One of the chief props to Churchill’s confidence during the early and dark days of the war with Germany was the knowledge that the British Government Code and Cipher School (GC&CS), located at Bletchley Park in Bedfordshire, was beginning to read Germany’s most secret military ciphers. These, enciphered on a compact, electrical switching machine known as ‘Enigma’, were believed by the German armed forces to be totally secure, since any single letter might be translated by the machine in any one of 200,000,000 different ways. It was calculated by its users that the effort to decipher an Enigma message would defeat any group of analysts, however large, using mathematical attack to test out the permutations.
In theory, the Germans were correct. Enigma, if strictly used, provided an impenetrable ciphering system. What the Germans did not recognize was that Enigma operators would be prone to taking short cuts, to the repetitive use of formula messages and to other procedural mistakes that would provide the British cryptologists with entry points into the Enigma traffic. Progress at first was slow, despite much help given bv the Poles, who were the first to break into Enigma, and some ciphers, used by meticulous German operators, were never penetrated. By the end of 1940, however, the mathematicians, chess champions, crossword solvers, etymologists and other exceptional brains collected at Bletchley had begun to break into several Enigma ciphers in ‘real time’ — near or at the time that German recipients were decrypting the messages intended for their sole use - and so were able to anticipate and forestall German operational moves. The Enigma secret, codenamed Ultra, was one of the most important of Britain’s — and later America’s — war-winning techniques.
Hut 6:Early Days
When the war broke out in 1939 I was in the Argentine, playing chess for the British team in the Olympiad. My great friend and rival, C. H. O’D. Alexander, was another member, as was Harry Golombek, later chess correspondent of The Times. We returned home immediately, and before long found ourselves at Bletchley Park, where we remained for the duration. We had been recruited by Gordon Welchman, an old friend of mine at Cambridge.
Welchman was a brilliant mathematician, a research fellow at Sidney Sussex. He and I came up to Trinity College, Cambridge, in the same term (October 1925). We became friends, and, as I lived in Cambridge with my mother (who was Cambridge born and bred, the daughter of Dr W. H. Besant, a well-known mathematical fellow of St John’s College), we saw a lot of Gordon at our home in Park Terrace. In due course Welchman married and came to live in Cambridge himself.
Alexander was three or four years younger than Welchman and myself, but I had known him ever since, in 1924, he had beaten me at Hastings in the British Boys’ Chess Championship, which I had won the previous year at the age of 16. Hugh Alexander was a scholar of King’s [College, Cambridge] and also a mathematician, though not, I fancy, in Welchman’s class. He came from Birmingham, where my sister (the eldest of our family) was a lecturer in English and Anglo-Saxon at the University. When he left Cambridge he became a schoolmaster and taught mathematics at Winchester College. But before long he married an Australian girl some years older than himself. She was unhappy in the cloistered atmosphere at Winchester, and eventually he gave up teaching at which he was very good, and took up a business career in London for which he was ill adapted; he was far too untidy even to look like a businessman.
Welchman had, I think, been recruited into intelligence work before the war broke out, and he had already done a good deal of research into the mysteries of the Enigma machine. These of course came easily enough to Alexander, but I am (in spite of my ancestry) almost innumerate. I therefore found Gordon Welchman’s patient explanations very difficult to follow, and to this day I could not claim that I fully understood how the machine worked, let alone what was involved in the problems of breaking and reading the Enigma cipher. Fortunately this did not matter too much, as I was able to make myself useful in other ways — principally because I had a working knowledge of German.
To return to the beginning of the war: fairly early in 1940 we three found ourselves installed at The Shoulder of Mutton Inn, Old Bletchley, about a mile from the entrance to Bletchley Park. Here Hugh and I were most comfortably looked after by an amiable landlady, Mrs Bowden. As an inn-keeper she did not seem to be unduly burdened by rationing, and we were able (among other privileges) to invite selected colleagues to supper on Sunday nights, which was a great boon. Welchman moved out fairly soon to live with his wife in the town, but Hugh and I happily remained at the Shoulder until the end of the war.
