diff --git a/include/ndn-cpp/security/certificate/certificate.hpp b/include/ndn-cpp/security/certificate/certificate.hpp
index 29626b3..0fdd213 100644
--- a/include/ndn-cpp/security/certificate/certificate.hpp
+++ b/include/ndn-cpp/security/certificate/certificate.hpp
@@ -14,7 +14,17 @@
 namespace ndn {
 
 class Certificate : public Data {
-  
+public:
+  /**
+   * The default constructor.
+   */
+  Certificate();
+
+  /**
+   * Create a Certificate from the content in the data packet.
+   * @param data The data packet with the content to decode.
+   */
+  Certificate(const Data& data);
 };
 
 }
diff --git a/include/ndn-cpp/security/certificate/identity-certificate.hpp b/include/ndn-cpp/security/certificate/identity-certificate.hpp
index 4d68231..41035a7 100644
--- a/include/ndn-cpp/security/certificate/identity-certificate.hpp
+++ b/include/ndn-cpp/security/certificate/identity-certificate.hpp
@@ -1,6 +1,7 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil -*- */
 /**
  * Copyright (C) 2013 Regents of the University of California.
+ * @author: Yingdi Yu <yingdi@cs.ucla.edu>
  * @author: Jeff Thompson <jefft0@remap.ucla.edu>
  * See COPYING for copyright and distribution information.
  */
@@ -14,7 +15,17 @@
 
 class IdentityCertificate : public Certificate
 {
+public:
+  /**
+   * The default constructor.
+   */
+  IdentityCertificate();
 
+  /**
+   * Create an IdentityCertificate from the content in the data packet.
+   * @param data The data packet with the content to decode.
+   */
+  IdentityCertificate(const Data& data);
 };
 
 }
diff --git a/include/ndn-cpp/security/identity/basic-identity-storage.hpp b/include/ndn-cpp/security/identity/basic-identity-storage.hpp
index 2e0db30..ee1a886 100644
--- a/include/ndn-cpp/security/identity/basic-identity-storage.hpp
+++ b/include/ndn-cpp/security/identity/basic-identity-storage.hpp
@@ -123,14 +123,14 @@
    * @param certificate The certificate to be added.
    */
   void
-  addAnyCertificate (const Certificate& certificate);
+  addAnyCertificate (const IdentityCertificate& certificate);
 
   /**
    * Add a certificate to the identity storage.
-   * @param certificate The certificate to be added.
+   * @param certificate The certificate to be added.  This makes a copy of the certificate.
    */
   virtual void 
-  addCertificate(const Certificate& certificate);
+  addCertificate(const IdentityCertificate& certificate);
 
   /**
    * Get a certificate from the identity storage.
diff --git a/include/ndn-cpp/security/identity/identity-manager.hpp b/include/ndn-cpp/security/identity/identity-manager.hpp
index 2dd9b6e..d461062 100644
--- a/include/ndn-cpp/security/identity/identity-manager.hpp
+++ b/include/ndn-cpp/security/identity/identity-manager.hpp
@@ -7,13 +7,16 @@
  */
 
 #ifndef NDN_IDENTITY_MANAGER_HPP
-#define	NDN_IDENTITY_MANAGER_HPP
+#define NDN_IDENTITY_MANAGER_HPP
 
-#include "../certificate/certificate.hpp"
+#include "../certificate/identity-certificate.hpp"
 #include "identity-storage.hpp"
 #include "../certificate/public-key.hpp"
 #include "private-key-storage.hpp"
 
