The Influence of ULTRA in the Second World War

In 1993 Sir Harry Hinsley kindly agreed to speak about Bletchley Park, where he worked during the Second World War. Sir Harry Hinsley is a distinguished historian who during the Second World War worked at Bletchley Park, where much of the allied forces code-breaking effort took place.

Keith Lockstone recorded the talk and edited the final document, Susan Cheesman transcribed the first draft and Sir Harry made comments and added amendments. Thanks to all involved.

Security Group Seminar

Speaker: Sir Harry Hinsley Date: Tuesday 19th October 1993 Place: Babbage Lecture Theatre, University of Cambridge Computer Laboratory Title: The Influence of ULTRA in the Second World War

Ross Anderson: It is a great pleasure to introduce today's speaker, Sir Harry Hinsley, who actually worked at Bletchley from 1939 to 1946 and then came back to Cambridge and became Professor of the History of International Relations and Master of St John's College. He is also the official historian of British Intelligence in World War II, and he is going to talk to us today about Ultra.

Sir Harry: As you have heard I've been asked to talk about Ultra and I shall say something about both sides of it, namely about the cryptanalysis and then on the other hand about the use of the product - of course Ultra was the name given to the product.

And I ought to begin by warning you, therefore, that I am not myself a mathematical or technical expert. I was privileged to be an assistant to the mathematicians led by Max Newman and Alan Turing, but I have never myself learned to master or even to approach mastering their art.

Ultra, of course, was the product of ciphers. It was used only for the product of the metering of the more important ciphers, and from the spring of 1941 at Bletchley we broke most ciphers to an unprecedented extent and with an unprecedented lack of delay. And there were two reasons for that success.

First of all, alone among the governments of those years, the British Government as early as the 1920's concentrated all its cryptanalytical effort in one place which it called the Government Code and Cipher School. And at Bletchley secondly, the staff rose from about 120 in 1939 to about 7,000 at the beginning of 1944.

Of course that staff was not entirely cryptanalytical, it consisted also of an immense amount of staff used, for example, for signalling the products to commands in the rest of England or abroad. And so it wasn't entirely cryptanalytical staff - it was a very mixed staff compared with pre-war.

In addition, those men and women, recruited mainly from universities, developed methods and machinery of a sophistication hitherto undreamt of, including as you all know the first operational electronic computer which was called Colossus.

Without those advances, at least the most difficult of the ciphers, which were (although I will make some qualifying remark about this in a minute) those based on the German Enigma and those still more complex systems which Germany introduced for ciphering non-morse transmissions, would have been for all practical purposes invulnerable.

Now the value of the resulting Ultra was all the greater because the enemy states - Germany, Italy and Japan - remained unaware of the British successes.

The main reason for that was that they didn't allow for that sophistication of method and machinery which the British brought to the attack on their ciphers. They didn't allow for that when they constructed their ciphers.

Nothing very surprising in that. As I have said, the methods and machinery developed were of a sophistication hitherto unthought of.

But it still remains necessary to say that, despite that sophistication, the German belief that the ciphers would remain invulnerable was almost right, almost correct.

The ciphers nearly escaped effective exploitation.

In the case of the Enigma (which was an electro-mechanical machine) the first solutions were made by hand by mathematicians relying on German operators' errors. The German airforce was always more untidy in its signalling than the other users of the Enigma.

And for that purpose (although you will understand it better than I do) they used perforated sheets and exploited these mistakes made by German operators.

For other keys than the airforce, especially those of the army and the navy, and especially for the regular and speedy solution of those keys, it was necessary to develop an effective answering cryptanalytical machine.

That was the key in the end to the prompt decrypting of the machine cyphers.

It was a machine called The Bombe - a name originally given to it by the Poles who invented an early prototype in the 1930's. The Bombe developed in Bletchley by Turing and Welshman and Babbage - all luminaries of the Cambridge scene - was helped a little by the Polish machine, but it was infinitely more powerful, about fifteen times more powerful than the Polish machine. And it was because of the greater difficulties of dealing with the Enigma that it had to be that powerful.

But it wasn't enough to have that machinery developed. Except in the case of the airforce keys, we had to capture them before the machinery - even this Bombe machinery - could break into them.

