diff --git a/model/sync-digest.cc b/model/sync-digest.cc
index 5ff9418..621b52b 100644
--- a/model/sync-digest.cc
+++ b/model/sync-digest.cc
@@ -120,6 +120,12 @@
   EVP_MD_CTX_destroy (m_context);
 }
 
+bool
+Digest::empty () const
+{
+  return m_buffer == 0;
+}
+
 void
 Digest::reset ()
 {
@@ -154,7 +160,8 @@
   if (m_buffer == 0)
     finalize ();
 
-  BOOST_ASSERT (sizeof (std::size_t) <= m_hashLength);
+  if (sizeof (std::size_t) > m_hashLength)
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
   
   // just getting first sizeof(std::size_t) bytes
   // not ideal, but should work pretty well
@@ -162,13 +169,10 @@
 }
 
 bool
-Digest::operator == (Digest &digest)
+Digest::operator == (const Digest &digest) const
 {
-  if (m_buffer == 0)
-    finalize ();
-
-  if (digest.m_buffer == 0)
-    digest.finalize ();
+  if (m_buffer == 0 || digest.m_buffer == 0)
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
   
   BOOST_ASSERT (m_hashLength == digest.m_hashLength);
 
@@ -221,6 +225,10 @@
 {
   string str;
   is >> str; // read string first
+
+  if (str.size () == 0)
+    throw DigestCalculationError () << errinfo_at_line (__LINE__);
+  
   // uint8_t padding = (3 - str.size () % 3) % 3;
   // for (uint8_t i = 0; i < padding; i++) str.push_back ('=');
 
diff --git a/model/sync-digest.h b/model/sync-digest.h
index f5c83f6..91b331e 100644
--- a/model/sync-digest.h
+++ b/model/sync-digest.h
@@ -42,6 +42,12 @@
   Digest ();
 
   /**
+   * @brief Check if digest is empty
+   */
+  bool
+  empty () const;
+  
+  /**
    * @brief Reset digest to the initial state
    */
   void
@@ -66,7 +72,12 @@
    * Side effect: Finalize will be called on `this' and `digest'
    */
   bool
-  operator == (Digest &digest);
+  operator == (const Digest &digest) const;
+
+  bool
+  operator != (const Digest &digest) const
+  { return ! (*this == digest); }
+  
 
   /**
    * @brief Add existing digest to digest calculation
diff --git a/model/sync-seq-no.h b/model/sync-seq-no.h
index fb9e6b8..a6005af 100644
--- a/model/sync-seq-no.h
+++ b/model/sync-seq-no.h
@@ -55,7 +55,7 @@
     , m_seq (seq)
   { }
 
-  inline Digest
+  inline const Digest&
   getDigest () const;
 
   /**
@@ -124,11 +124,10 @@
   m_digest << m_session << m_seq;
 }
   
-Digest
+const Digest&
 SeqNo::getDigest () const
 {
-  Digest digest;
-  return digest;
+  return m_digest;
 }