The Influence of ULTRA in the Second World War

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The text of a speech presented by Sir Harry Hinsley, wartime codebreaker, at CambridgeUniversity's Computer Security roup!

The Influence of ULTRA in the Second World War" Changed #$th %ovember &$ (

)ast year Sir Harry Hinsley kindly agreed to speak about *letchley +ark, where he worked duringthe Second orld ar! e are pleased to present a transcript of his talk!

Sir Harry Hinsley is a distinguished historian who during the Second orld ar worked at*letchley +ark, where much of the allied forces code-breaking effort took place! e are pleasedto include here a transcript of his talk, and would like to thank Susan Cheesman for typing the firstdraft and .eith )ockstone for adding Sir Harry's comments and amendments!

Security Group Seminar

Speaker: Sir Harry Hinsley

Date: Tuesday 19th October 1993

Place: Babbage Lecture Theatre !o"puter Laboratory

Title: The #n$luence o$ %LT&' in the Second (orld (ar

Ross Anderson:/t is a great pleasure to introduce today's speaker, Sir Harry Hinsley, whoactually worked at *letchley from &0 to &1$ and then came back to Cambridge and became+rofessor of the History of /nternational 2elations and 3aster of St 4ohn's College! He is also theofficial historian of *ritish /ntelligence in orld ar //, and he is going to talk to us today aboutUltra!

Sir Harry:5s you have heard /'ve been asked to talk about Ultra and / shall say something aboutboth sides of it, namely about the cryptanalysis and then on the other hand about the use of theproduct - of course Ultra was the name given to the product!

5nd / ought to begin by warning you, therefore, that / am not myself a mathematical or technicalexpert! / was privileged to be an assistant to the mathematicians led by 3ax %ewman and 5lanTuring, but / have never myself learned to master or even to approach mastering their art!

Ultra, of course, was the product of ciphers! /t was used only for the product of the metering of themore important ciphers, and from the spring of &1& at *letchley we broke most ciphers to anunprecedented extent and with an unprecedented lack of delay! 5nd there were two reasons forthat success!

6irst of all, alone among the governments of those years, the *ritish overnment as early as the's concentrated all its cryptanalytical effort in one place which it called the overnment Codeand Cipher School! 5nd at *letchley secondly, the staff rose from about in &0 to about8,777 at the beginning of &11!

9f course that staff was not entirely cryptanalytical, it consisted also of an immense amount ofstaff used, for example, for signalling the products to commands in the rest of :ngland or abroad!

5nd so it wasn't entirely cryptanalytical staff - it was a very mixed staff compared with pre-war!


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/n addition, those men and women, recruited mainly from universities, developed methods andmachinery of a sophistication hitherto undreamt of, including as you all know the first operationalelectronic computer which was called Colossus!

ithout those advances, at least the most difficult of the ciphers, which were ;although / will makesome


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assets - essentially captured material!

That was the :nigma! %ow with the non-morse machine which we called 6ish, the first successeswere again obtained by hand methods! 5nd those hand methods by mathematicians againexploited erman operators' errors! /n fact, the actual understanding of the machine theoretically;in other words as opposed to breaking it every day=, the actual understanding of how it workedwas obtained because the ermans went through a long series of experiments with it on the airbefore they brought it into operational use!

5nd it was these experimental transmissions which primarily were the errors which gave theentree!

*ut it was obvious that any regular or at all reasonably speedy decryption would be impossible ifagain machine methods were not developed! /n particular that they would be impossible withoutmachines because the different 6ish ciphers proliferated, >ust like the different :nigma keysproliferated, and you were dealing with a lot of ciphers concurrently!

5nd so in this case, as you know, the machine was developed which came to be called Colossus!

/t was of course a much more complex - it wasn't a mere electronic electro-mechanical thing - it

was the first computer! /t had to be like that in proportion to the fact that the 6ish was far morecomplex than the :nigma!

%ow it will be clear from what / have said that the problem wasn't merely to master the machines!The ermans recognised when they were constructing ;in their view= an invulnerable set ofmachines, that of course in wartime they would be open to capture and therefore locally andtemporarily they will be read!

*ut they also felt that the mere local ;by which / mean reading one key instead of another key=and temporary reading ;in other words that would complete your fundamental knowledge of themachine= wouldn't help you to read it regularly and daily! 5nd they were right, without thismachinery that would have been impossible!

5nd in particular it would be impossible because each of the :nigma and the 6ish were used bythe ermans as the basis not merely for one cipher each, not merely one :nigma and one 6ish,but as the basis for a wide range of different ciphers, each cipher having its different key!

