diff --git a/include/ndn-cpp/security/identity/private-key-storage.hpp b/include/ndn-cpp/security/identity/private-key-storage.hpp
index 29461bb..9777d64 100644
--- a/include/ndn-cpp/security/identity/private-key-storage.hpp
+++ b/include/ndn-cpp/security/identity/private-key-storage.hpp
@@ -20,6 +20,8 @@
 
 class PrivateKeyStorage {
 public:
+  struct Error : public std::runtime_error { Error(const std::string &what) : std::runtime_error(what) {} };
+
   /**
    * The virtual destructor.
    */    
@@ -51,12 +53,12 @@
    * @param digestAlgorithm the digest algorithm.
    * @return The signature, or a null pointer if signing fails.
    */  
-  virtual ConstBufferPtr
+  virtual Block
   sign(const uint8_t *data, size_t dataLength,
        const Signature &signature,
        const Name& keyName, DigestAlgorithm digestAlgorithm = DIGEST_ALGORITHM_SHA256) = 0;
 
-  virtual ConstBufferPtr
+  virtual Block
   sign(const Data &data,
        const Signature &signature,
        const Name& keyName, DigestAlgorithm digestAlgorithm = DIGEST_ALGORITHM_SHA256) = 0;