When we first came to GC&CS, Bletchley Park was a tiny organization, probably not more than thirty strong. It consisted of a few old-time professionals who had worked in Room 40 [Naval Intelligence] at the Admiralty in the First World War, such, for example, as Dillwyn Knox, a Fellow of King’s who died during the war, and A. G. Denniston, and new recruits such as Welchman and Alan Turing. Knox had, so I understood, been defeated by the Enigma, and the main credit for solving the Enigma and subsequently exploiting its success, should (subject to the Poles) probably go to the other two.
Turing was a strange and ultimately a tragic figure. But as an admirable biography of him has been written [Alan Hodges, Alan Turing: The Enigma of Intelligence, 1983], I shall say no more here. Welchman, on the other hand has, I think, never received his just deserts, quite apart from being ridiculously persecuted on security grounds for revealing, some forty years after the event, how the job of breaking the Enigma had been done. Welchman was a visionary, and a very practical visionary at that. In spite of Knox’s failure, he always believed the Enigma could be broken. He also realized the enormous importance of the success, and took it for granted that, when the phoney war ended, the Germans would rely principally on the Enigma for their military communications. He foresaw much of what would be involved in the way of expansion of staff, machinery (the bombes [see p. 361]), and all the other necessary substructure. And he had the fire in his belly that enabled him to cajole higher authority into supplying our wants. If Gordon Welchman had not been there, I doubt if Ultra would have played the part that it undoubtedly did in shortening the war.
The first essential, once the initial breakthrough had been made, was the expansion of staff. This was done to begin with on an ‘old-boy’ basis. Welchman knew a few undergraduate mathematicians whom he had supervised at Sidney Sussex. I lived in Cambridge and had the advantage of a close connection with Newnham College, where my sister Alda had been Vice-Principal until her untimely death in 1938. So I was able to recruit a few girls from both Newnham and Girton Colleges, such as Mary Wilson, Wendy Hinde, Margaret Usborne, and Jane Reynolds (later Jane Monroe), who formed, together with a number of undergraduates from men’s colleges, the invaluable nucleus of the original Hut 6. At about the same time I made a very profitable foray to the Scottish Universities.
As a result of these efforts, we were able, even during the period of the phoney war, to put ourselves on to a three-shift basis, and there of course we stayed throughout the war. This was during the winter and spring of 1940, and the decrypts themselves were mostly practice by the Germans — nursery rhymes and the like. But the exercise gave us invaluable practice as well as the Germans, and provided us with a battery of cribs, of which we were able to make invaluable use when the war became real.
This happened, as I remember, with the invasion of Norway, about the end of April and beginning of May 1940. The keys [German enciphering systems] we had been breaking were a practice key and the Red (general) key of the German Air Force. The Red key became of vital importance immediately, and remained so all through the war and in all the main theatres of war except Africa. Another key was the Army — Air Force key, the Yellow, used only in Norway.
The Red was the great standby that kept Hut 6 going. I cannot remember any period when we were held up for more than a few days at a time. Indeed, if we had been, it was by no means certain that we would have been able to get started again. The fact that we nearly always broke the Red, often quite early in the day, may give the impression that it was as easy as falling off a log; indeed Hut 3, our intelligence opposite numbers to whom all our decrypts were sent immediately, took it for granted that they would receive their daily ration. But there was in fact no certainty at all about it.
This leads me to stress how extraordinarily lucky we were. The Germans regarded the Enigma as a perfectly secure machine, proof against cryptanalysts however talented and ingenious they might be. Several times during the war, when it became inescapably clear that a great deal of intelligence was finding its way to the Allies, the Germans set up commissions of inquiry into the security of the Enigma. They always came to the conclusion that the machine was invulnerable and that the leakage must be due to secret agents. And the fact is that, had it not been for human error, compounded by a single design quirk, the Enigma was intrinsically a perfectly secure machine. If the wireless operators who enciphered the messages had followed strictly the procedures laid down, the messages might have been unbreakable. But of course that was the whole trouble. It is easy enough in principle to observe the relatively simple rules, such as, for example, avoiding enciphering the address and title of the recipient in the same way at the beginning and the end of the message. But if you are a harassed machine operator, it is so much easier to do what you have always done than consciously to force yourself to put your ‘Xs’ in different positions, or not to put ‘Xs’ in at all for punctuation, that it is very difficult to blame those concerned.
