Programming Satan's Computer

Cryptographic protocols are used in distributed systems to identify users and authenticate transactions. They may involve the exchange of about 2–5 messages, and one might think that a program of this size would be fairly easy to get right. However, this is absolutely not the case: bugs are routinely found in well known protocols, and years after they were first published. The problem is the presence of a hostile opponent, who can alter messages at will. In effect, our task is to program a computer which gives answers which are subtly and maliciously wrong at the most inconvenient possible moment. This is a fascinating problem; and we hope that the lessons learned from programming Satan’s computer may be helpful in tackling the more common problem of programming Murphy’s.