And that is where people like myself who were non-mathematical came into the story. It was because I was in close touch with Turing, for example, that I was fully aware of what he had to have before the machine which he had developed could exercise its powers.

And I was able to arrange - with other people of course, including the navy - how to capture that. I stress that because, both in the case of the navy and of the army, and at dates which are later than we realise (I will give you the dates later), we needed in addition to this superb mathematics, which was assisted of course by superb Post Office engineering, we needed also these side assets - essentially captured material.

That was the Enigma. Now with the non-morse machine which we called Fish, the first successes were again obtained by hand methods. And those hand methods by mathematicians again exploited German operators' errors. In fact, the actual understanding of the machine theoretically (in other words as opposed to breaking it every day), the actual understanding of how it worked was obtained because the Germans went through a long series of experiments with it on the air before they brought it into operational use.

And it was these experimental transmissions which primarily were the errors which gave the entree.

But it was obvious that any regular or at all reasonably speedy decryption would be impossible if again machine methods were not developed. In particular that they would be impossible without machines because the different Fish ciphers proliferated, just like the different Enigma keys proliferated, and you were dealing with a lot of ciphers concurrently.

And so in this case, as you know, the machine was developed which came to be called Colossus.

It was of course a much more complex - it wasn't a mere electronic electro-mechanical thing - it was the first computer. It had to be like that in proportion to the fact that the Fish was far more complex than the Enigma.

Now it will be clear from what I have said that the problem wasn't merely to master the machines. The Germans recognised when they were constructing (in their view) an invulnerable set of machines, that of course in wartime they would be open to capture and therefore locally and temporarily they will be read.

But they also felt that the mere local (by which I mean reading one key instead of another key) and temporary reading (in other words that would complete your fundamental knowledge of the machine) wouldn't help you to read it regularly and daily. And they were right, without this machinery that would have been impossible.

And in particular it would be impossible because each of the Enigma and the Fish were used by the Germans as the basis not merely for one cipher each, not merely one Enigma and one Fish, but as the basis for a wide range of different ciphers, each cipher having its different key.

At one time the Germans were operating concurrently about fifty Enigmas, some in the army, some in the airforce, some in the navy, some in the railways, some in the secret service. And so you were faced not merely with understanding the machine and with breaking a key regularly, but with breaking fifty sometimes regularly at once, or as many of them as you could without delay.

And Fish similarly rose from just one link, one cipher, one key to about 22 cipher links, all quite separate except they were using the same machine. And remember that each of them - the Enigma daily and the Fish at varying interludes, usually every few days - changed the keys.

So you are on constant alert - every day you had to start again at midnight, and you had to start on perhaps 30 Enigmas or 5 Fishes and so you could see the mere load put it beyond any manual solution.

That was one reason, that in spite of their confidence which was not far from being fully justified, that was one reason why the German confidence was proved to be unfounded. The other was (perhaps it is no less important) the fact that steps were taken to avoid arousing enemy suspicion.

The British imposed strict secrecy of course on the Ultra production process. Strict regulations about its distribution - who should be indoctrinated - strict regulations against carelessness by users when using it.

Those regulations were pretty effective. There were from time to time cases in the war where the Germans did sufficiently suspect to have an enquiry. There were cases when the Italians suspected and advised the Germans to have an enquiry.

I ought to say that everything I have been saying about the complex nature of the attack on the ciphers hardly applied to the Italian ciphers - and this is where I am going to bring my ironical remark in about ciphers which will interest you people as machinery experts.

The Italians only ran one machine and it was a baby really compared with the Enigma. It was a machine built by a firm called Hagelin (we called it C-38). The American armed forces used it occasionally, but it was easily broken. We broke it. It didn't come into use until the beginning of '41 and we broke it by June '41.

It was a very valuable cipher for shipping in relation to North African operations but it wasn't a cryptanalytical problem of the kind I have been describing in the case of the German ciphers.

Ironically the Italians, except for that one cipher and also for one they used for their diplomats, didn't use machines. They used book ciphers, and ironically we couldn't read the Italian book ciphers for the army, the navy and the airforce after they brought new ones in between June and November 1940 preparatory to or as a consequence of their own entry into the war.

