I also had a lot of fun with ASN.1 a few years ago when I was writing an SMS message codec for an anti-spam/fraud system. Its actually a really flexible system

I also had a lot of fun with ASN.1 a few years ago when I was writing an SMS message codec for an anti-spam/fraud system. Its actually a really flexible system and though it has its issues, it also has some really good design decisions. I always found it to be quite interesting, at least. I do have some less than fond memories staring and BER encoded SMS messages in Wireshark, however...