private string AuthenticateUser(string username, string password, ref SearchResultEntry results)
{
    string errorMessage = string.Empty;
    CertificateErrorMessage = string.Empty;
    IsServerCertificateReceived = false;

    System.DirectoryServices.Protocols.LdapConnection ldapConnection = null;
    try
    {
        // Create the new LDAP connection
        LdapDirectoryIdentifier ldi = new LdapDirectoryIdentifier(Convert.ToString(CoreData.SnmpcToolkit.LdapServerIPAddress), 636);
        ldapConnection = new System.DirectoryServices.Protocols.LdapConnection(ldi);
        ldapConnection.AuthType = AuthType.Basic;
        ldapConnection.SessionOptions.SecureSocketLayer = true;
        ldapConnection.SessionOptions.VerifyServerCertificate = new VerifyServerCertificateCallback(ServerCallback);
        NetworkCredential nc = new NetworkCredential("CN=" + username + "," + CoreData.SnmpcToolkit.LdapDistinguishedName, password);

        X509Certificate clientCertificate = CertificateHelper.FetchWindowsClientCertificate();

        if (clientCertificate != null)
        {
            ldapConnection.ClientCertificates.Add(clientCertificate);
        }

        ldapConnection.Timeout = new TimeSpan(0, 0, 20);
        ldapConnection.Bind(nc);

        // Searching for role associated with the logged in user.
        SearchRequest request = new SearchRequest(CoreData.SnmpcToolkit.LdapDistinguishedName, "(sAMAccountName=" + username + ")", System.DirectoryServices.Protocols.SearchScope.Subtree);
        SearchResponse response = (SearchResponse)ldapConnection.SendRequest(request);

        if (response.Entries.Count == 1)
        {
            results = response.Entries[0];
        }
    }
    catch (Exception ex)
    {
        errorMessage = ex.Message;
    }

    return errorMessage;
}