The Bletchley Park Codebreakers
Praise for Action this Day
‘Absolutely the best book ever written about codebreaking at Bletchley Park.’ Louis Kruh, Editor, Cryptologia
‘A sizeable contribution to the informed understanding of a world war.’ M. R. D. Foot, Times Literary Supplement
‘Peppered with priceless anecdotal and technical detail. Fascinating stuff.’ New Scientist
‘This gem … offers a pleasing combination of scholarship and memoirs.’ Mark E. Stout, The CIA’s Studies in Intelligence
‘Greatly extends our knowledge about the work done at BP [Bletchley Park].’ Jürgen Rohwer, Journal of Intelligence History
THE BLETCHLEY PARK
CODEBREAKERS
HOW ULTRA SHORTENED THE WAR AND
LED TO THE BIRTH OF THE COMPUTER
EDITED BY RALPH ERSKINE
AND MICHAEL SMITH
Originally published as Action This Day: From the Breaking of the Enigma Code to the Birth of the Modern Computer
– Bantam Press, 2001
(This edition is updated with a new chapter and additional material.)
CONTENTS
Title Page
Acknowledgements
Letter from the Bletchley Park codebreakers to Churchill
Dramatis Personae
1. Bletchley Park in Pre-War Perspective
Christopher Andrew
2. The Government Code and Cypher School and the First Cold War
Michael Smith
3. Reminiscences on the Enigma
Hugh Foss
4. Breaking Air Force and Army Enigma
Ralph Erskine
5. Hut 6 From the Inside
Derek Taunt
6. Breaking Italian Naval Enigma
Mavis Batey
7. A Biographical Fragment: 1942–5
John Chadwick
8. An Undervalued Effort: How the British Broke Japan’s Codes
Michael Smith
9. Solving JN-25 at Bletchley Park: 1943–5
Edward Simpson
10. Most Helpful and Co-operative: GC&CS and the Development of American Diplomatic Cryptanalysis, 1941–2
David Alvarez
11. Breaking German Naval Enigma on Both Sides of the Atlantic
Ralph Erskine
12. Hut 8 From the Inside
Rolf Noskwith
13. Bletchley Park and the Birth of the Very Special Relationship
Stephen Budiansky
14. Mihailović or Tito? How the Codebreakers Helped Churchill Choose
John Cripps
15. Traffic Analysis: A Log-reader’s Tale
James W. Thirsk
16. Bletchley Park, Double Cross and D-Day
Michael Smith
17. How Dilly Knox And His Girls Broke
the Abwehr Enigma
Keith Batey
18. Breaking Tunny and the Birth of Colossus
Shaun Wylie
19. Colossus and the Dawning of the Computer Age
B. Jack Copeland
20. Enigma’s Security: What the Germans Really Knew
Ralph Erskine
21. From Amateurs to Professionals: GC&CS and Institution-Building in Signals Intelligence
Philip H. J. Davies
22. Cold War Codebreaking and Beyond: the Legacy of Bletchley Park
Richard J. Aldrich
23. Bletchley Park in Post-War Perspective
Christopher Andrew
Appendix I: The very simple cipher which ‘Snow’, the first Double Cross agent, was given by his German controllers
Appendix II: Wehrmacht Enigma Indicating Systems, except the Kriegsmarine’s Kenngruppenbuch System
Appendix III: The Naval Enigma Kenngruppenbuch Indicator System – used with the main wartime ciphers
Appendix IV: Cillies
Appendix V: Enciphering by JN-25
Appendix VI: Recovery by Differencing
Appendix VII: Bayes, Hall’s Weights and the Standardising of Judgements
Notes and references
Glossary and abreviations
Notes on Contributors
Index
Plates
Copyright
ACKNOWLEDGEMENTS
The editors are grateful to the contributors for so readily agreeing to donate their work to the Bletchley Park Trust (which is a registered charity), and for meeting a very demanding timetable without complaint.
Ralph Erskine is indebted to his wife, Joan, for her forbearance when he is engrossed in Sigint history, especially during the editing of this book. Michael Smith would like to thank his wife, Hayley, and his family for their patience. The editors found the project a rewarding one, and hope that readers will enjoy the result.
On 21 October 1941, four of the leading codebreakers at Bletchley Park wrote to the Prime Minister, Winston Churchill:
Secret and Confidential
Prime Minister only
Hut 6 and Hut 8,
(Bletchley Park) 21st October 1941
Dear Prime Minister,
Some weeks ago you paid us the honour of a visit, and we believe that you regard our work as important. You will have seen that, thanks largely to the energy and foresight of Commander Travis, we have been well supplied with the ‘bombes’ for the breaking of the German Enigma codes. We think, however, that you ought to know that this work is being held up, and in some cases is not being done at all, principally because we cannot get sufficient staff to deal with it. Our reason for writing to you direct is that for months we have done everything that we possibly can through the normal channels, and that we despair of any early improvement without your intervention. No doubt in the long run these particular requirements will be met, but meanwhile still more precious months will have been wasted, and as our needs are continually expanding we see little hope of ever being adequately staffed.
