security: Added headers for der encoding.  Added CertificateExtension and CertificateSubjectDescription.
diff --git a/include/ndn-cpp/security/certificate/certificate-subject-description.hpp b/include/ndn-cpp/security/certificate/certificate-subject-description.hpp
new file mode 100644
index 0000000..90ff7fe
--- /dev/null
+++ b/include/ndn-cpp/security/certificate/certificate-subject-description.hpp
@@ -0,0 +1,70 @@
+/* -*- 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.
+ */
+
+#ifndef NDN_CERTIFICATE_SUBJECT_DESCRIPTION_HPP
+#define NDN_CERTIFICATE_SUBJECT_DESCRIPTION_HPP
+
+#include "../../common.hpp"
+#include "../../encoding/oid.hpp"
+
+namespace ndn {
+
+namespace der { class DerNode; }
+  
+/**
+ * A CertificateSubjectDescription represents the SubjectDescription entry in a Certificate.
+ */
+class CertificateSubjectDescription {
+public:
+  /**
+   * Create a new CertificateSubjectDescription.
+   * @param oid The oid of the subject description entry.
+   * @param value The value of the subject description entry.
+   */
+  CertificateSubjectDescription(std::string oid, std::string value)
+  : oid_(oid), value_(value)
+  {
+  }
+
+  /**
+   * Create a new CertificateSubjectDescription.
+   * @param oid The oid of the subject description entry.
+   * @param value The value of the subject description entry.
+   */
+  CertificateSubjectDescription(OID oid, std::string value)
+  : oid_(oid), value_(value)
+  {
+  }
+  
+  /**
+   * Encode the object into a DER syntax tree.
+   * @return The encoded DER syntax tree.
+   */
+  ptr_lib::shared_ptr<der::DerNode> 
+  toDer();
+
+  std::string
+  getOidString()
+  {
+    return oid_.toString();
+  }
+
+  const std::string &
+  getValue() const
+  {
+    return value_;
+  }
+  
+private:
+  OID oid_;
+  std::string value_;
+};
+
+}
+
+#endif