diff --git a/src/security/tpm/back-end-file.hpp b/src/security/tpm/back-end-file.hpp
index db58907..5609cba 100644
--- a/src/security/tpm/back-end-file.hpp
+++ b/src/security/tpm/back-end-file.hpp
@@ -134,7 +134,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace tpm
diff --git a/src/security/tpm/back-end-mem.hpp b/src/security/tpm/back-end-mem.hpp
index 4bfa212..469b466 100644
--- a/src/security/tpm/back-end-mem.hpp
+++ b/src/security/tpm/back-end-mem.hpp
@@ -114,7 +114,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace tpm
diff --git a/src/security/tpm/back-end-osx.hpp b/src/security/tpm/back-end-osx.hpp
index 29465c8..8310984 100644
--- a/src/security/tpm/back-end-osx.hpp
+++ b/src/security/tpm/back-end-osx.hpp
@@ -143,7 +143,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace tpm
diff --git a/src/security/tpm/tpm.cpp b/src/security/tpm/tpm.cpp
index fad73b9..0ed1062 100644
--- a/src/security/tpm/tpm.cpp
+++ b/src/security/tpm/tpm.cpp
@@ -133,13 +133,14 @@
 }
 
 ConstBufferPtr
-Tpm::exportPrivateKey(const Name& keyName, const char* pw, size_t pwLen)
+Tpm::exportPrivateKey(const Name& keyName, const char* pw, size_t pwLen) const
 {
   return m_backEnd->exportKey(keyName, pw, pwLen);
 }
 
 bool
-Tpm::importPrivateKey(const Name& keyName, const uint8_t* pkcs8, size_t pkcs8Len, const char* pw, size_t pwLen)
+Tpm::importPrivateKey(const Name& keyName, const uint8_t* pkcs8, size_t pkcs8Len,
+                      const char* pw, size_t pwLen)
 {
   try {
     m_backEnd->importKey(keyName, pkcs8, pkcs8Len, pw, pwLen);
diff --git a/src/security/tpm/tpm.hpp b/src/security/tpm/tpm.hpp
index 82ea656..dde9d14 100644
--- a/src/security/tpm/tpm.hpp
+++ b/src/security/tpm/tpm.hpp
@@ -22,11 +22,11 @@
 #ifndef NDN_SECURITY_TPM_TPM_HPP
 #define NDN_SECURITY_TPM_TPM_HPP
 
-#include "../../common.hpp"
 #include "../security-common.hpp"
 #include "../../name.hpp"
 #include "../key-params.hpp"
 #include "key-handle.hpp"
+
 #include <unordered_map>
 
 namespace ndn {
@@ -154,12 +154,6 @@
    */
   Tpm(const std::string& scheme, const std::string& location, unique_ptr<BackEnd> impl);
 
-  BackEnd*
-  getBackEnd()
-  {
-    return m_backEnd.get();
-  }
-
   /**
    * @brief Create key for @p identityName according to @p params.
    *
@@ -189,7 +183,7 @@
    * @throw BackEnd::Error the key does not exist or it cannot be exported.
    */
   ConstBufferPtr
-  exportPrivateKey(const Name& keyName, const char* pw, size_t pwLen);
+  exportPrivateKey(const Name& keyName, const char* pw, size_t pwLen) const;
 
   /**
    * @brief Import a private key.
@@ -231,7 +225,7 @@
 
   mutable std::unordered_map<Name, unique_ptr<KeyHandle>> m_keys;
 
