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;
 };