diff --git a/model/sync-digest.cc b/model/sync-digest.cc
new file mode 100644
index 0000000..65e3b44
--- /dev/null
+++ b/model/sync-digest.cc
@@ -0,0 +1,93 @@
+/* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil -*- */
+/*
+ * Copyright (c) 2012 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program 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 program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Zhenkai Zhu <zhenkai@cs.ucla.edu>
+ *         卞超轶 Chaoyi Bian <bcy@pku.edu.cn>
+ *	   Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ */
+
+#include "sync-digest.h"
+#include <string.h>
+
+#include "ns3/assert.h"
+
+namespace ns3 {
+namespace Sync {
+
+Digest::Digest ()
+  : m_buffer (0)
+  , m_hashLength (0)
+{
+  m_context = EVP_MD_CTX_create ();
+
+  int ok = EVP_DigestInit_ex (m_context, EVP_sha1 (), 0);
+  if (!ok)
+    throw DigestCalculationError ();
+}
+
+Digest::~Digest ()
+{
+  if (m_buffer != 0)
+    delete [] m_buffer;
+
+  EVP_MD_CTX_destroy (m_context);
+}
+
+void
+Digest::Finalize ()
+{
+  if (m_buffer != 0) return;
+
+  m_buffer = new uint8_t [HASH_SIZE];
+
+  int ok = EVP_DigestFinal_ex (m_context,
+			       m_buffer, &m_hashLength);
+  if (!ok)
+    throw DigestCalculationError ();
+}
+  
+std::size_t
+Digest::getHash ()
+{
+  if (m_buffer == 0)
+    Finalize ();
+
+  NS_ASSERT (sizeof (std::size_t) <= m_hashLength);
+  
+  // just getting first sizeof(std::size_t) bytes
+  // not ideal, but should work pretty well
+  return reinterpret_cast<std::size_t> (m_buffer);
+}
+
+bool
+Digest::operator == (Digest &digest)
+{
+  if (m_buffer == 0)
+    Finalize ();
+
+  if (digest.m_buffer == 0)
+    digest.Finalize ();
+  
+  NS_ASSERT (m_hashLength == digest.m_hashLength);
+
+  return memcmp (m_buffer, digest.m_buffer, m_hashLength) == 0;
+}
+
+
+} // Sync
+} // ns3
+