Seizing the Enigma: The Race to Break the German U-Boat Codes, 1933-1945
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SEIZING THE ENIGMA
The Race to Break the German U-boat Codes
1939–1943
David Kahn
Frontline Books, London
NAVAL INSTITUTE PRESS
Annapolis, Maryland
First published in 1991 by Houghton Mifflin Company of Boston, Massachusetts.
Second edition published in 1998 by Barnes & Noble of New York
This edition published in 2012 by Frontline Books,
an imprint of
Pen & Sword Books Ltd.,
47 Church Street, Barnsley, S. Yorkshire, S70 2AS.
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Copyright © David Kahn 1991, 1998
The right of David Kahn to be identified as the author of this work has been asserted by him
in accordance with the Copyrights, Designs and Patents Act 1988.
UK edition: ISBN 978-1-84832-636-1
US edition: ISBN 978-1-59114-807-4
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CONTENTS
List of Maps
Preface
Preface to the 2012 edition
1. A Staff School Memory
2. The Wreck of the Magdeburg
3. The Man, the Machine, the Choice
4. The Codebreaker and the Spy
5. Racing German Changes
6. Failure at Broadway Buildings
7. Phantoms
8. The Rotors
9. Royal Flags Wave Kings Above
10. In the Locked Drawer of the Krebs
11. Kisses
12. A Trawler Surprised
13. The Staff School Memory
14. “All This Rubbish?”
15. The Great Man Himself
16. When Sailors Look for Leaks
17. Blackout ’42
18. The George Cross
19. Enter the Americans
20. SC 127
21. The Cavity Magnetron Clue
22. The U-Tankers
23. The Reckoning
Appendix: Enciphering with Naval Enigma
Notes
Bibliography
Index
List of Maps
1. The stranding of the Magdeburg, August 26, 1914
2. Where the Krebs fought, was boarded, and sank, March 3, 1941
3. The patrol area of the München, May 1941
4. The patrol area of the Lauenburg, June 1941
5. The actual and replanned routes of Convoy SC 127, April 1943
PREFACE
THIS BOOK RECOUNTS THE SECRET HISTORY OF WORLD WAR II’s Battle of the Atlantic. It exposes the chief hidden factor that helped the Allies win it: they intercepted, solved, and read the coded radio messages between Admiral Karl Dönitz, Hitler’s commander of submarines, and his U-boats at sea. The solutions gave the British and Americans intelligence about the locations and movements of the U-boats, enabling the Allies to divert their convoys around wolfpacks and to sink subs. This was of fundamental importance because whoever won the Battle of the Atlantic would win the war. The struggle between Allied ships bringing supplies to Britain and German submarines seeking to sever that lifeline was the longest battle of the greatest war of all time, beginning on its first day and ending on its last. Winston Churchill has evoked its significance in words that, though familiar, still ring with high drama:
The Battle of the Atlantic was the dominating factor all throughout the war. Never for one moment could we forget that everything happening elsewhere, on land, at sea, or in the air, depended ultimately on its outcome, and amid all other cares, we viewed its changing fortunes day by day with hope or apprehension.…
Amid the torrent of violent events one anxiety reigned supreme. Battles might be won or lost, enterprises might succeed or miscarry, territories might be gained or quitted, but dominating all our power to carry on the war, or even keep ourselves alive, lay our mastery of the ocean routes and the free approach and entry to our ports.
The Germans used a cipher machine called the Enigma to put messages into secret form. Contrary to popular opinion formed since the 1974 publication of Group Captain F. W. Winterbotham’s The Ultra Secret, however, the Enigma used by the German navy—unlike that used by the air force—generally withstood British codebreaking for the first two years of the war. It was not until the British captured key documents from German warships that they were able to break the naval Enigma continuously. This book tells, for the first time, the story of those seizures and the role they played in helping win the Battle of the Atlantic.
The book differs in several other ways from earlier accounts of the Enigma. It depicts the codebreakers, especially those working on the naval Enigma, behind their closed doors and relates case histories of the role of the decodes in the defeat of the U-boats. It focuses upon personalities and rests as much as possible upon primary sources, namely documents and interviews. It shows how much of a “near-run thing” the cryptologic battle was and seeks to explain why the Allies won it. Finally, it weighs the effect of codebreaking on the war at sea.
I believe that this book tells an essentially complete story of the failures and successes of British codebreaking, thanks in large part to the magisterial history of British intelligence in World War II by Sir Harry Hinsley and his coauthors. Those three volumes are based upon primary sources and upon Hinsley’s own wartime experiences evaluating German naval intercepts. In some areas, such as the circuitry and mechanism of the cryptanalytic testing machines called bombes, my descriptions are based on interviews and reconstructions. This and other portions of my text dealing with technical matters are not intended to provide detailed instruction in Enigma cryptanalysis, though they give, I hope, enough information for readers who wish to pursue the matter to carry through their own analyses. This material may seem dry, but to leave it out would obscure a main point of the book: the fearful difficulty of the work of the cryptanalysts, which could not succeed without outside help.
