xref linkend="auth-password"/> for details about migrating to
          another password type.
         </para>
        </warning>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationGSS</term>
      <listitem>
       <para>
        The frontend must now initiate a GSSAPI negotiation. The frontend
        will send a GSSResponse message with the first part of the GSSAPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSSPI</term>
      <listitem>
       <para>
        The frontend must now initiate an SSPI negotiation. The frontend
        will send a GSSResponse with the first part of the SSPI
        data stream in response to this. If further messages are needed,
        the server will respond with AuthenticationGSSContinue.
       </para>
      </listitem>

     </varlistentry>
     <varlistentry>
      <term>AuthenticationGSSContinue</term>
      <listitem>
       <para>
        This message contains the response data from the previous step
        of GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI
        or a previous AuthenticationGSSContinue). If the GSSAPI
        or SSPI data in this message
        indicates more data is needed to complete the authentication,
        the frontend must send that data as another GSSResponse message. If
        GSSAPI or SSPI authentication is completed by this message, the server
        will next send AuthenticationOk to indicate successful authentication
        or ErrorResponse to indicate failure.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSASL</term>
      <listitem>
       <para>
        The frontend must now initiate a SASL negotiation, using one of the
        SASL mechanisms listed in the message. The frontend will send a
        SASLInitialResponse with the name of the selected mechanism, and the
        first part of the SASL data stream in response to this. If further
        messages are needed, the server will respond with
        AuthenticationSASLContinue. See <xref linkend="sasl-authentication"/>
        for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>AuthenticationSASLContinue</term>
      <listitem>
       <para>
        This message contains challenge data from the previous step of SASL
        negotiation (AuthenticationSASL, or a previous
        AuthenticationSASLContinue). The frontend must respond with a
        SASLResponse message.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>