+// TODO: Implement Time values.
+class Time;
+
 namespace ndn {
 
 /**
@@ -98,11 +101,35 @@
   }
 
   /**
+   * Create an identity certificate for a public key managed by this IdentityManager.
+   * @param keyName The name of public key to be signed.
+   * @param signerCertificateName The name of signing certificate.
+   * @param notBefore The notBefore value in the validity field of the generated certificate.
+   * @param notAfter The notAfter vallue in validity field of the generated certificate.
+   * @return The name of generated identity certificate.
+   */
+  Name
+  createIdentityCertificate(const Name& keyName, const Name& signerCertificateName, const Time& notBefore, const Time& notAfter);
+
+  /**
+   * Create an identity certificate for a public key supplied by the caller.
+   * @param keyName The name of public key to be signed.
+   * @param publickey The public key to be signed.
+   * @param signerCertificateName The name of signing certificate.
+   * @param notBefore The notBefore value in the validity field of the generated certificate.
+   * @param notAfter The notAfter vallue in validity field of the generated certificate.
+   * @return The generated identity certificate.
+   */
+  ptr_lib::shared_ptr<IdentityCertificate>
+  createIdentityCertificate
+    (const Name& keyName, const PublicKey& publickey, const Name& signerCertificateName, const Time& notBefore, const Time& notAfter); 
+    
+  /**
    * Add a certificate into the public key identity storage.
-   * @param certificate The certificate to to added.
+   * @param certificate The certificate to to added.  This makes a copy of the certificate.
    */
   void
-  addCertificate(const Certificate& certificate)
+  addCertificate(const IdentityCertificate& certificate)
   {
     identityStorage_->addCertificate(certificate);
   }
@@ -116,27 +143,27 @@
 
   /**
    * Add a certificate into the public key identity storage and set the certificate as the default for its corresponding identity.
-   * @param certificate The certificate to be added.
+   * @param certificate The certificate to be added.  This makes a copy of the certificate.
    */
   void
-  addCertificateAsIdentityDefault(const Certificate& certificate);
+  addCertificateAsIdentityDefault(const IdentityCertificate& certificate);
 
   /**
    * Add a certificate into the public key identity storage and set the certificate as the default of its corresponding key.
-   * certificate the certificate to be added
+   * @param certificate The certificate to be added.  This makes a copy of the certificate.
    */
   void
-  addCertificateAsDefault(const Certificate& certificate);
+  addCertificateAsDefault(const IdentityCertificate& certificate);
 
   /**
    * Get a certificate with the specified name.
    * @param certificateName The name of the requested certificate.
    * @return the requested certificate which is valid.
    */
-  ptr_lib::shared_ptr<Certificate>
+  ptr_lib::shared_ptr<IdentityCertificate>
   getCertificate(const Name& certificateName)
   {
-    return identityStorage_->getCertificate(certificateName, false);
+    return ptr_lib::make_shared<IdentityCertificate>(*identityStorage_->getCertificate(certificateName, false));
   }
     
   /**
@@ -144,10 +171,10 @@
    * @param certificateName The name of the requested certificate.
    * @return the requested certificate.
    */
-  ptr_lib::shared_ptr<Certificate>
+  ptr_lib::shared_ptr<IdentityCertificate>
   getAnyCertificate(const Name& certificateName)
   {
-    return identityStorage_->getCertificate(certificateName, true);
+    return ptr_lib::make_shared<IdentityCertificate>(*identityStorage_->getCertificate(certificateName, true));
   }
     
   /**
@@ -172,16 +199,15 @@
     return identityStorage_->getDefaultCertificateNameForIdentity(getDefaultIdentity());
   }
         
-#if 0
   /**
-   * sign blob based on certificate name
-   * @param blob the blob to be signed
-   * @param certificateName the signing certificate name
-   * @return the generated signature
+   * Sign the byte array data based on the certificate name.
+   * @param data The data to be signed.
+   * @param dataLength the length of data.
+   * @param certificateName The signing certificate name.
+   * @return The generated signature.
    */
-  Ptr<Signature>
-  signByCertificate(const Blob& blob, const Name& certificateName);
-#endif
+  ptr_lib::shared_ptr<Signature>
+  signByCertificate(const uint8_t* data, size_t dataLength, const Name& certificateName);
     
