The Bletchley Park Codebreakers (12 page)

The Enigma machine was believed by the German experts to be unbreakable if used correctly, even if the enemy had specimen machines and knew the methods adopted for using them. Indeed, it would have been secure if used properly but a) official usages could be insecure, and b) even if they were secure, misuse by operators could be fatal. I can speak only of air force and army and liaison between them, which were dealt with in Hut 6. A regimen of ‘need to know’ was rigidly observed at Bletchley Park, and while heads of departments would discuss their problems when they affected each other (e.g. the navy cryptanalysts in Hut 8, led by Hugh Alexander, the only English chess player at Bletchley Park ranking above Stuart Milner-Barry, shared the use of the bombes with Hut 6), the rank-and-file did not discuss their work outside the confines of their own huts. The form of Enigma attacked in Hut 6 – most particularly Red – had been used before 1 May 1940 in an inherently insecure way, so long as enough messages (say one hundred or so) were sent on any one day’s key. After that date a change of usage completely shut out the theoretical vulnerability and we became dependent on operators’ misuse and the introduction of machinery to help us exploit known habits. The essential principle was to maintain the continuity of breaking: ‘nothing succeeds like success’ in this game. The clean break of continuity in method of attack left Hut 6 in a Catch-22 situation: we needed continuity to achieve breaks, but we needed plenty of breaks to establish continuity (and of course we needed bombes – and by the end of 1940 there were at most only two). How could Hut 6 bridge this gap and restore continuity?

Two essential features of Enigma, its reciprocity (if A encoded to V, V encoded to A) and the fact that a letter could not encipher to itself, were exploited by cryptanalysts both in fitting cribs against cipher text and in drawing up menus – but also crucially in the design of the bombes which tested the menus to find positions of the wheels consistent with them, from the million possibilities.

Although Alan Turing did not impinge directly on my Hut 6 experience, it was his genius that made our success possible. And he gave a clear rebuttal of Hardy’s ‘harmlessness of real mathematics’! Turing, a research fellow of King’s College, Cambridge, was the most brilliant and most ‘real’ of Bletchley Park’s mathematicians attacking Enigma. His subject was the logical foundation of mathematics, which had been thrown into turmoil a decade earlier by the Czech mathematician Gödel, who had proved that any consistent system of mathematics, defined by axioms, must be incomplete – which means that there must exist meaningful statements in it that cannot be either proved or disproved. This raised the so-called ‘decision problem’: given a meaningful statement in such a system, was there any standard procedure for establishing whether it was provable or unprovable.’ Turing had shown that the answer to this problem was ‘No’. He did this by describing a routine process (or ‘thought machine’) by which the truth of any provable statement could be established in a finite sequence of steps selected from a small number of possibilities. He then showed the existence of meaningful statements which the machine could not decide as true or false. This ‘thought machine’ became known as a ‘Turing machine’. The same mind conceived the basic design of the far from abstract bombes which were the indispensable weapons against Enigma and in fighting the war.

It is time to revert to May 1940. There were two forms of operator breach of regulations which gave us a chance. The first (the ‘Herivel tip’) depended on a considerable number of operators taking the same short cut. The other (the ‘cilli’) depended on one operator making two independent breaches of discipline in a single sequence of
half-a-dozen
or more consecutive messages (see Appendix IV). John Herivel, a young mathematical recruit from Sidney Sussex College, put himself into the position of a sleepy Enigma operator setting up his machine at the beginning of the day, and asked himself what he might do to save some bother. If enough operators followed the same procedure, their actions could be detected from scrutiny of the first message sent by all operators that day. This would reduce the range of likely
Ringstellungen
from over 17,000 to perhaps six. With cillies (not ‘sillies’, as Welchman misremembered them in his
The Hut Six Story
forty years later), each breach of the rules was by itself both harmless and undetectable, but the two together could not only give evidence about the day’s wheel-order but also (especially if supported
by a Herivel tip) lead to a breaking of the key without recourse to the bombes (i.e. by hand methods). So, by one means and another. Hut 6 survived until the arrival of the bombes made it much easier to achieve the essential continuity.