Book ciphers proved to be invulnerable when the machinery proved not to be invulnerable!

And as I say the Italians occasionally, who rather looked down on machine ciphers, warned the Germans that they thought that there was evidence that the way the allies behaved suggested that maybe they were reading the German ciphers. And the Germans said 'Pooh, pooh, we are alright!' Apart from occasional suspicions, those precautions I described, as used by the Allies, worked.

They were wholly justifiable ones. Any confirmation reaching the Germans that this whiff of suspicion that this system that they had constructed wasn't safe, would have led to not easy but not impossible steps to render it safe.

But of course the precautions complicated the task of establishing the value that Ultra had in the war.

Contemporary reports and the memoirs and histories that have been published before the records about Ultra became available, of course allow for, incorporate, the contribution Ultra made to decisions and the course of events, but they don't acknowledge it. Because either the writers of the reports and the memoirs didn't know about it, or they were not able to mention it.

So that historians now have to identify that contribution from the written records about the war and it is a straightforward job to do that now that Ultra is available. You can see - we know what Ultra went to what commands, we know what time it arrived, we know what other intelligence they had at the time the Ultra arrived, we know what decisions and orders followed from its arrival, and frequently we have discussions on record about what they thought about what they ought to do about it before they reached their decisions.

It is not enough to establish accurately the availability of the Ultra and to reach reasonable conclusions about its influence on British and American assessments and decisions. You have also got to consider the consequences of those assessments and decisions on the war.

Let me give an example of the distinction. Once you have identified the Ultra (which you now can from the decrypts in the Public Record Office) you can see pretty clearly (if you have also got the record of the war) that Ultra was the main reason why the British were able to reduce the depredations of the U- Boats in the Atlantic in the second half of 1941.

But what was the value of that effect in the North Atlantic in that second half of 1941 on the course of the war? What was the consequence of that use of Ultra on the course of the war? And those effects too are already incorporated into the record, which shows that the U-Boats were defeated in the North Atlantic in the second half of 1941.

Of course in order to assess the true significance of Ultra we have got to assume that it didn't exist in the North Atlantic at that time. We have got to strip it out of the record in order to get its true significance into focus.

This is what historians call counter-factual history. To calculate something assuming that some factors in it didn't exist. And I am sure it is a process well known to mathematicians and other people like yourselves.

But it is still the case that there is a great deal of danger in using counter-factual history unless you use it very carefully. For example it is very common among historians to use counter-factual history either from a desire to shock or because the user in question hasn't got any judgement. And you have therefore got to use it in relation to the possibilities that were practically available in the circumstances that you are considering.

There is no danger whatever in reconstructing the course of the war on the assumption that Ultra hadn't existed.

As I have said the story of its acquisition is of near legendary, even science fiction proportions, because it might so easily not have taken place. You are not making a huge assumption when you start playing with the record of the war on the basis that it hadn't been solved, it hadn't been obtained.

It was by no means fortuitous or miraculous. It was the consequence of forces deliberately brought together to solve it. But it was far from being inevitable that the forces would succeed. The proposition that we might have had to fight the war without Ultra is a reasonable and necessary element in the assessment of its true significance.

On the other hand if you apply counter-factual history and use this proposition that Ultra might not have existed, you are undertaking a pretty bold enterprise in hypothesis and speculation and you must control that exercise by a constant reference to the straightforward facts about what Ultra actually did do.

If you apply that check, then I think we can draw two pretty sound conclusions. First of all, though we did obtain it in such amounts, amounts rising to 2,000 of those Italian Hagelin decrypts a day at the peak of the Mediterranean War and to 30,000 a month rising to 90,000 a month of Enigma and Fish decrypts combined - that is a very big number of decrypts. It is still the case that those volumes and the speed with which they were got out were not fully established until the second half of 1941.

Up 'till June '41 the successes were confined to decrypts of the German airforce Enigma and some of the Italian book ciphers which were quite readable before Italy came into the war and for a month or two afterwards.

Those helped to produce isolated allied successes like the Battle of Matapan when we defeated the Italian fleet, and wouldn't have done so but for a few Enigma and Italian signals which gave enough warning to the British Alexandria fleet.

