diff --git a/src/security/key.hpp b/src/security/key.hpp
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+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2013-2015 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_KEY_HPP
+#define NDN_SECURITY_KEY_HPP
+
+#include "identity-certificate.hpp"
+#include "certificate-container.hpp"
+
+namespace ndn {
+namespace security {
+
+class PibImpl;
+class Identity;
+class KeyContainer;
+
+/**
+ * @brief represents a key
+ *
+ * Key is at the second level in PIB's Identity-Key-Certificate hierarchy.
+ * An Key has a Name (identity + keyId), and contains one or more certificates,
+ * one of which is set as the default certificate of this key.  A certificate
+ * can be directly accessed from a Key object.
+ *
+ * @throw PibImpl::Error when underlying implementation has non-semantic error.
+ */
+class Key
+{
+public:
+  friend class Identity;
+  friend class KeyContainer;
+
+public:
+  /**
+   * @brief Default Constructor
+   *
+   * Key created using this default constructor is just a place holder.
+   * It must obtain an actual instance from Identity::getKey(...).  A typical
+   * usage would be for exception handling:
+   *
+   *   Key key;
+   *   try {
+   *     key = Identity.getKey(...);
+   *   }
+   *   catch (Pib::Error&) {
+   *     ...
+   *   }
+   *
+   * A Key instance created using the constructor is invalid. Calling a
+   * member method on an invalid Key instance may cause an std::domain_error.
+   */
+  Key();
+
+  /// @brief Get the name of the key.
+  const Name&
+  getName() const;
+
+  /// @brief Get the name of the belonging identity.
+  const Name&
+  getIdentity() const;
+
+  /// @brief Get the key id of the key.
+  const name::Component&
+  getKeyId() const;
+
+  /// @brief Get public key
+  const PublicKey&
+  getPublicKey() const;
+
+  /**
+   * @brief Add a certificate.
+   *
+   * @param certificate The certificate to add.
+   */
+  void
+  addCertificate(const IdentityCertificate& certificate);
+
+  /**
+   * @brief Remove a certificate.
+   *
+   * @param certName The name of the certificate to delete.
+   */
+  void
+  removeCertificate(const Name& certName);
+
+  /**
+   * @brief Get a certificate.
+   *
+   * @return the certificate
+   * @throws Pib::Error if the certificate does not exist.
+   */
+  IdentityCertificate
+  getCertificate(const Name& certName);
+
+  /// @brief Get all the certificates for this key.
+  CertificateContainer
+  getCertificates();
+
+  /**
+   * @brief Set the default certificate.
+   *
+   * @param certName The name of the default certificate of the key.
+   * @return the default certificate
+   * @throws Pib::Error if the certificate does not exist.
+   */
+  const IdentityCertificate&
+  setDefaultCertificate(const Name& certName);
+
+  /**
+   * @brief Set the default certificate.
+   *
+   * If the certificate does not exist, add it and set it as the default certificate of the key.
+   * If the certificate exists, simply set it as the default certificate of the key.
+   *
+   * @param certificate The certificate to add.
+   * @return the default certificate
+   */
+  const IdentityCertificate&
+  setDefaultCertificate(const IdentityCertificate& certificate);
+
+  /**
+   * @brief Get the default certificate for this Key.
+   *
+   * @throws Pib::Error if the default certificate does not exist.
+   */
+  const IdentityCertificate&
+  getDefaultCertificate();
+
+  /// @brief Check if the Key instance is valid
+  operator bool() const;
+
+  /// @brief Check if the Key instance is invalid
+  bool
+  operator!() const;
+
+NDN_CXX_PUBLIC_WITH_TESTS_ELSE_PRIVATE:
+  /**
+   * @brief Create a Key with @p identityName and @p keyId.
+   *
+   * If the key/identity does not exist in the backend, create it in backend.
+   *
+   * @param identityName The name of the Identity.
+   * @param keyId The key id of the key.
+   * @param publicKey The public key to add.
+   * @param impl The actual backend implementation.
+   */
+  Key(const Name& identityName, const name::Component& keyId,
+      const PublicKey& publicKey, shared_ptr<PibImpl> impl);
+
+  /**
+   * @brief Create an KeyEntry with @p identityName and @p keyId.
+   *
+   * @param identityName The name of the Identity.
+   * @param keyId The key id of the key.
+   * @param impl The actual backend implementation.
+   * @throws Pib::Error if the key does not exist.
+   */
+  Key(const Name& identityName, const name::Component& keyId, shared_ptr<PibImpl> impl);
+
+  /**
+   * @brief Check the validity of this instance
+   *
+   * @throws std::domain_error if the instance is invalid
+   */
+  void
+  validityCheck() const;
+
+private:
+  Name m_id;
+  name::Component m_keyId;
+  Name m_keyName;
+  PublicKey m_key;
+
+  bool m_hasDefaultCertificate;
+  IdentityCertificate m_defaultCertificate;
+
+  bool m_needRefreshCerts;
+  CertificateContainer m_certificates;
+
+  shared_ptr<PibImpl> m_impl;
+};
+
+} // namespace security
+} // namespace ndn
+
+#endif // NDN_SECURITY_PIB_HPP