diff --git a/common/rfb/CSecurityVncAuth.cxx b/common/rfb/CSecurityVncAuth.cxx
new file mode 100644
index 0000000..ba5a30b
--- /dev/null
+++ b/common/rfb/CSecurityVncAuth.cxx
@@ -0,0 +1,74 @@
+/* Copyright (C) 2002-2005 RealVNC Ltd.  All Rights Reserved.
+ * 
+ * This is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ * 
+ * This software is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this software; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307,
+ * USA.
+ */
+//
+// CSecurityVncAuth
+//
+// XXX not thread-safe, because d3des isn't - do we need to worry about this?
+//
+
+#include <string.h>
+#include <stdio.h>
+#include <rfb/CConnection.h>
+#include <rfb/UserPasswdGetter.h>
+#include <rfb/Password.h>
+#include <rfb/CSecurityVncAuth.h>
+#include <rfb/util.h>
+extern "C" {
+#include <rfb/d3des.h>
+}
+
+
+using namespace rfb;
+
+static const int vncAuthChallengeSize = 16;
+
+
+CSecurityVncAuth::CSecurityVncAuth(UserPasswdGetter* upg_)
+  : upg(upg_)
+{
+}
+
+CSecurityVncAuth::~CSecurityVncAuth()
+{
+}
+
+bool CSecurityVncAuth::processMsg(CConnection* cc)
+{
+  rdr::InStream* is = cc->getInStream();
+  rdr::OutStream* os = cc->getOutStream();
+
+  // Read the challenge & obtain the user's password
+  rdr::U8 challenge[vncAuthChallengeSize];
+  is->readBytes(challenge, vncAuthChallengeSize);
+  PlainPasswd passwd;
+  upg->getUserPasswd(0, &passwd.buf);
+
+  // Calculate the correct response
+  rdr::U8 key[8];
+  int pwdLen = strlen(passwd.buf);
+  for (int i=0; i<8; i++)
+    key[i] = i<pwdLen ? passwd.buf[i] : 0;
+  deskey(key, EN0);
+  for (int j = 0; j < vncAuthChallengeSize; j += 8)
+    des(challenge+j, challenge+j);
+
+  // Return the response to the server
+  os->writeBytes(challenge, vncAuthChallengeSize);
+  os->flush();
+  return true;
+}