5t one time the ermans were operating concurrently about fifty :nigmas, some in the army,some in the airforce, some in the navy, some in the railways, some in the secret service! 5nd soyou were faced not merely with understanding the machine and with breaking a key regularly, butwith breaking fifty sometimes regularly at once, or as many of them as you could without delay!

5nd 6ish similarly rose from >ust one link, one cipher, one key to about ## cipher links, all ustified,that was one reason why the erman 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 *ritish imposed strict secrecy of course on the Ultra production process! Strict regulationsabout its distribution - who should be indoctrinated - strict regulations against carelessness by


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users when using it!

Those regulations were pretty effective! There were from time to time cases in the war where theermans did sufficiently suspect to have an en


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record of the war= that Ultra was the main reason why the *ritish were able to reduce thedepredations of the U- *oats in the 5tlantic in the second half of &1&!

*ut what was the value of that effect in the %orth 5tlantic in that second half of &1& on thecourse of the war hat was the conse


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cover the thousand miles or the seven hundred and fifty miles!

So 3atapan was one success! The sinking of the *ismark was another! 5gain / am speaking ofthe period before 4une '1&! She was sunk in 3ay '1& >ust before the turn! The defeat of the/talians in :ast 5frica and in %orth 5frica! Those were 5llied successes, but they were slightlyisolated successes!

5gain in the same period Ultra did something to mitigate *ritish disasters! /t greatly assisted the*ritish forces that were sent to reece, to retreat without serious loss when it become obviousthat they couldn't hold a line against the scale of the erman invasion!

/t gave us - here was another disaster - all the information reorexceptions to the lack of success by the 5xis against 5llied ciphers! 9ne was that they did havesome success in reading a *ritish naval cipher which was for a longish time also shared with the

5merican navy in relation to convoy escorting!

They were successful in reading that for a long period from &17 to the end of '1#! 5nd the otherwas that they didn't exactly capture but they managed to extract of copy of the cipher that wasbeing used by the 5merican 3ilitary 5ttache in Cairo for a period when 2ommel was at his mostdangerous! 5nd from that too the ermans obtained some great advantage!

*ut generally speaking, except possibly in relation to the convoy cipher, there was never anygreat cryptanalytical rivalry! The ermans were completely outclassed in terms of Ultra! The/talians also made very little progress against any important allied cipher!

/n 4une &1& however, ;we survived 'till then with very little value from the Ultra=, the end of thewar still four years away! 5nd that is such a length of time that we might be tempted to >ump to theother conclusion and say that far from producing by itself on its own the defeat of the 5xis, itmade only a marginal contribution to it!

Here we are, we start getting this Ultra coming onto stream in 4une '1& as opposed to the slighttrickle before that date, and yet you have still got four years of war! How can it have made muchdifference

*ut that second conclusion can / think be as firmly dismissed as the one / have been discussingabout how Ultra didn't really win the war!

The second real conclusion that stands out is that Ultra was decisive in shortening the war fromthe time, beginning in the summer of &1&, the cryptanalytical successes were extended from the


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erman airforce :nigma keys to the :nigmas used by the navy and the army and the secretservice, to the non-morse ciphers of the erman High Command which came on stream in mid&1&, and to a new /talian machine cipher, the one / have mentioned which also was brought intoforce beginning of '1& and broken in the summer of '1&! 5nd to the ciphers of the /talian anderman and especially 4apanese :mbassies!

The 4apanese :mbassies in :urope were in the second half of the war to prove of immense/ntelligence value because they were repeating back to Tokyo their versions of ermanassessments and their knowledge of erman intentions! They were almost as valuable on somesub>ects ;like for example the %ormandy )andings= as were the direct Ultra from the ermanhorse's mouth!

6rom the moment we began that expansion you can see that the influence is continuous! / havespoken of the amount of Ultra there was! The lack of delay, the fact that they were obtained withvery little delay was e


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it, be surprising if Ultra hadn't contributed to the very considerable shortening of the war, given thefact that on the other side the enemy is blind and his /ntelligence is increasingly deterioratingbecause of the 5llied possession of the superiority in /ntelligence!

/ will give you an example of that! e read all the :nigma signals of the erman 5bwehr whichmeant that we captured every spy that arrived in the United .ingdom by having advanceknowledge of his arrival! hich meant that we could turn such as we needed and use them tosend messages we wanted the 5bwehr to receive, and monitor the reception and the reaction ofthe 5bwehr! 5ll that signal /ntelligence underlay the effective use of what was called theDoublecross 9peration for the purposes both of stopping erman reception of /ntelligence ;otherthan false /ntelligence= and also of creating deception by sending them false /ntelligence!

So given that they were so blind and we were getting this increasing amount with less and lessdelay, it would be surprising if it hadn't, from the middle of '1&, contributed pretty appreciably tothe difficulties of the enemy and to the accurate appreciations of the 5llies!

