For 35 years, the cryptographic community has created the impression that anonymous communication is always possible. Chaum's dining cryptographer's solution is regarded as achieving unconditional security. Chaum's MIX approach, namely, applying a uniformly random permutation on the plaintexts to be sent, is often given as a definition for anonymity. However, people working in law enforcement know that attempts by whistle-blowers and criminals to remain at large, often fail. The content of the message may reveal the name of the sender or may leak for whom the message is really intended. To understand this issue, we give a definition of perfect anonymity. We contrast our definition with Chaum's approach and discuss the impact on Chaum's dining cryptographer's solution.

The 2004 Turing Lecture was held on 21 January 2004. It was given by Fred Piper, who is the joint author of Cipher Systems (1982), one of the first books to be published on the subject of protection of communications, Secure Speech Communications (1985), Cryptography: A Very Short Introduction (2002), and an ISACA research monograph on digital signatures (1999). He has been a member of a number of DTI advisory committees and is a member of the Board of Trustees for Bletchley Park. The annual Turing Lecture, organized by the British Computer Society and the Institute of Electrical Engineering, commemorates the central seminal figure in the computer revolution, Alan Turing, whose outstanding originality and vision made it possible, in work originating in the mid 1930s. Although it is now hard to see what the limits of the computer revolution might eventually be, it was Turing himself who pointed out to us the very existence of such theoretical limitations.