In claiming credit, as I have done, for Turing and Welchman for the initial breakthrough in the Enigma, I should stress the original and vital contribution made by the Poles. The Poles had always been brilliant cryptographers, and had been breaking and reading some Enigma ciphers since the early 1930s. When the war broke out, however, the Germans made a major change in the machine which put the Poles out of business; but shortly before the war and in anticipation of it, the French Intelligence Chief (Colonel Bertrand) and ourselves had got together with the Poles, who not only gave us an Enigma machine, but shared with us all their knowledge and experience. They had a group of distinguished mathematicians, including one, Marian Rejewski, and another, Colonel Langer, whom I met after the war. It was always a mystery to me that the Polish contingent was not incorporated at Bletchley during the war, where they would no doubt have made an invaluable contribution; but in fact they were sidetracked in France and had to be evacuated when the Germans overran the whole of the country. I can only assume there were security doubts, and I believe the Poles continued to operate their own organization; but I feel there must have been a sad waste of resources somewhere.
So much has been written on the technical side, including, of course, Gordon Welchman’s own The Hut Six Story [1982], and many erudite productions by Polish cryptanalysts, that it would be pointless to add to it. But for the non-technical layman who would like to have some idea of what was going on, it might be of interest if, as a non-technical layman myself, I were to try to describe things as I saw them.
The Enigma, an electric machine, looked like a typewriter with a typewriter keyboard. But it could be set up in millions of different ways, so that if you typed out an ordinary sentence on it you would get nothing but a jumble of nonsense groups. In order to turn these nonsense groups back into the original German you therefore had to have a machine set up exactly as the original operator had it. Merely having a machine did you no good at all, and since the Germans changed their machine settings every twenty-four hours, you were helpless unless you knew in advance, or could in some way work out, what the new setting for the next day was to be, out of the millions available.
Since we did not know, we had to start from scratch every day; and the basis of breaking was known as the ‘crib’. Take, for example, a routine message like keine besonderen Ereignisse(‘no special developments’). This might appear in the enciphered version as:
Text: KEINE BESON DEREN EREIG NISSE Cipher: ACDOU LMNRS TDOPS FCIMN RSTDO
This jumble would form the core of the message. But, of course, most messages had to have an address and a signatory; thus there would have to be two or three groups beforehand and two or three at the end, so that, even if you knew that somewhere in a short message (e.g. twenty groups of five) there lurked the five groups which represented keine besonderen Ereignisse, you did not know which five consecutive groups they were.
Here we were greatly assisted by an idiosyncrasy of the Enigma machine. This was that a letter ‘could never go to itself’: in other words, if you tapped A on your machine, you might get any letter appearing except A.
As you will see, this was a great help in eliminating possible candidates. If you look back at keine besonderen Ereignisse, you will see that in the enciphered version beginning A C D O U there is nowhere a clash of letters between the German and the enciphered groups. But if, for example, you were looking for the most likely position in your twenty groups for the keine besonderen Ereignisse sequence, and you noticed that under the B in besonderen was another B, you would know for a certainty that that could not be the right position for the core of the message, and would have to look elsewhere.
Let us assume, however, that you have guessed the right position for the five keine besonderen Ereignisse groups. The problem is now to set up your own machine in an identical position in all respects to the German machine. If you could do that, you would have broken the Enigma Red key for that day; and then you could decipher all the messages sent over the air by the Germans for that day in that particular key. But how was this problem to be resolved?
In theory, of course, it could be done by running through all possible settings of the machine until, as by a miracle, the tapping of ACDOU LMNRS etc. produced KEINE BESONDEREN EREIGNISSE. But in practice this was completely impossible. It might have taken years to run through all the possible permutations and combinations, and the war might well have been over before we had succeeded with even one key for one day. However, given that we were confident that somewhere in a jumble of consecutive five-letter groups was concealed a text of intelligible German — twenty-five letters such as in keinebesonderen Ereignisse should normally be quite sufficient — that would put us well on the road to success. So long a stretch would enormously cut down the number of machine settings that would have to be tried; and modern technology had, in fact, provided a machine (known as the bombe) which would normally provide the correct setting of the Enigma within a few hours or so.