We realize that there is a tremendous demand for labour of all kinds and that its allocation is a matter of priorities. The trouble to our mind is that as we are a very small section with numerically trivial requirements it is very difficult to bring home to the authorities finally responsible either the importance of what is done here or the urgent necessity of dealing promptly with our requests. At the same time we find it hard to believe that it is really impossible to produce quickly the additional staff that we need, even if this meant interfering with the normal machinery of allocations.
We do not wish to burden you with a detailed list of our difficulties, but the following are the bottlenecks which are causing us the most acute anxiety.
1. Breaking of Naval Enigma (Hut 8)
Owing to shortage of staff and the overworking of his present team the Hollerith section here under Mr Freeborn has had to stop working night shifts. The effect of this is that the finding of the naval keys is being delayed at least twelve hours every day. In order to enable him to start night shifts again Freeborn needs immediately about twenty more untrained Grade III women clerks. To put himself in a really adequate position to deal with any likely demands he will want a good many more.
A further serious danger now threatening us is that some of the skilled male staff, both with the British Tabulating Company at Letchworth and in Freeborn’s section here, who have so far been exempt from military service, are now liable to be called up.
2. Military and Air Force Enigma (Hut 6)
We are intercepting quite a substantial proportion of wireless traffic in the Middle East which cannot be picked up by our intercepting stations here. This contains among other things a good deal of new ‘Light Blue’ intelligence. Owing to shortage of trained typists, however, and the fatigue of our present decoding staff, we cannot get all this traff
ic decoded. This has been the state of affairs since May. Yet all that we need to put matters right is about twenty trained typists.
3. Bombe testing. Hut 6 and Hut 8
In July we were promised that the testing of the ‘stories’ produced by the bombes would be taken over by the WRNS in the bombe hut and that sufficient WRNS would be provided for this purpose. It is now late in October and nothing has been done. We do not wish to stress this so strongly as the two preceding points, because it has not actually delayed us in delivering the goods. It has, however, meant that staff in Huts 6 and 8 who are needed for other jobs have had to do the testing themselves. We cannot help feeling that with a Service matter of this kind it should have been possible to detail a body of WRNS for this purpose, if sufficiently urgent instructions had been sent to the right quarters.
4. Apart altogether from staff matters, there are a number of other directions in which it seems to us that we have met with unnecessary impediments. It would take too long to set these out in full, and we realize that some of the matters involved are controversial. The cumulative effect, however, has been to drive us to the conviction that the importance of the work is not being impressed with sufficient force upon those outside authorities with whom we have to deal.
We have written this letter entirely on our own initiative. We do not know who or what is responsible for our difficulties, and most emphatically we do not want to be taken as criticizing Commander Travis who has all along done his utmost to help us in every possible way. But if we are to do our job as well as it could and should be done it is absolutely vital that our wants, small as they are, should be promptly attended to. We have felt that we should be failing in our duty if we did not draw your attention to the facts and to the effects which they are having and must continue to have on our work, unless immediate action is taken.
A. M. Turing
W. G. Welchman
C H O’D Alexander
P. S. Milner-Barry
We are, Sir, Your obedient servants,
On receipt of this letter, the Prime Minister minuted as follows to General Ismay on 22 October 1941 (reproduced in facsimile on the opposite page; PRO HW 1/155):
Secret
In a locked box
Gen. Ismay
Make sure they have all they want on extreme priority and report to me that this has been done.
WSC
22.x
DRAMATIS PERSONAE
Hugh Alexander joined Hut 6 in February 1940. He became deputy head of Hut 8 in March 1941, and its head during the autumn of 1942. He later worked on Japanese naval codes and cipher machines.
Gustave Bertrand was a member of French military intelligence, in its cryptology section. He supplied vital documents on Enigma to Polish intelligence and the Government Code and Cypher School (GC&CS).
Frank Birch joined the German naval section at Bletchley Park in September 1939, and later became the head of the naval section.
Joshua (Josh) Cooper joined GC&CS in October 1925 to specialize in Russian codes and ciphers, later becoming head of the air section.
Alastair Denniston was the operational head of GC&CS from its establishment until February 1942. He was then appointed the Deputy Director (Civil) (DD(C)) in charge of the section responsible for diplomatic and commercial codebreaking. He retired in 1945.
Thomas (Tommy) Flowers was the engineer at the Post Office Research Station who designed Colossus.
Hugh Foss joined GC&CS in December 1924. He devised a method to solve non-plugboard Enigma in the late 1920s, and was head of the Japanese naval section in 1942 and 1943.
William Friedman was the founder of the US Army Signal Intelligence Service (SIS), and its head until 1941. He was appointed the Director of Communications Research in November 1942.
Harold (Doc) Keen was an engineer at the British Tabulating Machine Company, which made the British bombe. He was responsible for the detailed design of the bombes.
Dillwyn (Dilly) Knox was the chief cryptographer (cryptanalyst) at GC&CS. He was severely ill from 1941 onwards, and died in February 1943. He specialized in breaking non-plugboard versions of Enigma.