The distances in the Mediterranean are such that unless you have got some notice you can't cover the thousand miles or the seven hundred and fifty miles.

So Matapan was one success. The sinking of the Bismark was another. Again I am speaking of the period before June '41. She was sunk in May '41 just before the turn. The defeat of the Italians in East Africa and in North Africa. Those were Allied successes, but they were slightly isolated successes.

Again in the same period Ultra did something to mitigate British disasters. It greatly assisted the British forces that were sent to Greece, to retreat without serious loss when it become obvious that they couldn't hold a line against the scale of the German invasion.

It gave us - here was another disaster - all the information required to destroy the German attack on Crete. We didn't destroy the attack but we made it an extremely damaging exercise for the Germans, which was done because the Ultra signals were so complete.

Some people think we should have prevented or destroyed the invasion - an air landing invasion. In fact Bletchley Park felt very strongly for the first time in the war that its product had not been used properly in the case of the Crete invasion. I think possibly that we were wrong now that we can see the evidence in more detail, but at least it helped to make it a disastrous operation for the Germans even though they actually got Crete as a consequence.

And so in all that story you can see that the British survived the war with little benefit from Intelligence until the Germans invaded Soviet Russia. And since Soviet Russia survived the German invasion, and that invasion was followed by the entry of the United States in December '41, we can safely conclude that Germany was going to be defeated in the long run, even if the enormous expansion of Ultra from the summer of 1941 had not from that date given the Allies this massive superiority in Intelligence which they retained until the end of the war.

They were hardly ever rivalled by Axis success in reading our ciphers. There were two major exceptions to the lack of success by the Axis against Allied ciphers. One was that they did have some success in reading a British naval cipher which was for a longish time also shared with the American navy in relation to convoy escorting.

They were successful in reading that for a long period from 1940 to the end of '42. And the other was that they didn't exactly capture but they managed to extract of copy of the cipher that was being used by the American Military Attache in Cairo for a period when Rommel was at his most dangerous. And from that too the Germans obtained some great advantage.

But generally speaking, except possibly in relation to the convoy cipher, there was never any great cryptanalytical rivalry. The Germans were completely outclassed in terms of Ultra. The Italians also made very little progress against any important allied cipher.

In June 1941 however, (we survived 'till then with very little value from the Ultra), the end of the war still four years away. And that is such a length of time that we might be tempted to jump to the other conclusion and say that far from producing by itself on its own the defeat of the Axis, it made only a marginal contribution to it.

Here we are, we start getting this Ultra coming onto stream in June '41 as opposed to the slight trickle before that date, and yet you have still got four years of war. How can it have made much difference?

But that second conclusion can I think be as firmly dismissed as the one I have been discussing about how Ultra didn't really win the war.

The second real conclusion that stands out is that Ultra was decisive in shortening the war from the time, beginning in the summer of 1941, the cryptanalytical successes were extended from the German airforce Enigma keys to the Enigmas used by the navy and the army and the secret service, to the non-morse ciphers of the German High Command which came on stream in mid 1941, and to a new Italian machine cipher, the one I have mentioned which also was brought into force beginning of '41 and broken in the summer of '41. And to the ciphers of the Italian and German and especially Japanese Embassies.

The Japanese Embassies in Europe were in the second half of the war to prove of immense Intelligence value because they were repeating back to Tokyo their versions of German assessments and their knowledge of German intentions. They were almost as valuable on some subjects (like for example the Normandy Landings) as were the direct Ultra from the German horse's mouth.

From the moment we began that expansion you can see that the influence is continuous. I have spoken of the amount of Ultra there was. The lack of delay, the fact that they were obtained with very little delay was equally important. After all, one of the crucial characteristics of Intelligence is that to be useful it must be quick.

In the case of the Enigmas we didn't exactly reach a position in which the new keys, having come into force at midnight, were broken by breakfast, but of, shall we say, twenty, twenty five Enigmas running concurrently (the number varied according to different stages of the war), we would be reading twenty to twenty five at most times. Of that twenty to twenty five the ones to which highest priority was given on the limited number of Bombes available would be out by breakfast. Which meant that the whole of the rest of that 24 hours' signals from the moment you broke the key for the day, the setting for the day, would be read instantaneously, as soon as the message was intercepted it would be decrypted.

