diff --git a/include/ndn-cpp/security/certificate/certificate-subject-description.hpp b/include/ndn-cpp/security/certificate/certificate-subject-description.hpp
index 90ff7fe..30af6bf 100644
--- a/include/ndn-cpp/security/certificate/certificate-subject-description.hpp
+++ b/include/ndn-cpp/security/certificate/certificate-subject-description.hpp
@@ -46,10 +46,10 @@
    * @return The encoded DER syntax tree.
    */
   ptr_lib::shared_ptr<der::DerNode> 
-  toDer();
+  toDer() const;
 
   std::string
-  getOidString()
+  getOidString() const
   {
     return oid_.toString();
   }
diff --git a/src/security/certificate/certificate-subject-description.cpp b/src/security/certificate/certificate-subject-description.cpp
index 9aa511f..3571f7f 100644
--- a/src/security/certificate/certificate-subject-description.cpp
+++ b/src/security/certificate/certificate-subject-description.cpp
@@ -14,7 +14,7 @@
 namespace ndn {
 
 ptr_lib::shared_ptr<der::DerNode> 
-CertificateSubjectDescription::toDer()
+CertificateSubjectDescription::toDer() const
 {
   ptr_lib::shared_ptr<der::DerSequence> root(new der::DerSequence());