Stewart Menzies (‘C’) became the head of MI6 in 1939 and, as such, was also the Director of GC&CS (Director General from March 1944).
Stuart Milner-Barry joined Hut 6 in early 1940, becoming its head in September 1943.
Maxwell (Max) Newman was the head of a GC&CS section known as ‘the Newmanry’, which attacked the Tunny cipher machine.
Marian Rejewski was the Polish cryptanalyst who first solved plugboard Enigma, in 1932.
Telford Taylor was a lawyer who joined US Army Intelligence in 1942. He made an extended visit to GC&CS in April 1943, and later worked in Hut 3.
John Tiltman was the head of the military wing at Bletchley Park, and GC&CS’s top cryptanalyst on non-machine ciphers. He was appointed chief cryptographer in 1944.
Alan Turing joined GC&CS in September 1939, and was head of Hut 8 until autumn 1942. He also worked on Tunny.
Edward Travis was a deputy to Alastair Denniston until February 1942, when he was appointed Deputy Director (Services) (DD(S)) in charge of the services’ side of GC&CS. He became the Director of GC&CS in March 1944 (when Menzies’s title was changed).
Gordon Welchman was the head of Hut 6 from its establishment until about the autumn of 1943, when he became the Assistant Director, Mechanicisation.
1
BLETCHLEY PARK IN PRE-WAR PERSPECTIVE
CHRISTOPHER ANDREW
Bletchley Park may well have been the best-kept secret in modern British history. The 10,000 men and women who worked there were, in Churchill’s famous phrase, ‘the geese who laid the golden eggs and never cackled’. Ultra, the intelligence produced at Bletchley Park from the breaking of high-grade enemy ciphers and the analysis of intercepted signals, was the best intelligence in the history of warfare. But if the secret had leaked out, Ultra would have been worthless. At the end of the Second World War most of those who had been ‘indoctrinated’ into Ultra believed that it would never be revealed. Not until the secret was declassified in the mid-1970s did the geese begin to cackle. A student at my Cambridge college told me how, together with his parents and his sister, he had watched the first BBC documentary on Bletchley Park which showed wartime Wrens (members of the Women’s Royal Naval Service) operating the ‘bombes’ used to break the German ‘Enigma’ machine ciphers. At the end of the programme, his mother turned to the rest of the family and told them, ‘That’s where I worked. That’s what I did.’ Until that moment neither her husband nor her children had had any idea that she had been a wartime codebreaker. The most extraordinary thing about this extraordinary episode is that, so far as the codebreakers were concerned, it was not unusual. Shortly before the publication in 1979 of the first volume of Sir Harry Hinsley’s official history, British Intelligence in the Second World War, he addressed a large reunion of Bletchley Park veterans and their spouses. After the address, the husbands of a number of former Wrens told Hinsley, ‘She never breathed a word to me.’
For the mostly youthful wartime recruits to Bletchley Park indoctrination into Ultra was an unforgettable emotional experience which had few, if any, previous parallels in the entire course of British history. Until their recruitment, hardly any of these people were even aware that Britain had a signals intelligence (Sigint) agency. Yet they suddenly found themselves, during Britain’s ‘finest hour’, in possession of a secret whose revelation might do irreparable damage to the war effort. No wonder that some, perhaps many, suffered from nightmares in which they unwittingly gave the secret away. The extraordinary success with which the Ultra secret was kept for so long reflected in part a national culture which embodied far greater respect for official secrecy and deference to authority than is imaginable today. Despite joining the anti-war movement only six years before he arrived at Bletchley Park, Alan Turing, one of the greatest of the wartime codebreakers, seems to have been untroubled even by peace
time doubts about the essential importance of official secrecy. During the Abdication Crisis of 1936, though at first ‘wholly in favour of the King [Edward VIII] marrying Mrs Simpson’, Turing had second thoughts after ‘It appeared that the King was extremely lax about state documents, leaving them about and letting Mrs Simpson and friends see them.’
In itself, however, a traditional national culture of official secrecy is an inadequate explanation for the extraordinary success with which the Ultra secret was maintained for so long. Some of the secrets of British codebreaking during the First World War had begun to leak out almost as soon as the war was over. In his great history of the war, The World Crisis, Winston Churchill, later among the staunchest defenders of the Ultra secret, vividly recalled his excitement while First Lord of the Admiralty for the first nine months of the war at receiving decrypted German naval radio messages which, on occasion, were delivered to him even in his bath, where he eagerly ‘grasped [them] with dripping hand’. During the 1920s, some of the early successes of Britain’s newly established peacetime Sigint agency, the Government Code and Cypher School (GC&CS), against the Soviet Union were revealed in both the press and government statements. Churchill’s own attitude to Sigint security at the time now seems remarkably naive. He wrote in the summer of 1920 that the ‘perfidy and treachery’ contained in Soviet diplomatic decrypts was such that their contents should be made public:
I have carefully weighed the pros and cons of this question, and I am convinced that the danger to the State which has been wrought by the intrigues of these revolutionaries and the disastrous effect which will be produced on their plans by the exposure of their methods outweighs all other considerations.