Fish was a bit slower. It didn't change daily like the Enigma. It varied, on different links it changed with different frequency. The average was that it changed about once every five or six days - the setting of the keys changed every five or six days.

Again the number we read varied from time to time but from the end of '42 when Fish on the German side got going strongly, we were generally reading four or five Fish links at any one time.

Generally we were reading them about seven, six, five days late - after their transmission. That didn't matter with Fish (at least it didn't matter so much as it would have done with Enigma) because, whereas the Enigma like that Italian machine was used for what you might call operational purposes below army level for something that was happening tomorrow or happening today, Fish was reserved for communications between the highest commands of the German Armed Forces. Between Berlin and the army groups or the armies. And then never lower than armies. It was a system that increasingly replaced the landline transmissions between Berlin and Kesselring in Rome and Von Rundstedt who was commander in the west in Paris by 1943.

They were on landlines normally but gradually with Fish being perfected as they thought and with landlines being damaged by bombing they put more onto the air.

So Fish was carrying Intelligence of a character that didn't really depend for its value on immediacy. It would carry long term estimates, or it would carry prolonged discussions between German Headquarters on the Russian Front or in Italy and Berlin about what was the best thing to do next. So you didn't have the immediacy requirement there as you did with the Enigma.

Speaking on the whole then we can see the fact that we were getting Ultra in the amounts I spoke of and with the speed that I emphasised as being a very important characteristic of valuable Intelligence. It was no use having Enigma a week late, and it wouldn't be much use having Fish more than a month late.

If you had that amount of decrypts with that small amount of delay, it would, I think, on the face of it, be surprising if Ultra hadn't contributed to the very considerable shortening of the war, given the fact that on the other side the enemy is blind and his Intelligence is increasingly deteriorating because of the Allied possession of the superiority in Intelligence.

I will give you an example of that. We read all the Enigma signals of the German Abwehr which meant that we captured every spy that arrived in the United Kingdom by having advance knowledge of his arrival. Which meant that we could turn such as we needed and use them to send messages we wanted the Abwehr to receive, and monitor the reception and the reaction of the Abwehr. All that signal Intelligence underlay the effective use of what was called the Doublecross Operation for the purposes both of stopping German reception of Intelligence (other than false Intelligence) and also of creating deception by sending them false Intelligence.

So given that they were so blind and we were getting this increasing amount with less and less delay, it would be surprising if it hadn't, from the middle of '41, contributed pretty appreciably to the difficulties of the enemy and to the accurate appreciations of the Allies.

Now the question remains how much did it shorten the war, leaving aside the contribution made to the campaigns in the Far East on which the necessary work hasn't been done yet. My own conclusion is that it shortened the war by not less that two years and probably by four years - that is the war in the Atlantic, the Mediterranean and Europe.

The detailed answer for those theatres begins in the Mediterranean. There, in the autumn of 1941 against Rommel it turned almost certain defeat into a stalemate. If not then, then certainly in the summer of 1941 after Rommel had returned to the Egyptian frontier, it made a decisive contribution to keeping him out of Egypt between his victory at the Battle of Gazala in 1942 and the British getting ready for their own victory at El Alamein.

It did this chiefly by killing off his seaborne supplies. Both the Italian machine cipher and the airforce Enigma and a bit of naval Enigma contributed decisively to starving Rommel of fuel and replacement hardware and ammunition.

Without that, the commander of our own forces at the time, General Auchinleck, concluded that Rommel would have got through to Egypt.

As you know at that time the Allies themselves were landing in North West Africa. If they had lost Egypt they might have abandoned the operation against North West Africa, especially as they would also have lost Malta if Egypt went, and decided to alter their strategy (we have to allow for this possibility) and go back and concentrate on the North Sea and the direct Second Front.

Now if they had stayed in the Mediterranean it would have taken them a least a year longer than it actually took them from Tunis and from the Western Desert to complete the conquest of North Africa and open the Mediterranean. That was successfully achieved in May '43.

It wouldn't have been achieved in less than a year beyond that, if we had gone on in spite of the loss of Egypt trying to do it. It wouldn't have been achieved in time to do the Normandy landings in 1944.

