diff --git a/include/ndn-cpp/encoding/oid.hpp b/include/ndn-cpp/encoding/oid.hpp
index bc0dd25..f217889 100644
--- a/include/ndn-cpp/encoding/oid.hpp
+++ b/include/ndn-cpp/encoding/oid.hpp
@@ -39,20 +39,20 @@
   }
 
   std::string 
-  toString();
+  toString() const;
 
-  bool operator == (const OID& oid)
+  bool operator == (const OID& oid) const
   {
     return equal(oid);
   }
 
-  bool operator != (const OID& oid)
+  bool operator != (const OID& oid) const
   {
     return !equal(oid);
   }
   
 private:
-  bool equal(const OID& oid);
+  bool equal(const OID& oid) const;
 
   std::vector<int> oid_;
 };
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/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/encoding/oid.cpp b/src/encoding/oid.cpp
index db1fbc3..633d1cf 100644
--- a/src/encoding/oid.cpp
+++ b/src/encoding/oid.cpp
@@ -34,11 +34,11 @@
   }
 }
 
-string OID::toString()
+string OID::toString() const
 {
   ostringstream convert;
   
-  vector<int>::iterator it = oid_.begin();
+  vector<int>::const_iterator it = oid_.begin();
   for(; it < oid_.end(); it++){
     if(it != oid_.begin())
       convert << ".";
@@ -48,7 +48,7 @@
   return convert.str();
 }
 
-bool OID::equal(const OID& oid)
+bool OID::equal(const OID& oid) const
 {
   vector<int>::const_iterator i = oid_.begin();
   vector<int>::const_iterator j = oid.oid_.begin();
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);
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());