Why were German operators, especially those in the
Luftwaffe
, so careless as to allow us to escape from our Catch-22 situation? I believe it was partly due to their belief that with over 150 million million million different possible ways of setting up the Enigma machine, the enemy’s task in choosing the right one was impossible. But here the idea of safety in large numbers falls down. There are over two million times as many different simple substitution ciphers as Enigma machine keys, yet any intelligent youngster, faced with a message of 250 letters (the standard length of an Enigma message) encoded by such a cipher could unravel it in half an hour. If the possible different Enigma keys were tested at the rate of one per second, it would take five million million years to try them all. So how could Alan Turing’s ingeniously designed bombe make any impact, however fast it ran.’ The answer was that it filtered out the Stecker, leaving only a million possibilities – say twelve hours running-time at twenty-five tests per second.

On arrival in Hut 6 in August 1941 I was assigned to Control, the first point on the conveyor-belt. Like all of us concerned with the breaking or interpretation of ciphers (as opposed to administration), those of us in Control covered all twenty-four hours every day in three shifts (as I remember 12-9, 9-4, 4-12). Those outside easy cycling range of Bletchley Park were conveyed to and from their billets by a fleet of estate cars (later augmented by superannuated coaches). My first billet was at the home of a railwayman’s family in Stony Stratford, some eight miles away up Watling Street, one of several dozen villages eventually colonized by Bletchley Park workers. Stony Stratford was celebrated for one thing only – as the reputed origin of the phrase ‘a cock and bull story’, supposedly originating from the pair of almost adjacent coaching inns, the Cock and the Bull, and immortalized in the final sentence of
Tristram Shandy
.

In Control, we kept in regular touch with the intercept stations and with both the codebreakers in the Hut 6 Watch and interpreters in Hut 3, making sure that frequencies important either cryptographically or intelligence-wise were double-banked, checking details and generally ensuring that the necessary raw material for the production of Ultra – the top-secret inside information about enemy formations and
intentions – was forthcoming. It was not the most glamorous or most mathematically ‘real’ activity, but gave a very good introduction to the various sub-departments in Hut 6 and their leading personalities. Our work suited the three-shift pattern – one just handed over the interception charts and the current situation to the next shift without any loss of continuity. But life was different in the Watch, to which I was transferred after a year or so in Control. By the end of the shift one might be in the middle of turning a re-calcitrant crib or re-encipherment into the form of a pair of runnable menus, and be rather unwilling to hand over in midstream to a fresh mind (which might well follow up the dead-ends which you had tried and rejected). Thus living in or near Bletchley was a real advantage, and members of the Watch were given priority when local billets were available. For the rest of my time at Bletchley Park I was not restricted by the need to catch an end-of-shift bus home.

This was even more useful in my final role as a member of the Q-watch (pronounced ‘Quatsch’, the German word for ‘nonsense’). The Qwatch was a back-room which tackled intractable and longer-term problems. My two colleagues there (for whom I acted as best man when they married in 1947) were Bob Roseveare – who had joined Bletchley Park straight from Marlborough College and had endless enthusiasm and energy – and Ione Jay, whose calm efficiency in keeping us both in order was essential to our effectiveness. We kept tabs on some rather sinister scientists at Peenemünde on the Baltic coast, whose interest in heavy water and rocketry emphasized the need for the allies to get their retaliation in first for the V3 weapon (which fortunately never appeared). We made contingency plans for expected horrors (such as the pluggable
Umkehrwalze)
which threatened but never interrupted the flow of Ultra. One of our more satisfying encounters was with the
Notschlüssel
, or emergency keys. A routine laid down by the Germans for creating a complete Enigma key from a long key-word had been uncovered, and we were able to reconstruct the key-words from broken keys (which showed clear indications of their origin if they had been generated in this way). We could use the key-words if they subsequently reappeared, sometimes written backwards.