If they had abandoned the idea of re-conquering North Africa, the most probable course would have been in fact what the Americans had always wanted to do, to do the cross Channel invasion more quickly than in fact occurred. It in fact occurred in June '44. They would have turned back and done that straight away, that was their obvious alternative.

What would have been the prospect for that undertaking if Ultra hadn't become available against the U-Boats in June 1941 and radically reduced their successes against the convoys.

We know that in that second half of 1941 their shipping successes were cut back to 120,000 tons a month average. That has to be compared not with the monthly average of 280,000 tons a months in the four months before June '41 but with the sinkings they would have achieved with their greater number of U-Boats.

It has been calculated that the Ultra saved about one and half million tons in September, October, November and December '41.

And even if Britain's essential imports had not without that reduction been reduced to a dangerously low level, the intermission that provided was invaluable in enabling the British to build up reserves in merchant shipping and develop anti-submarine defences.

So that when the U-Boats returned to the Atlantic after their first defeat (they did that in the autumn of 1942), they had been delayed in making a decisive thrust for more than a year. Now when they returned they had been supplied with an advanced Enigma, one that instead of using three wheels concurrently used four wheels, which as you can see noticeably increased the mathematical difficulties of solving the key.

In fact Bletchley couldn't solve it from February to December 1942. Mercifully for us (though not for the Americans) most of the U-Boats were on the Atlantic American coast at that time, but as they came back to the North Atlantic convoys they were still using this cipher and they brought about another crisis in the Atlantic.

It again was the Ultra which brought them under control. The figures of sinkings of Allied shipping reached the highest in the war in March '43. They had been brought down by May '43 to lower proportions than ever before in the war as a result of this return of Ultra to the scene.

And so you can see that the problem of undertaking the Normandy landings if those two defeats and controls of the U-Boats hadn't occurred would have been very pronounced.

Then there was the contribution of Enigma to the Normandy Landings themselves (I can't go into detail and will answer questions if you need). I think it is no exaggeration to say that even if the U-Boats had prevented it from being attempted only until '45, we would have found it an infinitely more difficult operation to do than in 1944. The Germans would have completed the Atlantic defences, they would be bombarding Britain with 'V' weapons on a massive scale, all of which was in the event cut off by the '44 Landings. And they would have had a much bigger Panzer Army to deal with the problems.

My own calculation is we wouldn't in fact have been able to do the Normandy Landings, even if we had left the Mediterranean aside, until at the earliest 1946, probably a bit later. It would have then taken much longer to break through in France and Germany than it did in fact take, which was a year from '44. And altogether therefore the war would have been something like two years longer, perhaps three years longer, possibly four years longer than it was.

I am sorry I have exceeded my length of time but I hope you will forgive me, and I will do what I can to answer questions.

Q. Would we have won the war without Ultra?

My own view is that given that the Soviets survived the German attack and the Americans came in as they did, the combined forces of Russia, America and the British would eventually have won the war. The long term relative strengths of Germany and those three counties were such that Germany was bound to loose in the end. But how lengthily and with what damage and destruction we should have succeeded I don't know. I think we would have won but it would have been a long and much more brutal and destructive war.

Q. Was Bletchley involved at all in cryptanalysis of the Russian theatre?

We read a large amount of German signals from the Russian front, but no work was done against Soviet signals after Germany invaded Russia, on account of the high priority given to Axis signals. On the other hand, co-operation with the Soviets was never as close as it was with the USA.

Of course when the Americans came into the war in December '41 we had already begun some development of a cryptanalytical partnership with them, and when they came into the war that partnership became almost so complete as to constitute a single joint cryptanalytical effort. Of course that effort involved division of labour and the division of labour is much directed by the interception facilities. For example, except for the Atlantic traffic the American coast couldn't intercept European, German and Italian signals. That was all being intercepted in the UK. Obvious solution - UK concentrates on decrypting, on cryptanalysis against German and Italian. America which can intercept the Pacific from the Pacific and also has headquarters in Brisbane and various places in the Pacific - America concentrates of working on the Japanese.

