diff --git a/src/security/pib/detail/key-impl.hpp b/src/security/pib/detail/key-impl.hpp
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+++ b/src/security/pib/detail/key-impl.hpp
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+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2013-2017 Regents of the University of California.
+ *
+ * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
+ *
+ * ndn-cxx library is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU Lesser General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later version.
+ *
+ * ndn-cxx library is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.
+ *
+ * You should have received copies of the GNU General Public License and GNU Lesser
+ * General Public License along with ndn-cxx, e.g., in COPYING.md file.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ *
+ * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
+ */
+
+#ifndef NDN_SECURITY_PIB_DETAIL_KEY_IMPL_HPP
+#define NDN_SECURITY_PIB_DETAIL_KEY_IMPL_HPP
+
+#include "../../../data.hpp"
+#include "../certificate-container.hpp"
+#include "../../security-common.hpp"
+
+namespace ndn {
+namespace security {
+namespace pib {
+
+class PibImpl;
+
+namespace detail {
+
+/**
+ * @brief Backend instance of Key
+ *
+ * An Key has only one backend instance, but may have multiple frontend handles.
+ * Each frontend handle is associated with the only one backend KeyImpl.
+ *
+ * @throw PibImpl::Error when underlying implementation has non-semantic error.
+ */
+class KeyImpl : noncopyable
+{
+public:
+  /**
+   * @brief Create a KeyImpl with @p keyName.
+   *
+   * If the key does not exist in the backend, create it in backend.
+   *
+   * @param keyName The name of the key.
+   * @param key The public key to add.
+   * @param keyLen The length of the key.
+   * @param impl The Pib backend implementation.
+   * @throw Pib::Error a key with the same @p keyName already exists.
+   */
+  KeyImpl(const Name& keyName, const uint8_t* key, size_t keyLen, shared_ptr<PibImpl> impl);
+
+  /**
+   * @brief Create a KeyImpl with @p keyName.
+   *
+   * @param keyName The name of the key.
+   * @param impl The Pib backend implementation.
+   * @throw Pib::Error the key does not exist.
+   */
+  KeyImpl(const Name& keyName, shared_ptr<PibImpl> impl);
+
+  /// @brief Get the name of the key.
+  const Name&
+  getName() const
+  {
+    return m_keyName;
+  }
+
+  /**
+   * @brief Get the name of the belonging identity.
+   */
+  const Name&
+  getIdentity() const
+  {
+    return m_identity;
+  }
+
+  /**
+   * @brief Get key type
+   */
+  KeyType
+  getKeyType() const
+  {
+    return m_keyType;
+  }
+
+  /**
+   * @brief Get public key bits
+   */
+  const Buffer&
+  getPublicKey() const
+  {
+    return m_key;
+  }
+
+  /**
+   * @brief Add @p certificate.
+   *
+   * If no default certificate is set before, the new certificate will be set as the default
+   * certificate of the key.
+   *
+   * @throw std::invalid_argument certificate name does not match key name
+   * @throw Pib::Error a certificate with the same name already exists
+   */
+  void
+  addCertificate(const v2::Certificate& certificate);
+
+  /**
+   * @brief Remove a certificate with @p certName
+   * @throw std::invalid_argument @p certName does not match key name
+   */
+  void
+  removeCertificate(const Name& certName);
+
+  /**
+   * @brief Get a certificate with @p certName
+   * @throw std::invalid_argument @p certName does not match key name
+   * @throw Pib::Error the certificate does not exist.
+   */
+  v2::Certificate
+  getCertificate(const Name& certName) const;
+
+  /// @brief Get all the certificates for this key.
+  const CertificateContainer&
+  getCertificates() const;
+
+  /**
+   * @brief Set an existing one with @p certName as the default certificate
+   * @throw std::invalid_argument @p certName does not match key name
+   * @throw Pib::Error the certificate does not exist.
+   * @return the default certificate
+   */
+  const v2::Certificate&
+  setDefaultCertificate(const Name& certName);
+
+  /**
+   * @brief Add @p certificate and set it as the default certificate of the key
+   * @throw std::invalid_argument @p certificate does not match key name
+   * @throw Pib::Error the certificate with the same name already exists.
+   * @return the default certificate
+   */
+  const v2::Certificate&
+  setDefaultCertificate(const v2::Certificate& certificate);
+
+  /**
+   * @brief Get the default certificate for this Key.
+   * @throw Pib::Error the default certificate does not exist.
+   */
+  const v2::Certificate&
+  getDefaultCertificate() const;
+
+private:
+  Name m_identity;
+  Name m_keyName;
+  Buffer m_key;
+  KeyType m_keyType;
+
+  mutable bool m_isDefaultCertificateLoaded;
+  mutable v2::Certificate m_defaultCertificate;
+
+  CertificateContainer m_certificates;
+
+  shared_ptr<PibImpl> m_impl;
+};
+
+} // namespace detail
+} // namespace pib
+} // namespace security
+} // namespace ndn
+
+#endif // NDN_SECURITY_PIB_DETAIL_KEY_IMPL_HPP