There was a strong sense of comradeship in Hut 6, and a feeling that we were all contributing to a great enterprise. This was strikingly expressed much later by Bill Bundy, the officer-in-charge of the USA contingent attached to Bletchley Park from August 1943. He was the
older brother of the better-known McGeorge Bundy; both brothers became high-level advisers to successive presidents. Bill had led a US Army unit known as the 6813th Signals Security Detachment to work at Bletchley Park alongside the indigenous staff, and with a handful of others he had joined Hut 6. In a BBC interview in 1999 Bill said: ‘Although I have done many interesting things and known many interesting people, my work at Bletchley was the most satisfying of my career.’ He had sensed the special ambience of the outfit, quite unlike that of the American stereotype of conventional British reserve and its difference from any other hierarchical organization in Britain or the USA (especially the American Army!). In a talk Bill gave in 1982 to the American Cryptogram Association he said:

This comment reflects an important point: when undertaking something as intricate and significant as Hut 6 did, you needed staff of high intelligence and integrity to tackle even simple, routine jobs accurately. This had been achieved in Hut 6 in two ways: by recruitment of suitable staff, either by the old boy/girl network or (from 1941) by C. P. Snow’s allocation organization; and by making sure that everyone in a particular department (such as Hut 6) knew what was going on throughout that unit and realized how important to its success was the part played by every individual member. Contrariwise, it was essential to maintain confidentiality outside the immediate circle, even within Bletchley Park. This explains why such a small part of what I now know about Bletchley Park comes from my memory at the time – one only discovered what colleagues in other departments were doing on a ‘need-to-know’ basis.

It is an extraordinary fact that, for at least thirty years after 1945, little hint of what was achieved, and none at all of how it had been done, became public knowledge. It is perhaps less surprising that the secrets were kept in wartime, as the dangers of ‘loose talk’ were appreciated by everyone. In fact, enemy awareness of our success
with Enigma was zero, as the many unsuccessful German attempts to discover why we were so well-informed attest. Among the 8,000-10,000 workers at Bletchley Park, only one, John Cairncross, was an authenticated spy, and he gave information only to our allies, the Russians. Perhaps he felt that they were being unjustly excluded from our secrets; in fact it was because of their known poor cipher security.

A short while ago I received a letter from an old Jesus College man, who had been a pupil of mine fifty years ago and had spent most of the interim years in the USA. He had read
Codebreakers,
to which I contributed a chapter, and commented:

If by ‘turf wars’ he meant struggles for individual or sectional territory, they were not mentioned because there weren’t any! I am not claiming a regime of universal love, but the fact was that no ‘strong, continuing, deliberate effort’ was needed to achieve harmony. We had a strong awareness of common purpose, and a recruitment process which produced a range not only of the diverse talents needed, but also of tolerant, understanding personalities (most notably that of Stuart Milner-Barry, our boss). Too much has been made of eccentrics at Bletchley Park. In fact there was no greater proportion of eccentrics than in the average Cambridge faculty (staff and students), though at Bletchley Park they dressed less conventionally and sported more beards and long hair than was usual at that time.

However, memory is notoriously selective and unreliable. I shared with Dennis Babbage (who was far more machine-literate than I) the same false memory of the detailed turnover sequence of the machine’s wheels. Welchman himself gave a garbled account of the phenomenon of cillies in his trail-breaking book, and many other inaccurate accounts have been given since. (In 1993 I was prevented from including the true story of cillies in my chapter in
Codebreakers
by the censor!) I have seen a reference to rifle practice in the woods between Bletchley Park and the railway line – a crazy place for amateur shooting – when
in fact the Home Guard practised in a deep brick-clay pit near at hand, which gave a passable imitation of the Somme in 1916.