As much as possible, I have used cryptologically precise terminology. Thus I distinguish between codes (which, to oversimplify, work by words) and ciphers (which work by letters). I usually write “encipher” in connection with ciphers and “encode” with codes. But sometimes, for simplicity or rhythm or sound, I use “code” and its derivatives when “cipher” or even “key” (a setting of the wheels or plugs of the Enigma, for exa
mple) would be correct: “codes” in the subtitle, which stands for “keys,” is an instance.
Among the many people who helped with this book I should like to thank first of all Tom Congdon, who has believed in it for many years and whose tough but sensitive editing improved the text. Robie Macauley of Houghton Mifflin had enough faith in the idea to contract for it. My agent, Max Becker, provided valuable moral support. John Sterling of Houghton Mifflin handled the publishing with energy and flair. Peg Anderson of that house did not a good, but a great job of copyediting, tightening and rearranging the text and suggesting improvements.
Others helped with the substance. Sir Harry Hinsley patiently answered torrents of questions. Dr. Jurgen Röhwer steered me right on the Battle of the Atlantic. Ralph Erskine generously shared his expertise on the naval Enigma. Dr. Cipher Deavours made complex cryptanalyses plain. Carl Ellison helped in this as well. Christine Kelly located many retired Royal Navy officers and men. My researchers Alexander Lesnoff-Caravaglia and Mary Z. Pain came up with what I needed in the Public Record Office. Ilan Berkner computed times of solution for messages. Franz Selinger provided names and addresses of crew members of the German weather ships. The late Patrick Beesly furnished information and encouragement.
Archivists and historians who greatly helped included John Taylor, Harry Rilley, Tim Mulligan, Bill Cunliffe, and Bob Wolfe at the National Archives, Dr. Dean Allard, Bernard Cavalcante, and Kathleen Lloyd at the U.S. Navy’s Operational Archives, Dr. Hansjoseph Maierhofer and Dr. Manfred Kehrig at the Militärarchiv, and David Brown at the Naval Historical Branch of the Ministry of Defence.
Richard Cornett of Newsday drew fine maps, and Bob Newman of that newspaper helped with art matters. Karen Bacon and Rita Porzelt typed a hard-to-read manuscript. I thank my colleagues at Newsday for their help and understanding: Jim Lynn, Peg Finucane, the late Lou Renzulli, Marty Hollander, Judy Bender, Jim Klurfeld, Ilene Barth, Mark Howard, and Eileen McDermott.
My friends Edward S. Miller and Dr. Louis Kruh encouraged me. Bernie Bookbinder provided valuable emotional support. Dr. Zita Brandes’s professional aid was indispensable. Others who helped include Dr. Robert N. Grant, Dr. Alec Douglas, and Gilbert Bloch, as well as all those who answered questions in interviews or in letters. Susanne Kahn discussed problems sympathetically; our sons Oliver and Michael reminded me of what really matters. My father, Jesse Kahn, offered advice, and together we remembered my mother, Florence Kahn, who died while the book was being written.
I am grateful to all for their help. The responsibility for errors lies with me, of course, and I shall appreciate any corrections that readers send.
GREAT NECK, NEW YORK
October 1990
PREFACE TO THE 2012
EDITION
BRITISH FORTITUDE MADE IT POSSIBLE FOR THE ALLIES TO WIN World War II. Russian manpower and American productivity actually won it, but they wouldn’t have been able to do so if Britain hadn’t first stood firm against Nazi Germany. And later the island gave the United States and Britain the platform on which they could build up their forces and from which they would launch their invasion of the Continent. The British people’s courage and determination made this possible, and is one of the great epics in world history, comparable (though with a better outcome) to Thermopylae.
But Britons required food to eat and raw materials to make the instruments of war. The dominions and colonies and allies overseas could furnish these items to the island kingdom only by sea. The Axis could choke off this supply by sinking the ships bringing those items to Britain. The United Kingdom therefore needed to ensure that these ships got through the U-boat blockade. It did not have enough destroyers and corvettes to do this, so it sought to turn U-boat convoy intelligence against itself by discovering it and using it to avoid the wolfpacks.
Admiral Karl Dönitz, the commander of U-boats, coordinated his attacks on the convoys by radio messages. But radio waves have the disadvantage that they can be heard not only by the intended recipients but by anybody. To prevent eavesdroppers from gaining information from the transmissions, U-boat command encrypted them – put them into secret form. It did so with a cipher machine, named the Enigma, then among the world’s best, that the German army and air force also used.