In fact, the Germans used a number of keys every day. Some we were able to break and some not. As I explained earlier, the Red was the great standby, but at times others were of even greater operational importance. There was, for example, the Brown. The Brown was the key used by Kampfgruppe [Luftwaffe battle group] 100, which was responsible for the so-called Baedeker raids on key targets such as Bristol, Birmingham, Coventry, and so on [in retaliation for RAF raids on equivalently historic German towns]. My recollection is that the Brown was a relatively easy key to break, and that we often broke it fairly early in the day. Obviously, the earlier the better, so that counter-measures could be put in hand: sometimes even squadrons in the air, or at least such anti-aircraft ground measures as were possible. Unfortunately, the Germans did not tell us what the targets were. They simply referred to them by numbers (Ziel [target] so-and-so), and we might or might not be able to guess what town was intended. There was an obvious risk that, if we took counter-measures, we should give away that we knew from Enigma that X was the target that day. There was a story, which can still be heard from time to time, that it was known that Coventry was to be the target for a devastating raid, and that Churchill forbade any special precautions for fear of giving away the all important secret that we were reading the Enigma. What Churchill would have done had he been confronted with this terrible dilemma I have, of course, no idea. But it so happens that we did not know the target for that night until too late; in fact, as I remember, we were expecting a very heavy raid on London.
While Brown was, I suppose, the most dramatic of the various keys with which we were concerned, it was when Rommel invaded Egypt that Hut 6 really came into its own. The German expeditionary force had its own keys, which we named after particular birds, and the most important bird was Chaffinch. I think we read Chaffinch pretty consistently after the spring of 1942. And since the Germans had no other regular means of communication with their forces abroad, we were given a pretty complete picture of their strength and disposition, as well as their intentions; and, as a by-product, a lot of information about their U-boat movements in the Mediterranean. (This was quite distinct from the U-boat warfare in the Atlantic, which was looked after by Hut 8 under Alexander and which dealt in the U-boats’ own naval Enigma key). I remember I made Chaffinch my own particular concern, if that day’s key had not been broken by the time I came on deck.
Hut 6 was the cryptanalytical section; Hut 3 the intelligence section. We broke the keys, the messages were deciphered as they came in by our deciphering section, and the contents were passed immediately to Hut 3. It was Hut 3’s responsibility to decide what they meant, to decide the degree of importance and urgency, and to report accordingly to London. Naturally, it was essential that we should keep in the closest touch with Hut 3, which was organized in watches just as we were; and the heads of our respective watches were in continual discussion about the priorities for breaking and deciphering. In the early days, before the production of bombes overtook the requirement, this sometimes produced very difficult discussions between Hut 6 and Hut 8, because we shared the same bombes. But, fortunately, Alexander and I were such close friends as well as colleagues, that I do not remember any serious dispute ever arising. In any case, I took it for granted that ultimately breaking the U-boat cipher must take priority, for failure to read it could lose us the war. But naturally we took guidance from Hut 3 about which of our Hut 6 ciphers took priority in our sphere.
Once again, we were very fortunate. Hut 3 was staffed rather differently from Hut 6; whereas Hut 6 had a high proportion of undergraduates (even one or two straight from school), those in Hut 3 were for the most part older. They were, naturally, German scholars and not cryptanalysts, many of them schoolmasters or young dons of their University (predominantly Cambridge as it happened). But they were presided over by Group-Captain Jones. Jones was not a scholar or an academic; I suppose he must have had some knowledge of German, but primarily he was a businessman coming, I think, from Lancashire. I do not know what brought him to Bletchley, but it proved a brilliant choice. He was a genuinely modest man who regarded himself as having little to contribute compared with the boffins with whom he was surrounded; in fact he was a first-rate administrator who was liked and trusted by everyone. After the war he stayed on in the service, and ended up as the head of GCHQ [Government Communications Headquarters] at Cheltenham.
I do not imagine that any war since classical times, if ever, has been fought in which one side read consistently the main military and naval intelligence of the other. Of course, the Germans read a fair amount of our own codes, but in nothing like the comprehensive and all-embracing manner in which we read theirs. It was rather like a game of bridge in which we were shown the opponents’ cards before the hand was played. One might well have expected that, with the cards so stacked on one side, so big an advantage would be reaped by the reading side that the result of the contest could hardly be in doubt. But, although the intelligence balance was enormously in our favour, its only really decisive effect was in the Battle of the Atlantic. It can, I think, be pretty confidently asserted that, had we not at the most crucial times and for long periods read the U-boat ciphers, we should have lost the war. I have seen it argued that, in the long run, it would have made no difference because the Americans would have more than made up any conceivable losses in our — and their — shipping; but this makes big assumptions about how the United Kingdom was to be kept from starving in the meantime. Certainly, when, as did happen in 1942, Hut 8 was held up for months on end on the U-boat ciphers, the losses in the Battle of the Atlantic rose in the most alarming way. This was particularly so after the entry of the United States into the war, before the Americans had had time to organize a convoy system on the American littoral.
