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_; }
diff --git a/src/security/certificate/certificate-extension.cpp b/src/security/certificate/certificate-extension.cpp
index f081f2a..d9297a9 100644
--- a/src/security/certificate/certificate-extension.cpp
+++ b/src/security/certificate/certificate-extension.cpp
@@ -15,7 +15,7 @@
 namespace ndn {
 
 ptr_lib::shared_ptr<der::DerNode> 
-CertificateExtension::toDer()
+CertificateExtension::toDer() const
 {
   ptr_lib::shared_ptr<der::DerSequence> root(new der::DerSequence);
     
@@ -33,7 +33,7 @@
 }
 
 Blob
-CertificateExtension::toDerBlob()
+CertificateExtension::toDerBlob() const
 {
   blob_stream blobStream;
   der::OutputIterator& start = reinterpret_cast<der::OutputIterator&>(blobStream);