%ow the


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a month average! That has to be compared not with the monthly average of #@7,777 tons amonths in the four months before 4une '1& but with the sinkings they would have achieved withtheir greater number of U-*oats!

/t has been calculated that the Ultra saved about one and half million tons in September, 9ctober,%ovember and December '1&!

5nd even if *ritain's essential imports had not without that reduction been reduced to adangerously low level, the intermission that provided was invaluable in enabling the *ritish tobuild up reserves in merchant shipping and develop anti-submarine defences!

So that when the U-*oats returned to the 5tlantic after their first defeat ;they did that in theautumn of &1#=, they had been delayed in making a decisive thrust for more than a year! %owwhen they returned they had been supplied with an advanced :nigma, one that instead of usingthree wheels concurrently used four wheels, which as you can see noticeably increased themathematical difficulties of solving the key!

/n fact *letchley couldn't solve it from 6ebruary to December &1#! 3ercifully for us ;though notfor the 5mericans= most of the U-*oats were on the 5tlantic 5merican coast at that time, but asthey came back to the %orth 5tlantic convoys they were still using this cipher and they brought

about another crisis in the 5tlantic!

/t again was the Ultra which brought them under control! The figures of sinkings of 5llied shippingreached the highest in the war in 3arch '10! They had been brought down by 3ay '10 to lowerproportions than ever before in the war as a result of this return of Ultra to the scene!

5nd so you can see that the problem of undertaking the %ormandy landings if those two defeatsand controls of the U-*oats hadn't occurred would have been very pronounced!

Then there was the contribution of :nigma to the %ormandy )andings themselves ;/ can't go intodetail and will answer


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e read a large amount of erman signals from the 2ussian front, but no work was done againstSoviet signals after ermany invaded 2ussia, on account of the high priority given to 5xis signals!9n the other hand, co-operation with the Soviets was never as close as it was with the US5!

9f course when the 5mericans came into the war in December '1& we had already begun somedevelopment of a cryptanalytical partnership with them, and when they came into the war thatpartnership became almost so complete as to constitute a single >oint cryptanalytical effort! 9fcourse that effort involved division of labour and the division of labour is much directed by theinterception facilities! 6or example, except for the 5tlantic traffic the 5merican coast couldn'tintercept :uropean, erman and /talian signals! That was all being intercepted in the U.!9bvious solution - U. concentrates on decrypting, on cryptanalysis against erman and /talian!

5merica which can intercept the +acific from the +acific and also has headust hadn't been the close relationship between the twocountries that existed historically between the *ritish and the 5mericans! The other was thatwhen we actually broke the ciphers - :nigma in the first instance, but 6ish later - that wererelevant to the :astern 6ront, they were coming in to us at a time when it was uncertain whether2ussia would survive! 5nd then later on when 2ussia had survived and we were reading moreciphers both 6ish and :nigma from the :astern 6ront, there was the problem that we knew fromthe :nigma that the ermans were reading 2ussian ciphers, so that if they had too much :nigmaintelligence in their ciphers you see the security risk was extremely high! Then fourthly the

2ussians were not collaborative! They wanted any intelligence we supplied but they wouldn't giveany in return! %ot that they had much Sigint, but they had a lot of other /ntelligence!

The answer to your


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who are slipping the news to us through *erne or somewhere like that, and we are getting itudge the erman reactions to the movements of the convoys!So we had this little intercept station and then the 2ussians locked them all up because theythought we were spying on them, so you had all sorts of little rows with them like that! 6rom timeto time when it wasn't vital we did say if you don't behave better than this we wont send you yourdaily summary! 5nd we stopped it for a short time, then we started again! *ut it had to be of thatcharacter - the collaboration!

G! /s there scope for counter-factual historians studying the siege of )eningrad - if they had hadaccess to the Ultra information that you could have given them!

5gain you have to bear in mind that there were problems! 6or example, one of the areas in whichwe found it extremely difficult to intercept erman signals because of radio conditions oratmosphere conditions or whatever it is, was the %orth Cape and the other was the )eningradarea! /t was very difficult to intercept from the )eningrad area because whatever fre


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officially, and we didn't know that he was sending the decrypts unofficially! Guite a complexproblemA

G! Did they never figure out that this was coming from decrypts

e are never ust described in the 3editerranean! e let captured ermans, people we had captured from U-

*oats write home from prison camp and we instructed our people when interrogated by ermans- our pilots for example - to propagate the view that we had absolutely miraculous radar whichcould detect a U-*oat even if it was submerged from hundreds of miles! 5nd the ermansbelieved it!

They had an en


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was unsafe, which is pretty marvellous really!

The Influence of ULTRA in the Second World War

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