Add a DER encoder and decoder. (Making use of an existing DER library, like Bouncy Castle's, would require bringing in a really big dependency. Also, many existing libraries are not very efficient.)

Links for reference:
http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
http://luca.ntop.org/Teaching/Appunti/asn1.html