The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

by Simon Singh

cryptanalysts began to invent their own shortcuts for finding the Enigma keys. For example, they cottoned on to the fact that the German Enigma operators would occasionally choose obvious message keys. For each message, the operator was supposed to select a different message key, three letters chosen at random. However, in the heat of battle, rather than straining their imaginations to pick a random key, the overworked operators would sometimes pick three consecutive letters from the Enigma keyboard (Figure 46), such as QWE or BNM. These predictable message keys became known as cillies. Another type of cilly was the repeated use of the same message key, perhaps the initials of the operator’s girlfriend—indeed, one such set of initials, C.I.L., may have been the origin of the term. Before cracking Enigma the hard way, it became routine for the cryptanalysts to try out the cillies, and their hunches would sometimes pay off. Cillies were not weaknesses of the Enigma machine, rather they were weaknesses in the way the machine was being used.