So far as the Army and Air Force were concerned, the advantage conferred on the Allies must have meant a very great saving of lives and, I imagine, a considerable shortening of the war. I would not suggest that it made the difference between victory and defeat. Indeed, in the early days the disparity in overall strength and readiness between the Germans and the Allies was such that no amount of knowing their intentions could prevent them carrying them out; only later, when the strengths became more equal, did the intelligence advantage become a match-winning factor, and remained so until the last days of Hitler.
I felt at the time — and looking back on it after more than half a century, I feel exactly the same — that I was extraordinarily lucky to have found myself in that particular job at that time; and I suspect that almost everybody concerned in the Enigma operation at every level felt the same. We were slightly ashamed, or at any rate unhappy, that we should be leading such relatively safe and comfortable lives when a large proportion of the population, whether civilian or in uniform, was in so much danger. But at the same time, we could not help realizing that our work, individually and collectively, was of enormous importance in the conduct of the war. For that reason alone there was a spirit of camaraderie which I think never failed us, even when the war generally seemed to be going badly, or when we found ourselves temporarily delayed in delivering the goods. There was a perpetual excitement about each day’s breaks, at whatever time of day or night they might come. To the chess player, it was rather like a long-running tournament with several rounds being played every day, and never any certainty that the luck would continue to hold. The friendships that were formed during the war have in many cases survived fifty years later, and that applies equally to some of the small but very high-powered American contingent, headed on the cryptanalytical side by Captain (as he then was) William Bundy. No more admirable representative of our great new ally could possibly have been selected. Many of us feared, I think, that the Americans, with their enormous superiority in manpower and resources of every kind, would quickly reduce Hut 6 — and no doubt Hut 8 as well — to a very subordinate and minor role. Nothing of the kind happened. Bundy and his colleagues were anxious only to learn and to help. They learnt very quickly and their help was invaluable. Above all, able and high-powered as they were, they were modesty itself in their demeanour. There could have been no happier partnership.
It goes without saying that, since we were frequently reading the enemy’s communications as soon as their intended recipients, some pretty dramatic messages were brought to me in Hut 6. I remember especially a message describing in considerable detail the German plans for the invasion of Crete in some fortnight’s time [May 1941], and also thinking to myself (quite wrongly, alas!), ‘Now, we really have got them this time.’ And also the desperate message from the German commander in the Battle of Normandy in 1944, which heralded the collapse of the German resistance in the Cotentin peninsula and enabled the American tanks to break for Paris. This kind of message, shown to us maybe in the middle of the night, gave one an extraordinary sensation of living with history. But of course it was not individual messages of this kind that mattered most, though the Prime Minister took the liveliest interest in them and No. 10 [Downing Street] had to be kept constantly in touch. It was the complete build-up which Hut 3 was able to construct of the whole German side of the fence that counted, and that could have been achieved in no other way.
The Cretan episode was, from the Hut 6 point of view, the greatest disappointment of the war. It seemed a near certainty that, with General Freyberg warned that the crucial point of the invasion was to be the airborne attack on the Maleme airport, and the time and every detail of the operation spelt out for us in advance, and given the appalling difficulty and danger of any airborne invasion in the best of circumstances, the attack would be ignominiously thrown back; and we awaited the operation with anxiety but also with a considerable degree of confidence.
In the event, it was a ‘damned close-run thing’. The Germans took Crete from the air, and we lost a great deal of shipping in trying to save it. The best that could be said about it, from the Allied point of view, was that, though the conquest of Crete was ultimately achieved, it was so enormously expensive that the Germans never attempted an airborne invasion on that scale again. We fully expected, I believe, that Crete would be followed by Malta; but it never came. So, since the loss of Malta would have been an appalling catastrophe, we can at least be assured that the defenders of Crete lost their lives in a good cause.