/// <summary>
        /// Continues authentication process.
        /// </summary>
        /// <param name="serverResponse">Server sent SASL response.</param>
        /// <returns>Returns challange request what must be sent to server or null if authentication has completed.</returns>
        /// <exception cref="ArgumentNullException">Is raised when <b>serverResponse</b> is null reference.</exception>
        /// <exception cref="InvalidOperationException">Is raised when this method is called when authentication is completed.</exception>
        public override byte[] Continue(byte[] serverResponse)
        {
            if (serverResponse == null)
            {
                throw new ArgumentNullException("serverResponse");
            }
            if (m_IsCompleted)
            {
                throw new InvalidOperationException("Authentication is completed.");
            }

            /* RFC 2831.
             *  The base64-decoded version of the SASL exchange is:
             *
             *  S: realm="elwood.innosoft.com",nonce="OA6MG9tEQGm2hh",qop="auth",
             *     algorithm=md5-sess,charset=utf-8
             *  C: charset=utf-8,username="******",realm="elwood.innosoft.com",
             *     nonce="OA6MG9tEQGm2hh",nc=00000001,cnonce="OA6MHXh6VqTrRk",
             *     digest-uri="imap/elwood.innosoft.com",
             *     response=d388dad90d4bbd760a152321f2143af7,qop=auth
             *  S: rspauth=ea40f60335c427b5527b84dbabcdfffd
             *  C:
             *  S: ok
             *
             *  The password in this example was "secret".
             */

            if (m_State == 0)
            {
                m_State++;

                // Parse server challenge.
                AUTH_SASL_DigestMD5_Challenge challenge = AUTH_SASL_DigestMD5_Challenge.Parse(Encoding.UTF8.GetString(serverResponse));

                // Construct our response to server challenge.
                m_pResponse = new AUTH_SASL_DigestMD5_Response(
                    challenge,
                    challenge.Realm[0],
                    m_UserName,
                    m_Password,
                    Guid.NewGuid().ToString().Replace("-", ""),
                    1,
                    challenge.QopOptions[0],
                    m_Protocol + "/" + m_ServerName
                    );

                return(Encoding.UTF8.GetBytes(m_pResponse.ToResponse()));
            }
            else if (m_State == 1)
            {
                m_State++;
                m_IsCompleted = true;

                // Check rspauth value.
                if (!string.Equals(Encoding.UTF8.GetString(serverResponse), m_pResponse.ToRspauthResponse(m_UserName, m_Password), StringComparison.InvariantCultureIgnoreCase))
                {
                    throw new Exception("Server server 'rspauth' value mismatch with local 'rspauth' value.");
                }

                return(new byte[0]);
            }
            else
            {
                throw new InvalidOperationException("Authentication is completed.");
            }
        }
        /// <summary>
        /// Continues authentication process.
        /// </summary>
        /// <param name="clientResponse">Client sent SASL response.</param>
        /// <returns>Retunrns challange response what must be sent to client or null if authentication has completed.</returns>
        /// <exception cref="ArgumentNullException">Is raised when <b>clientResponse</b> is null reference.</exception>
        public override byte[] Continue(byte[] clientResponse)
        {
            if (clientResponse == null)
            {
                throw new ArgumentNullException("clientResponse");
            }

            /* RFC 2831.
             *  The base64-decoded version of the SASL exchange is:
             *
             *  S: realm="elwood.innosoft.com",nonce="OA6MG9tEQGm2hh",qop="auth",
             *     algorithm=md5-sess,charset=utf-8
             *  C: charset=utf-8,username="******",realm="elwood.innosoft.com",
             *     nonce="OA6MG9tEQGm2hh",nc=00000001,cnonce="OA6MHXh6VqTrRk",
             *     digest-uri="imap/elwood.innosoft.com",
             *     response=d388dad90d4bbd760a152321f2143af7,qop=auth
             *  S: rspauth=ea40f60335c427b5527b84dbabcdfffd
             *  C:
             *  S: ok
             *
             *  The password in this example was "secret".
             */

            if (m_State == 0)
            {
                m_State++;

                AUTH_SASL_DigestMD5_Challenge callenge = new AUTH_SASL_DigestMD5_Challenge(new string[] { m_Realm }, m_Nonce, new string[] { "auth" }, false);

                return(Encoding.UTF8.GetBytes(callenge.ToChallenge()));
            }
            else if (m_State == 1)
            {
                m_State++;

                try{
                    AUTH_SASL_DigestMD5_Response response = AUTH_SASL_DigestMD5_Response.Parse(Encoding.UTF8.GetString(clientResponse));

                    // Check realm and nonce value.
                    if (m_Realm != response.Realm || m_Nonce != response.Nonce)
                    {
                        return(Encoding.UTF8.GetBytes("rspauth=\"\""));
                    }

                    m_UserName = response.UserName;
                    AUTH_e_UserInfo result = OnGetUserInfo(response.UserName);
                    if (result.UserExists)
                    {
                        if (response.Authenticate(result.UserName, result.Password))
                        {
                            m_IsAuthenticated = true;

                            return(Encoding.UTF8.GetBytes(response.ToRspauthResponse(result.UserName, result.Password)));
                        }
                    }
                }
                catch {
                    // Authentication failed, just reject request.
                }

                return(Encoding.UTF8.GetBytes("rspauth=\"\""));
            }
            else
            {
                m_IsCompleted = true;
            }

            return(null);
        }