diff --git a/src/util/digest.cpp b/src/util/digest.cpp
index 68c53ce..516255f 100644
--- a/src/util/digest.cpp
+++ b/src/util/digest.cpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -21,7 +21,11 @@
 
 #include "digest.hpp"
 #include "string-helper.hpp"
-#include "security/detail/openssl.hpp"
+#include "../security/detail/openssl.hpp"
+#include "../security/transform/digest-filter.hpp"
+#include "../security/transform/stream-sink.hpp"
+#include "../security/transform/stream-source.hpp"
+#include "../security/v1/cryptopp.hpp"
 
 #include <sstream>
 
@@ -175,13 +179,118 @@
   return os;
 }
 
-template
-class Digest<CryptoPP::SHA256>;
 
-template
+////////////////////////////////////////
+
+
+Sha256::Sha256()
+{
+  reset();
+}
+
+Sha256::Sha256(std::istream& is)
+  : m_output(make_unique<OBufferStream>())
+  , m_isEmpty(false)
+  , m_isFinalized(true)
+{
+  namespace tr = security::transform;
+
+  tr::streamSource(is) >> tr::digestFilter(DigestAlgorithm::SHA256) >> tr::streamSink(*m_output);
+}
+
+void
+Sha256::reset()
+{
+  namespace tr = security::transform;
+
+  m_input = make_unique<tr::StepSource>();
+  m_output = make_unique<OBufferStream>();
+  m_isEmpty = true;
+  m_isFinalized = false;
+
+  *m_input >> tr::digestFilter(DigestAlgorithm::SHA256) >> tr::streamSink(*m_output);
+}
+
+ConstBufferPtr
+Sha256::computeDigest()
+{
+  if (!m_isFinalized) {
+    BOOST_ASSERT(m_input != nullptr);
+    m_input->end();
+    m_isFinalized = true;
+  }
+
+  return m_output->buf();
+}
+
+bool
+Sha256::operator==(Sha256& digest)
+{
+  const Buffer& lhs = *computeDigest();
+  const Buffer& rhs = *digest.computeDigest();
+
+  if (lhs.size() != rhs.size()) {
+    return false;
+  }
+
+  // constant-time buffer comparison to mitigate timing attacks
+  return CRYPTO_memcmp(lhs.get(), rhs.get(), lhs.size()) == 0;
+}
+
+Sha256&
+Sha256::operator<<(Sha256& src)
+{
+  auto buf = src.computeDigest();
+  update(buf->get(), buf->size());
+  return *this;
+}
+
+Sha256&
+Sha256::operator<<(const std::string& str)
+{
+  update(reinterpret_cast<const uint8_t*>(str.data()), str.size());
+  return *this;
+}
+
+Sha256&
+Sha256::operator<<(const Block& block)
+{
+  update(block.wire(), block.size());
+  return *this;
+}
+
+Sha256&
+Sha256::operator<<(uint64_t value)
+{
+  update(reinterpret_cast<const uint8_t*>(&value), sizeof(uint64_t));
+  return *this;
+}
+
+void
+Sha256::update(const uint8_t* buffer, size_t size)
+{
+  if (m_isFinalized)
+    BOOST_THROW_EXCEPTION(Error("Digest has been already finalized"));
+
+  BOOST_ASSERT(m_input != nullptr);
+  m_input->write(buffer, size);
+  m_isEmpty = false;
+}
+
+std::string
+Sha256::toString()
+{
+  auto buf = computeDigest();
+  return toHex(*buf);
+}
+
 std::ostream&
-operator<<(std::ostream& os, Digest<CryptoPP::SHA256>& digest);
-
+operator<<(std::ostream& os, Sha256& digest)
+{
+  auto buf = digest.computeDigest();
+  printHex(os, *buf);
+  return os;
+}
 
 } // namespace util
 } // namespace ndn
diff --git a/src/util/digest.hpp b/src/util/digest.hpp
index 41e45a5..40d0100 100644
--- a/src/util/digest.hpp
+++ b/src/util/digest.hpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -22,10 +22,11 @@
 #ifndef NDN_UTIL_DIGEST_HPP
 #define NDN_UTIL_DIGEST_HPP
 
