Seductive Cynthia, the Manchester Baby and brain-boxes who saved the world from huts in Bucks

Place names loom very large in Britain. Names such as Fleet Street, Whitehall and Scotland Yard continue to name organisations long after the organisations have moved from or overflowed their original locations. Bletchley Park, the centre of British code-breaking in the second world war, did not quite achieve such status; the British code-breakers, now quartered in Cheltenham, no longer call themselves Bletchley Park. Nonetheless, if you say "Bletchley Park", many more people will know that you are talking about code-breaking than if you say "Cheltenham" or even "GCHQ" (Government Communications Headquarters).

Bletchley Park is not a very large place: a country house on a few acres, where a few thousand people lived for a few years during the second world war. But somehow it is a very large subject. It was at the convergence of many histories: military history, of course, but also the history of modern military technology, of Anglo-American cooperation, of intelligence, of cryptography, of mathematics and of many fascinating individuals. The subject has long needed a comprehensive coordinated historical treatment that explains to the mythical "educated reader" just the right amount of each of the prerequisite subjects. Action This Day , which takes its name from the words used in a Churchill memo ordering that Bletchley immediately get the resources it required, is not that book. It is, however, a remarkable collection of essays - some written by participants, others by historians - that leaves one in awe of the complexity of Bletchley Park and its impact on both the world war and our postwar world.

The story starts well before the 1940s. Signals intelligence - although it has a long pre-history - was born in the first world war, which was the first war to be fought with the help of radio. Radio transformed warfare, first at sea, later on land. Before radio, the first sea lord at the British Admiralty, who commanded the most powerful fleet in the world, would dispatch a flotilla and expect news of its mission only weeks or months later. With radio, he could reach any ship within hours and maintain a comprehensive picture of the locations and actions of the British fleet throughout the world. Radio was so valuable that no military force could forgo it. However, it had a serious drawback. Everyone could listen to radio, and sometimes the enemy could receive your transmissions better than your own side. As a direct result, cryptography was immediately elevated from being a secondary security measure - generally applied to messages already travelling in dispatch cases or at worst on wires - to being the only applicable security measure, or at least the primary security measure. The effect was to overload the code clerks and begin the process of automation that has dominated cryptography ever since. One of the first products of this process was the Enigma crypto-system, and another was standing, peacetime signals intelligence organisations.

Action This Day follows the story from its beginnings after 1918 through the second world war and into the subsequent four decades of cold war. It begins and concludes with essays by historians, "Bletchley Park in pre-war perspective" by Christopher Andrew and "Cold war code-breaking and beyond" by Richard Aldrich. In between, we are treated to an exposition of code-breaking's geographic and cultural breadth. The impression conveyed by much popular coverage of second-world-war cryptography is: the Brits broke the German codes; the Yanks broke the Japanese codes. Though more true than false, this summary obscures a very important fact. To attack communications you must first intercept them. The British had assets in the Far East that not only made the breaking of Japanese communications as vital for them as for the Americans, but also gave them better access to many of those communications than was available to the Americans. So, British work was vital to American success against the Japanese just as, particularly later in the war, the American contribution was vital to British success against the Germans. The web of cooperation and competition among the British and American organisations was complex; and the essays reflect this. A group on one side of the Atlantic was often closer to its opposite number on the other side than to other groups located on its own side.

A second impression readily taken from the superficial treatment normally accorded to Bletchley Park in popular presentations is that its achievement, at least its primary achievement, lay in breaking the German Enigma crypto-system - often with the suggestion that Enigma was the only important German system or at least the highest-level German system. While Action This Day treats Enigma at length, it relieves the reader of the false impression that Enigma was the only or the most important target. In fact, Enigma was a mid-level crypto-system employed, in various versions, by infantry, ships and aircraft. Communications between Berlin and the major German military commands were protected by encrypted teletype systems - in many ways the most advanced of their era - operating on quite different principles from Enigma.