But there are overlaps. For example we have a cryptanalytical annexe at Bletchley, we have it in Singapore, it moved to Hong Kong, it moved to Ceylon, and from there it pitches in its bit by serving the decrypts direct to the American Headquarters. Similarly because the U-Boat traffic can be heard both in America and in Britain, the two sides - Bletchley and the American Navy - swopped keys. They have got a direct line. They say 'we will take 4th June, you take tomorrow 5th June,' and so they split the keys and swopped solutions. So there was an almost total amalgamation of resources and a logical division of labour.

Q. Is it not the case that the arrival of the atom bomb in 1945 would have bought a quicker solution?

This is a problem because strict, sensible, proper counter-factual history can't really take into account something like that. It is speculation. But of course if my scenario is right and the war was still struggling on and we had the bomb which presumably we would still have had, the problem of whether to drop it on Germany would have arisen. And in some respects the dropping of it on Germany would be more justified than the dropping of it on Japan because Japan was visibly on her knees when we dropped it on her, but in my scenario Germany would have been far from on her knees. So yes the prospects of it being dropped as the solution are quite high. I would mention it in a speculative scenario.

Q. How closely did Bletchley work with the Russians on decryption?

It couldn't be as close as the collaboration I have described with the Americans for a variety of reasons. One is of course that there just hadn't been the close relationship between the two countries that existed historically between the British and the Americans. The other was that when we actually broke the ciphers - Enigma in the first instance, but Fish later - that were relevant to the Eastern Front, they were coming in to us at a time when it was uncertain whether Russia would survive. And then later on when Russia had survived and we were reading more ciphers both Fish and Enigma from the Eastern Front, there was the problem that we knew from the Enigma that the Germans were reading Russian ciphers, so that if they had too much Enigma intelligence in their ciphers you see the security risk was extremely high. Then fourthly the Russians were not collaborative. They wanted any intelligence we supplied but they wouldn't give any in return. Not that they had much Sigint, but they had a lot of other Intelligence.

The answer to your question is with all those difficulties we couldn't have so close a collaboration with the Russians as we had with Washington, but we started sending them a summary of signal intelligence a week after they were attacked by the Germans in '41. We sent it via the British Military Mission in Moscow where there were people to hand it over to the Russian General Staff.

We had to have a cover for it, had to explain to them that this is the horse's mouth but it is coming to us (this is the kind of cover we used) it is coming to us from very high ranking German officers who are slipping the news to us through Berne or somewhere like that, and we are getting it quickly because we have got pretty direct connections with Switzerland.

A steady stream of information about German intentions and dispositions - Airforce and Army on the Eastern Front - were sent to them. They were interrupted from time to time when the Russians were being particularly beastly. For example at the top of Russia, Murmansk at the Kola Inlet where all the convoys taking arms to Russia and supplies were going, we had to keep seamen, sailors, both to man the Allied facilities, unloading facilities - of course the Russians were there too but we had to keep some British sailors there. And then we persuaded the Russians to let us have an Intercept station there because half of the traffic around the top of the North Cape was difficult to intercept even in Northern Scotland.

And of course we covered the risk that they would suspect that we were reading Enigma or that they would do more than suspect that, by saying that the value of the traffic to us was that it enabled us, by traffic analysis, to judge the German reactions to the movements of the convoys. So we had this little intercept station and then the Russians locked them all up because they thought we were spying on them, so you had all sorts of little rows with them like that. From time to time when it wasn't vital we did say if you don't behave better than this we wont send you your daily summary. And we stopped it for a short time, then we started again. But it had to be of that character - the collaboration.

Q. Is there scope for counter-factual historians studying the siege of Leningrad - if they had had access to the Ultra information that you could have given them.

Again you have to bear in mind that there were problems. For example, one of the areas in which we found it extremely difficult to intercept German signals because of radio conditions or atmosphere conditions or whatever it is, was the North Cape; and the other was the Leningrad area. It was very difficult to intercept from the Leningrad area because whatever frequency they were using relative to the distances and the ionosphere we never could cover the Leningrad area properly. Caucaucus on the other hand, Central Front, we could hear then as clear as a bell.

