diff --git a/include/ndn-cpp/security/certificate/certificate-extension.hpp b/include/ndn-cpp/security/certificate/certificate-extension.hpp
index c67434d..b47b880 100644
--- a/include/ndn-cpp/security/certificate/certificate-extension.hpp
+++ b/include/ndn-cpp/security/certificate/certificate-extension.hpp
@@ -56,10 +56,10 @@
    * @return the encoded DER syntax tree
    */
   ptr_lib::shared_ptr<der::DerNode> 
-  toDer();
+  toDer() const;
 
   Blob
-  toDerBlob();
+  toDerBlob() const;
 
   inline const OID& 
   getOid() const { return extensionId_; }