Bletchley Park's attack on Germany's highest-level systems gave rise to perhaps its best-known contribution to the world of today: its role in the development of the digital computer. The high-grade German systems were much closer than Enigma to those in use today and are better thought of as sending streams of bits than streams of characters. In order to break these systems, Max Newman, Alan Turing and others developed a series of binary computing machines, the most famous of which is Colossus. These machines were not quite computers in the modern sense because their programs were on plugboards rather than in electronic memory. But they were the direct ancestors of the Manchester Baby, a leading candidate for the first true digital computer.

Each of the book's 22 essays belongs to one of two categories. The majority (more than two-thirds) are studies by historians working from documentary sources - many declassified only in the past few years. The remaining ones are memoirs by Bletchley Park alumni. Two of these people were familiar to me long before I knew of their work in the second world war. John Chadwick worked on Italian naval codes and later Japanese translation while at Bletchley Park and, like all such work, what he did went unreported for decades. But in the 1950s, Chadwick became well known for a different kind of code-breaking when he collaborated with Michael Ventris in unravelling an ancient mystery, how to decipher the Cretan script known as Linear B. Shaun Wylie's work at Bletchley Park is discussed in his essay, "Breaking Tunny and the birth of Colossus". But for me, Wylie was first known as the author of a book from which I learned algebraic topology, while at a summer school in Berkeley, California, 40 years ago.

Wylie could not get code-breaking out of his blood. In 1958, he left Cambridge again to become chief mathematician at GCHQ, a post he held until the early 1970s. Towards the end of his tenure, James Ellis, Clifford Cocks and Malcolm Williamson, all at GCHQ, made seminal, if secret contributions in my own field: public-key cryptography. Although most of their discoveries in the 1970s preceded ours, some by half a decade, the official establishment did not put them into practice until after the public community had published and begun to apply its own discoveries. In my attempt to understand GCHQ's response to this "disruptive" technology, I finally made the acquaintance of Wylie, who was by then retired from GCHQ.

One contributor of whom I had not previously heard is Mavis Batey, a Bletchley Park alumna married to another contributor. She is described as a historian of landscape and literature, author of such intriguing titles as Jane Austen and the English Landscape . In her essay, "Breaking Italian naval Enigma", Batey claims for Bletchley Park the credit for recovering - by purely cryptanalytic means - a set of Italian naval codebooks, whose theft earlier authors have credited to a seductive spy operating under the code name Cynthia (real name Amy Elizabeth Thorpe). At this point, it would have been very helpful if the editors had undertaken some additional historical investigation. Batey writes: "I was able to scupper the idea that we had been given codebooks captured by Cynthia or anyone else." Be that as it may, Bletchley Park's non-receipt of the codebooks does not mean that Cynthia did not steal them. To know more about their real fate would not only be entertaining; it might provide insight into the interaction of code-breaking with other elements of British intelligence.

Two of the personal memoirs piqued my interest by harking back to one of the earliest published works about Bletchley Park: The Hut 6 Story by Gordon Welchman, who worked with Turing to construct the "bombe", the specialised computing machine used to break Enigma. The essay by Derek Taunt, "Hut 6 from the inside", helps to flesh out the history of a pivotal component of Bletchley Park. A complementary essay, "Hut 8 from the inside", describes Rolf Noskwith's experiences breaking the German naval Enigma. The navy was more careful than the other German services in its use of the Enigma machine, and so naval Enigma presented a more difficult problem than its army and airforce cousins.

In addition to the essays, which form the book's core, there is supporting material that unfortunately is not covered in the two-level index. Four appendices deal with cryptanalytical issues too technical for the main body of the book; and there is an extensive set of notes, a glossary and two sets of capsule biographies (both of the contributors and of the other dramatis personae).

More than 50 years have passed since the events forming the high point of Bletchley Park's history. Documents are still being declassified and alumni are still writing their memoirs. Perhaps eventually a comprehensive history, one that will make Bletchley Park clear to a reader such as I who has trouble remembering the intricate ebb and flow of the second world war, will be written. In the meantime, we are fortunate to have Action This Day .

Whitfield Diffie is chief security officer, Sun Microsystems. He is involved in typesetting and editing a work critical to Bletchley Park history that was written in 1945 but declassified and released to the Public Record Office only in 2000: the General Report on Tunny with Emphasis on Statistical Methods by I. J. Good, Donald Michie and Geoffrey Timms.