Q. If the collaboration had been as close as with the Americans . . .

It would have been an advantage if it had been as close as with the Americans, it is quite true. But on the other hand the risks which I briefly portrayed were quite considerable. And we did our best to make sure that they knew about all the important forthcoming development. Don't forget they had very good intelligence of their own, not primarily Sigint but they had very good air reconnaissance and air superiority after a certain time, and they had an enormous espionage system behind the German lines. So they weren't without information. But we did do our best to make sure that they got crucial early notice whenever we got it ourselves.

It was a big dilemma and one that was fought about. Churchill wanted to risk it and let them have more. Naturally the Ultra authorities didn't want to risk it because everything hangs on it you see, so there was a tussle all the time about how much to send.

Q. The was a programme recently on Kursk - one might say that a Russian counter-factual historian would say that if we didn't have the Ultra which we got in various ways, then we wouldn't have been able to win the battle of Kursk and Hitler would have been able to carve up Russia. This is perhaps another case . . .

Another case. Stalingrad of course is another one. Those two battles were crucial, especially Stalingrad. Again it wasn't only through us they were getting . . . we did give them the central facts in advance of Kursk. But as we now know, we didn't know at the time, the one single Russian agent in Bletchley was at that time (just that short period of time before and after Kursk in '43) actually giving them decrypts through the Russian Embassy in London. So all sorts of complications about the story. He didn't know that they were getting the supply from London officially, and we didn't know that he was sending the decrypts unofficially. Quite a complex problem!

Q. Did they never figure out that this was coming from decrypts?

We are never quite clear. Certainly when this man at Bletchley who only surfaced after the war - the secret about him only surfaced after the war - they knew that what they were getting from him was decrypts. They must then have known that our summaries were decrypts. But that didn't alter our practice because we didn't know that he was sending them the decrypts.

Q. How was the Ultra Information disguised so that the Germans couldn't work out that you were decrypting them?

How did we disguise what we had got you mean?

Q. How did you disguise, for example, that a particular submarine was going through a particular area? How could you disguise it so that it wasn't obvious that you'd intercepted it?

Let me give you an example of how we took the precautions, using it without on the other hand giving grounds for suspicion to the other side. The most dramatic example comes from the Mediterranean where we sank at two or three stages in the war something from 40% to 60% of every ship that left the north shore of the Mediterranean for North Africa. 60% of the shipping was sunk, for example, just before the Battle of Alamein and again just before Gazala when Rommel was stopped.

Every one of those ships before it was attacked and sunk had to be sighted by a British aeroplane or submarine which had been put in a position in which it would sight it without it knowing that it had been put in that position, and had made a sighting signal which the Germans and the Italians had intercepted. That was the standard procedure. As a consequence of that the Germans and the Italians assumed that we had 400 submarines whereas we had 25. And they assumed that we had a huge reconnaissance airforce on Malta, whereas we had three aeroplanes!

But solemnly that procedure had to be followed by the commanders. When they in their little centre in Cairo or, as it was later on Algiers, said we can't sink all those seventeen ships today, which five are we going to take first and which five will we take second, when they were doing this they had to arrange that procedure before they hit a single ship.

Similar precautions were taken in the Atlantic, but there the problem was different. That is why the Germans got most suspicious about the Atlantic. The great feature there was that the Enigma was used in the first instance not to fight the U- Boats but to evade them. And the problem was how could you evade them without their noticing. You have a situation on the graph in which the number of U-Boats at sea in the Atlantic is going up, and the number of convoys they see is going down!

How do you cover that? We did cover it but it was done by a different system from what I have just described in the Mediterranean. We let captured Germans, people we had captured from U-Boats write home from prison camp and we instructed our people when interrogated by Germans - our pilots for example - to propagate the view that we had absolutely miraculous radar which could detect a U-Boat even if it was submerged from hundreds of miles. And the Germans believed it.

They had an enquiry saying 'surely it must be possible that it is the Enigma that isn't safe.' And the cipher men come back and say 'it can't be the Enigma.' So somebody gets up and says 'well, it must be this bloody radar that we have heard about.' And so they decided. But you see different solutions had to be adopted for each particular situation. But these were the kind of precautions that were taken I think with great success. I mean they never really did tumble to the idea that it was unsafe, which is pretty marvellous really.