   /**
    * Sign data packet based on the certificate name.
@@ -211,7 +237,7 @@
    * @param keyName The name of the public key.
    * @return The generated certificate.
    */
-  ptr_lib::shared_ptr<Certificate>
+  ptr_lib::shared_ptr<IdentityCertificate>
   selfSign(const Name& keyName);
   
   ptr_lib::shared_ptr<IdentityStorage> identityStorage_;
diff --git a/include/ndn-cpp/security/identity/identity-storage.hpp b/include/ndn-cpp/security/identity/identity-storage.hpp
index db6ab0e..d514dc8 100644
--- a/include/ndn-cpp/security/identity/identity-storage.hpp
+++ b/include/ndn-cpp/security/identity/identity-storage.hpp
@@ -15,6 +15,7 @@
 namespace ndn {
 
 class Certificate;
+class IdentityCertificate;
 class Data;
 
 /**
@@ -117,10 +118,10 @@
 
   /**
    * Add a certificate to the identity storage.
-   * @param certificate The certificate to be added.
+   * @param certificate The certificate to be added.  This makes a copy of the certificate.
    */
   virtual void 
-  addCertificate(const Certificate& certificate) = 0;
+  addCertificate(const IdentityCertificate& certificate) = 0;
 
   /**
    * Get a certificate from the identity storage.
diff --git a/include/ndn-cpp/security/identity/memory-identity-storage.hpp b/include/ndn-cpp/security/identity/memory-identity-storage.hpp
index ba0ea5a..cf8f318 100644
--- a/include/ndn-cpp/security/identity/memory-identity-storage.hpp
+++ b/include/ndn-cpp/security/identity/memory-identity-storage.hpp
@@ -113,10 +113,10 @@
 
   /**
    * Add a certificate to the identity storage.
-   * @param certificate The certificate to be added.
+   * @param certificate The certificate to be added.  This makes a copy of the certificate.
    */
   virtual void 
-  addCertificate(const Certificate& certificate);
+  addCertificate(const IdentityCertificate& certificate);
 
   /**
    * Get a certificate from the identity storage.
diff --git a/include/ndn-cpp/security/key-chain.hpp b/include/ndn-cpp/security/key-chain.hpp
index 97db9d8..40e24fd 100644
--- a/include/ndn-cpp/security/key-chain.hpp
+++ b/include/ndn-cpp/security/key-chain.hpp
@@ -116,7 +116,7 @@
    * @param certificate The certificate to to added.
    */
   void
-  installIdentityCertificate(const Certificate& certificate)
+  installIdentityCertificate(const IdentityCertificate& certificate)
   {
     identityManager_->addCertificate(certificate);
   }
@@ -134,7 +134,7 @@
   /**
    * Get a certificate with the specified name.
    * @param certificateName The name of the requested certificate.
-   * @return the requested certificate.
+   * @return the requested certificate which is valid.
    */
   ptr_lib::shared_ptr<Certificate>
   getCertificate(const Name& certificateName)
@@ -154,6 +154,28 @@
   }
 
   /**
+   * Get an identity certificate with the specified name.
+   * @param certificateName The name of the requested certificate.
+   * @return the requested certificate which is valid.
+   */
+  ptr_lib::shared_ptr<IdentityCertificate>
+  getIdentityCertificate(const Name& certificateName)
+  {
+    return identityManager_->getCertificate(certificateName);
+  }
+
+  /**
+   * Get an identity certificate even if the certificate is not valid anymore.
+   * @param certificateName The name of the requested certificate.
+   * @return the requested certificate.
+   */
+  ptr_lib::shared_ptr<IdentityCertificate>
+  getAnyIdentityCertificate(const Name& certificateName)
+  {
+    return identityManager_->getAnyCertificate(certificateName);
+  }
+
+  /**
    * Revoke a key
    * @param keyName the name of the key that will be revoked
    */