Implement stub for PrivateKeyStorage::sign.
diff --git a/ndn-cpp/security/identity/identity-manager.cpp b/ndn-cpp/security/identity/identity-manager.cpp
index 8d36d30..30da18f 100644
--- a/ndn-cpp/security/identity/identity-manager.cpp
+++ b/ndn-cpp/security/identity/identity-manager.cpp
@@ -1,4 +1,5 @@
 /**
+ * @author: Yingdi Yu <yingdi@cs.ucla.edu>
  * @author: Jeff Thompson
  * See COPYING for copyright and distribution information.
  */