-#include "../encoding/buffer.hpp"
-#include "../encoding/block.hpp"
-#include "../security/v1/cryptopp.hpp"
 #include "concepts.hpp"
+#include "crypto.hpp"
+#include "../encoding/block.hpp"
+#include "../encoding/buffer-stream.hpp"
+#include "../security/transform/step-source.hpp"
 
 namespace ndn {
 namespace util {
@@ -194,10 +195,153 @@
 std::ostream&
 operator<<(std::ostream& os, Digest<Hash>& digest);
 
+
 /**
- * @brief A digest using SHA256 as the hash function.
+ * @brief Provides stateful SHA-256 digest calculation.
+ *
+ * Example:
+ * @code
+ * Sha256 digest;
+ * digest.update(buf1, size1);
+ * digest.update(buf2, size2);
+ * ...
+ * ConstBufferPtr result = digest.computeDigest();
+ * @endcode
  */
-typedef Digest<CryptoPP::SHA256> Sha256;
+class Sha256
+{
+public:
+  class Error : public std::runtime_error
+  {
+  public:
+    explicit
+    Error(const std::string& what)
+      : std::runtime_error(what)
+    {
+    }
+  };
+
+  /**
+   * @brief Create an empty SHA-256 digest.
+   */
+  Sha256();
+
+  /**
+   * @brief Calculate SHA-256 digest of the input stream @p is.
+   */
+  explicit
+  Sha256(std::istream& is);
+
+  /**
+   * @brief Check if digest is empty.
+   *
+   * An empty digest means nothing has been taken into calculation.
+   */
+  bool
+  empty() const
+  {
+    return m_isEmpty;
+  }
+
+  /**
+   * @brief Discard the current state and start a new digest calculation.
+   */
+  void
+  reset();
+
+  /**
+   * @brief Finalize and return the digest based on all previously supplied inputs.
+   */
+  ConstBufferPtr
+  computeDigest();
+
+  /**
+   * @brief Check if the supplied digest is equal to this digest.
+   * @note This method invokes computeDigest() on both operands, finalizing the digest.
+   */
+  bool
+  operator==(Sha256& digest);
+
+  /**
+   * @brief Check if the supplied digest is not equal to this digest.
+   * @note This method invokes computeDigest() on both operands, finalizing the digest.
+   */
+  bool
+  operator!=(Sha256& digest)
+  {
+    return !(*this == digest);
+  }
+
+  /**
+   * @brief Add existing digest to the digest calculation.
+   * @param src digest to combine with
+   *
+   * The result of this combination is `sha256(sha256(...))`
+   *
+   * @note This method invokes computeDigest() on @p src, finalizing the digest.
+   * @throw Error the digest has already been finalized
+   */
+  Sha256&
+  operator<<(Sha256& src);
+
+  /**
+   * @brief Add a string to the digest calculation.
+   * @throw Error the digest has already been finalized
+   */
+  Sha256&
+  operator<<(const std::string& str);
+
+  /**
+   * @brief Add a block to the digest calculation.
+   * @throw Error the digest has already been finalized
+   */
+  Sha256&
+  operator<<(const Block& block);
+
+  /**
+   * @brief Add a uint64_t value to the digest calculation.
+   * @throw Error the digest has already been finalized
+   */
+  Sha256&
+  operator<<(uint64_t value);
+
+  /**
+   * @brief Add a raw buffer to the digest calculation.
+   * @param buffer the input buffer
+   * @param size the size of the input buffer
+   * @throw Error the digest has already been finalized
+   */
+  void
+  update(const uint8_t* buffer, size_t size);
+
+  /**
+   * @brief Convert digest to std::string.
+   * @note This method invokes computeDigest(), finalizing the digest.
+   */
+  std::string
+  toString();
+
+  /**
+   * @brief Compute a one-time SHA-256 digest.
+   * @param buffer the input buffer
+   * @param size the size of the input buffer
+   * @return SHA-256 digest of the input buffer
+   */
+  static ConstBufferPtr
+  computeDigest(const uint8_t* buffer, size_t size)
+  {
+    return crypto::computeSha256Digest(buffer, size);
+  }
+
+private:
+  unique_ptr<security::transform::StepSource> m_input;
+  unique_ptr<OBufferStream> m_output;
+  bool m_isEmpty;
+  bool m_isFinalized;
+};
+
+std::ostream&
+operator<<(std::ostream& os, Sha256& digest);
 
 } // namespace util
 } // namespace ndn