diff --git a/common/rfb/SSecurityVncAuth.cxx b/common/rfb/SSecurityVncAuth.cxx
new file mode 100644
index 0000000..29a3b96
--- /dev/null
+++ b/common/rfb/SSecurityVncAuth.cxx
@@ -0,0 +1,87 @@
+/* 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.
+ */
+//
+// SSecurityVncAuth
+//
+// XXX not thread-safe, because d3des isn't - do we need to worry about this?
+//
+
+#include <rfb/SSecurityVncAuth.h>
+#include <rdr/RandomStream.h>
+#include <rfb/SConnection.h>
+#include <rfb/Password.h>
+#include <rfb/Configuration.h>
+#include <rfb/LogWriter.h>
+#include <rfb/util.h>
+#include <rfb/Exception.h>
+#include <string.h>
+#include <stdio.h>
+extern "C" {
+#include <rfb/d3des.h>
+}
+
+
+using namespace rfb;
+
+static LogWriter vlog("SVncAuth");
+
+
+SSecurityVncAuth::SSecurityVncAuth(VncAuthPasswdGetter* pg_)
+  : sentChallenge(false), responsePos(0), pg(pg_)
+{
+}
+
+bool SSecurityVncAuth::processMsg(SConnection* sc)
+{
+  rdr::InStream* is = sc->getInStream();
+  rdr::OutStream* os = sc->getOutStream();
+
+  if (!sentChallenge) {
+    rdr::RandomStream rs;
+    rs.readBytes(challenge, vncAuthChallengeSize);
+    os->writeBytes(challenge, vncAuthChallengeSize);
+    os->flush();
+    sentChallenge = true;
+    return false;
+  }
+
+  while (responsePos < vncAuthChallengeSize && is->checkNoWait(1))
+    response[responsePos++] = is->readU8();
+
+  if (responsePos < vncAuthChallengeSize) return false;
+
+  PlainPasswd passwd(pg->getVncAuthPasswd());
+
+  if (!passwd.buf)
+    throw AuthFailureException("No password configured for VNC Auth");
+
+  // Calculate the expected 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);
+
+  // Check the actual response
+  if (memcmp(challenge, response, vncAuthChallengeSize) != 0)
+    throw AuthFailureException();
+
+  return true;
+}