-  unique_ptr<BackEnd> m_backEnd;
+  const unique_ptr<BackEnd> m_backEnd;
 
   friend class v2::KeyChain;
 };
diff --git a/src/security/transform/base64-decode.hpp b/src/security/transform/base64-decode.hpp
index 0ef0a46..ea69b8d 100644
--- a/src/security/transform/base64-decode.hpp
+++ b/src/security/transform/base64-decode.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -76,7 +76,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/base64-encode.hpp b/src/security/transform/base64-encode.hpp
index 7e9d99d..d62b5d8 100644
--- a/src/security/transform/base64-encode.hpp
+++ b/src/security/transform/base64-encode.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -78,7 +78,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/block-cipher.hpp b/src/security/transform/block-cipher.hpp
index 885bd43..320e184 100644
--- a/src/security/transform/block-cipher.hpp
+++ b/src/security/transform/block-cipher.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -86,7 +86,6 @@
   isConverterEmpty() const;
 
 private:
-
   void
   initializeAesCbc(const uint8_t* key, size_t keyLen,
                    const uint8_t* iv, size_t ivLen,
@@ -94,7 +93,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/digest-filter.hpp b/src/security/transform/digest-filter.hpp
index 952c4a7..0ccc81b 100644
--- a/src/security/transform/digest-filter.hpp
+++ b/src/security/transform/digest-filter.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -58,7 +58,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/hmac-filter.hpp b/src/security/transform/hmac-filter.hpp
index 0cec1fd..002fda8 100644
--- a/src/security/transform/hmac-filter.hpp
+++ b/src/security/transform/hmac-filter.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -59,7 +59,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/private-key.hpp b/src/security/transform/private-key.hpp
index eae0787..3eb1d7a 100644
--- a/src/security/transform/private-key.hpp
+++ b/src/security/transform/private-key.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -243,7 +243,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 /**
diff --git a/src/security/transform/public-key.hpp b/src/security/transform/public-key.hpp
index 169b9e0..db08cad 100644
--- a/src/security/transform/public-key.hpp
+++ b/src/security/transform/public-key.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -127,7 +127,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace transform
diff --git a/src/security/transform/signer-filter.hpp b/src/security/transform/signer-filter.hpp
index dc4ffff..a6d06c5 100644
--- a/src/security/transform/signer-filter.hpp
+++ b/src/security/transform/signer-filter.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -58,7 +58,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/security/transform/verifier-filter.hpp b/src/security/transform/verifier-filter.hpp
index 7fed9fc..4bc63e3 100644
--- a/src/security/transform/verifier-filter.hpp
+++ b/src/security/transform/verifier-filter.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -60,7 +60,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>
diff --git a/src/util/network-monitor.hpp b/src/util/network-monitor.hpp
index b48d5e9..a9cf60b 100644
--- a/src/util/network-monitor.hpp
+++ b/src/util/network-monitor.hpp
@@ -108,7 +108,7 @@
   Signal<NetworkMonitor> onNetworkStateChanged;
 
 private:
-  std::unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 } // namespace util
diff --git a/tests/unit-tests/security/tpm/back-end-wrapper-file.hpp b/tests/unit-tests/security/tpm/back-end-wrapper-file.hpp
index 3e403ce..64f5a0a 100644
--- a/tests/unit-tests/security/tpm/back-end-wrapper-file.hpp
+++ b/tests/unit-tests/security/tpm/back-end-wrapper-file.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -38,7 +38,7 @@
 public:
   BackEndWrapperFile()
     : m_tmpPath(boost::filesystem::path(UNIT_TEST_CONFIG_PATH) / "TpmFileTest")
-    , m_impl(new BackEndFile(m_tmpPath.string()))
+    , m_impl(make_unique<BackEndFile>(m_tmpPath.string()))
   {
   }
 
@@ -60,8 +60,8 @@
   }
 
 private:
-  boost::filesystem::path m_tmpPath;
-  unique_ptr<BackEnd> m_impl;
+  const boost::filesystem::path m_tmpPath;
+  const unique_ptr<BackEnd> m_impl;
 };
 
 } // namespace tests
diff --git a/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp b/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
index ea10295..692ba65 100644
--- a/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
+++ b/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -36,8 +36,8 @@
 {
 public:
   BackEndWrapperMem()
+    : m_impl(make_unique<BackEndMem>())
   {
-    m_impl = unique_ptr<BackEnd>(new BackEndMem);
   }
 
   BackEnd&
@@ -53,7 +53,7 @@
   }
 
 private:
-  unique_ptr<BackEnd> m_impl;
+  const unique_ptr<BackEnd> m_impl;
 };
 
 } // namespace tests