This book might suit a keen teenager. After the usual gentle introduction with Caesar shifts, there is a look at classic ciphers such as Vigen re and Playfair, with side trips into double transposition and book codes (and later on, code books). The emphasis in every case is on the methods of cryptanalysis, because none of these systems remains in the least bit secure. The attacks do not just use letter frequencies (though William Friedman's Index of Coincidence is mysteriously absent), advanced techniques such as "crib dragging" are also presented. Each worked example is followed by only one problem, for which, unwisely, the solution is at the back of the book - which all rather spoils the teenager's challenge.
The Enigma is gamely tackled, with explanations of how the Poles broke the double indicator system. Those interested in this coding machine will already have devoured one of the many existing books on Bletchley Park that has all this material, and a great deal more, in its appendices. Here, it is in the main text, but the neat stories about "pinches" and "gardening" are absent, along with the techniques of "cillis", "rodding", "banburismus" and "Herivel's tip". It is boring to be shown the Enigma without being told how breaking its traffic undoubtedly shortened the war.
The book finally scores by tackling the cryptanalysis of some Hagelin machines and the SZ42 (whose traffic is so much better known as Fish or Tunny that you are surprised the author never mentions it). But it is too little too late and the book slides downhill into modern times.
Diffie-Hellman key exchange is baldly presented, but the author cannot hide the complexity from our teenager by sticking modular arithmetic into an appendix, because the maths is now what it is all about.
There is a reasonable explanation of the RSA crypto-system and the difficulty of factoring primes, but then we get to digital encryption standard and what a howler in suggesting that this is secure. "If a computer could test one key in a microsecond, it could try them all in 2,300 years. This is clearly impractical." But, the Electronic Frontier Foundation built just such a machine in 1998 that cracks any DES key in ten days and everyone thinks that the US National Security Agency has been doing it much faster for a decade or more.
A disappointing omission is the story of James Ellis and Clifford Cocks' invention of "secure non-secret digital encryption" in the classified world of the Government Communications Headquarters, several years before Whitfield Diffie and Martin Hellman published their own scheme. When in 1974 GCHQ needed a second opinion, it was Robert Churchhouse whom they approached, and he has assisted them with more besides. So perhaps we should read this book completely differently. Are Britain's spooks still vainly trying to manipulate just how much cryptanalysis the world should know while broadcasting the message that single DES is still to be trusted?
As for the teenager, who is apparently to be brainwashed, my advice would be to read The Codebreakers by David Kahn instead. It is so much more fun.
Richard Clayton is a researcher in the security group, University of Cambridge.
Codes and Ciphers: Julius Caesar, the Enigma, and the Internet
Author - Robert Churchhouse
Publisher - Cambridge University Press
Pages - 240
Price - £42.50 and £15.95
ISBN - 0 521 81054 X and 00890 5