diff --git a/nsync/sync-digest.cc b/nsync/sync-digest.cc
new file mode 100644
index 0000000..c9ff12d
--- /dev/null
+++ b/nsync/sync-digest.cc
@@ -0,0 +1,262 @@
+/* -*- 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>
+ *         Chaoyi Bian <bcy@pku.edu.cn>
+ *	   Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ */
+
+#include "sync-digest.h"
+#include <string.h>
+
+#include <boost/assert.hpp>
+#include <boost/throw_exception.hpp>
+typedef boost::error_info<struct tag_errmsg, std::string> errmsg_info_str;
+typedef boost::error_info<struct tag_errmsg, int> errmsg_info_int;
+
+// for printing, may be disabled in optimized build
+
+// #ifdef DIGEST_BASE64
+// #include <boost/archive/iterators/base64_from_binary.hpp>
+// #include <boost/archive/iterators/binary_from_base64.hpp>
+// #endif
+
+#include "boost-archive.h"
+
+using namespace boost;
+using namespace boost::archive::iterators;
+using namespace std;
+
+// Other options: VP_md2, EVP_md5, EVP_sha, EVP_sha1, EVP_sha256, EVP_dss, EVP_dss1, EVP_mdc2, EVP_ripemd160
+#define HASH_FUNCTION EVP_sha256
+#define HASH_FUNCTION_LEN 32
+
+
+// #ifndef DIGEST_BASE64
+
+template<class CharType>
+struct hex_from_4_bit
+{
+  typedef CharType result_type;
+  CharType operator () (CharType ch) const
+  {
+    const char *lookup_table = "0123456789abcdef";
+    // cout << "New character: " << (int) ch << " (" << (char) ch << ")" << "\n";
+    BOOST_ASSERT (ch < 16);
+    return lookup_table[static_cast<size_t>(ch)];
+  }
+};
+
+typedef transform_iterator<hex_from_4_bit<std::vector<uint8_t>::const_iterator::value_type>,
+                           transform_width<std::vector<uint8_t>::const_iterator, 4, 8, std::vector<uint8_t>::const_iterator::value_type> > string_from_binary;
+
+
+template<class CharType>
+struct hex_to_4_bit
+{
+  typedef CharType result_type;
+  CharType operator () (CharType ch) const
+  {
+    const signed char lookup_table [] = {
+      -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      0, 1, 2, 3, 4, 5, 6, 7, 8, 9,-1,-1,-1,-1,-1,-1,
+      -1,10,11,12,13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      -1,10,11,12,13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1,
+      -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
+    };
+
+    // cout << "New character: " << hex << (int) ch << " (" << (char) ch << ")" << "\n";
+    signed char value = -1;
+    if ((unsigned)ch < 128)
+      value = lookup_table [(unsigned)ch];
+    if (value == -1)
+      BOOST_THROW_EXCEPTION (Sync::Error::DigestCalculationError () << errmsg_info_int ((int)ch));
+
+    return value;
+  }
+};
+
+typedef transform_width<transform_iterator<hex_to_4_bit<string::const_iterator::value_type>, string::const_iterator>, 8, 4> string_to_binary;
+
+namespace Sync {
+
+Digest::Digest ()
+{
+  m_context = EVP_MD_CTX_create ();
+
+  reset ();
+}
+
+Digest::~Digest ()
+{
+  EVP_MD_CTX_destroy (m_context);
+}
+
+bool
+Digest::empty () const
+{
+  return m_buffer.empty ();
+}
+
+bool
+Digest::isZero () const
+{
+  if (m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest has not been yet finalized"));
+
+  return (m_buffer.size () == 1 && m_buffer[0] == 0);
+}
+
+
+void
+Digest::reset ()
+{
+  m_buffer.clear ();
+
+  int ok = EVP_DigestInit_ex (m_context, HASH_FUNCTION (), 0);
+  if (!ok)
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("EVP_DigestInit_ex returned error")
+                           << errmsg_info_int (ok));
+}
+
+
+void
+Digest::finalize ()
+{
+  if (!m_buffer.empty ()) return;
+
+  m_buffer.resize (HASH_FUNCTION_LEN);
+
+  unsigned int tmp;
+  int ok = EVP_DigestFinal_ex (m_context,
+			       &m_buffer[0], &tmp);
+  if (!ok)
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("EVP_DigestFinal_ex returned error")
+                           << errmsg_info_int (ok));
+}
+
+std::size_t
+Digest::getHash () const
+{
+  if (isZero ()) return 0;
+
+  if (sizeof (std::size_t) > m_buffer.size ())
+    {
+      BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                             << errmsg_info_str ("Hash is not zero and length is less than size_t")
+                             << errmsg_info_int (m_buffer.size ()));
+    }
+
+  // just getting first sizeof(std::size_t) bytes
+  // not ideal, but should work pretty well
+  return *(reinterpret_cast<const std::size_t*> (&m_buffer[0]));
+}
+
+bool
+Digest::operator == (const Digest &digest) const
+{
+  if (m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest1 is empty"));
+
+  if (digest.m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest2 is empty"));
+
+  return m_buffer == digest.m_buffer;
+}
+
+
+void
+Digest::update (const uint8_t *buffer, size_t size)
+{
+  // cout << "Update: " << (void*)buffer << " / size: " << size << "\n";
+
+  // cannot update Digest when it has been finalized
+  if (!m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest has been already finalized"));
+
+  bool ok = EVP_DigestUpdate (m_context, buffer, size);
+  if (!ok)
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("EVP_DigestUpdate returned error")
+                           << errmsg_info_int (ok));
+}
+
+
+Digest &
+Digest::operator << (const Digest &src)
+{
+  if (src.m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest has not been yet finalized"));
+
+  update (&src.m_buffer[0], src.m_buffer.size ());
+
+  return *this;
+}
+
+std::ostream &
+operator << (std::ostream &os, const Digest &digest)
+{
+  BOOST_ASSERT (!digest.m_buffer.empty ());
+
+  ostreambuf_iterator<char> out_it (os); // ostream iterator
+  // need to encode to base64
+  copy (string_from_binary (digest.m_buffer.begin ()),
+        string_from_binary (digest.m_buffer.end ()),
+        out_it);
+
+  return os;
+}
+
+std::istream &
+operator >> (std::istream &is, Digest &digest)
+{
+  string str;
+  is >> str; // read string first
+
+  if (str.size () == 0)
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Input is empty"));
+
+  // uint8_t padding = (3 - str.size () % 3) % 3;
+  // for (uint8_t i = 0; i < padding; i++) str.push_back ('=');
+
+  // only empty digest object can be used for reading
+  if (!digest.m_buffer.empty ())
+    BOOST_THROW_EXCEPTION (Error::DigestCalculationError ()
+                           << errmsg_info_str ("Digest has been already finalized"));
+
+  digest.m_buffer.clear ();
+
+  copy (string_to_binary (str.begin ()),
+        string_to_binary (str.end ()),
+        std::back_inserter (digest.m_buffer));
+
+  return is;
+}
+
+
+} // Sync