diff --git a/src/util/digest.cpp b/src/util/digest.cpp
index 156d372..664df2b 100644
--- a/src/util/digest.cpp
+++ b/src/util/digest.cpp
@@ -32,6 +32,19 @@
 }
 
 template<typename Hash>
+Digest<Hash>::Digest(std::istream& is)
+  : m_isInProcess(false)
+  , m_isFinalized(true)
+{
+  using namespace CryptoPP;
+
+  m_buffer = make_shared<Buffer>(m_hash.DigestSize());
+  FileSource(is, true,
+             new HashFilter(m_hash,
+                            new ArraySink(m_buffer->get(), m_buffer->size())));
+}
+
+template<typename Hash>
 void
 Digest<Hash>::reset()
 {
diff --git a/src/util/digest.hpp b/src/util/digest.hpp
index b0189a4..0fdb3a3 100644
--- a/src/util/digest.hpp
+++ b/src/util/digest.hpp
@@ -64,6 +64,13 @@
   Digest();
 
   /**
+   * @brief Create digest of the input stream @p
+   * @param is input stream
+   */
+  explicit
+  Digest(std::istream& is);
+
+  /**
    * @brief Discard the current state and start a new digest.
    */
   void