diff --git a/model/sync-digest.h b/model/sync-digest.h
index d0c76f5..adf4d96 100644
--- a/model/sync-digest.h
+++ b/model/sync-digest.h
@@ -124,6 +124,8 @@
 
 struct DigestCalculationError : virtual boost::exception { };
 
+typedef boost::shared_ptr<Digest> DigestPtr;
+typedef boost::shared_ptr<const Digest> DigestConstPtr;
 
 Digest &
 Digest::operator << (const std::string &str)