In the 1930s, however, the Poles, fearful of German revanchism and aided by spy information and mathematics, which neither the French nor the British cryptanalysts used, reconstructed the Enigma machine. Shortly before World War II, they gave reconstructed Enigmas to the British and to the French. It didn’t help. Strength, not intelligence, mattered. Blitzkrieg defeated the Poles and the French; Britain survived on its island. But it needed to know what the Germans were intending, and one source of that intelligence could come from solving German radio messages. Britain had set up a codebreaking center in Bletchley Park, a country house near Milton Keynes in Buckinghamshire. There cryptanalysts sought to solve German messages.
At first they had little success. The Enigma cryptosystem was too good. But machines are used by humans, and if they are not used well, they can fail. That is what happened with the Enigma used by the Luftwaffe. In enciphering a message, a Luftwaffe code clerk was to choose three letters to set the Enigma’s three codewheels for encipherment. (These three letters were repeated and transmitted in a secret way to the deciphering clerk so he could read the message.) Each message was supposed to have a random setting, making each message’s encipherment different from any other and so harder for an enemy to break. But instead of choosing three letters at random, such as HWX, the Luftwaffe encipherers often chose girlfriends’ names, such as ANN, or curses, or patriotic words, or letters near those of the preceding message so the setting up would be less work. This greatly reduced the number of settings that the British codebreakers had to try to solve the intercept. As a consequence, and with other help, the British began reading Luftwaffe intercepts in the spring of 1940, helping the few win the Battle of Britain.
Later on the young mathematical genius Alan Turing invented a mechanism—called a bombe, because it expanded upon a Polish device of that name—that tested guessed German plaintexts against intercepts to find the Enigma settings. This was improved by another Cambridge mathematician, Gordon Welchman, through what was called a diagonal board. These devices enabled the British sometimes to determine the Enigma key for some messages and so translate all messages enciphered in that key. But, as with the guessed Luftwaffe three-letter settings, running the bombes to find the right key took hours, sometimes days. And sometimes the guessed plaintexts were wrong. So determining the key, which would enable the British to read all messages enciphered in that key, happened only rarely.
And they could not crack German army or Kriegsmarine messages. The army code clerks were more disciplined than the Luftwaffe’s in choosing the setting letters. And the German navy didn’t permit its code clerks to choose the codewheel settings at all. Basically it listed the settings that could be used—they were random groups, such as DOK—and indicated them by letter groups that bore no relation to the setting groups. These indicators the enciphering code clerk converted to secret form according to a table in a book (this is simplified here) and radioed them to the decipherer, who unconverted them, set his Enigma wheels accordingly, and deciphered the message from gibberish to German. This system, which left almost nothing to chance and so to human blunders, was so well thought-out that the British cryptanalysts could not break into the Kriegsmarine messages. They could almost never guess plaintexts that would enable them to use the Turing bombe. So they could not learn what the U-boats had been ordered to do and what they were reporting back to headquarters about their successes and failures.
Yet the very existence of Britain all but required the defeat of the U-boats. Everything depended ultimately upon that. Warship escorts, air cover, and enough freighters would chiefly determine the outcome. But knowing what the German submarines were planning and where their wolfpacks would assemble would enable the Allied convoys to avoid t
hem. Later such knowledge would enable Allied warships to sink U-boats. Codebreaking could provide that knowledge. But the Allies didn’t have it. They couldn’t crack the naval Enigma.
Then a brilliant Cambridge undergraduate, Harry Hinsley, had the idea of stealing the Enigma keying books from some isolated German weather ships in the North Sea. These two dramatic seizures were followed by the rescue, by two Royal Navy sailors, of a new edition of the Short Weather Code from a sinking U-boat in the Mediterranean. Though they drowned, they saved the codebook. It enabled Bletchley to find cribs to weather messages and so read naval Enigma. This book tells the story of those captures, which greatly helped the Britain to win the Battle of the Atlantic and so not to lose but to win World War II.
An important correction to the popular account must be made. A film, U-571, purported to tell the story of the captures. It is dramatic, exciting—and false. First, it maintains that what was captured was an Enigma machine. No. Captured by the British in the war were the keying books. These made naval Enigma breakable in time for convoys to be rerouted around wolfpacks. But it is simpler for viewers to understand that a coding machine was seized than to explain the intricacies of naval Enigma keying. The second, and grosser, falsehood in the film affirmed that the capture in the film was an American exploit. This was a Hollywood invention to make the film more appealing to an American audience. The capture was, from inception to conclusion, a British triumph. The film deprives the British of their just reward. Their triumph must not be forgotten.
What does the story of the Enigma seizures mean? If Britain had not solved the naval Enigma, would it have lost the war? No. Though it is impossible to prove a counterfactual, I believe that Britain would have fought on in 1941 even without naval Enigma solutions just as it had in 1940. The war certainly would have taken longer, cost more lives, imposed greater hardships, but Hitler could never have won. The powers against him were too great. The atom bomb would have been detonated over Berlin instead of Hiroshima. The great value of the Enigma captures is that, by shortening the war, they helped to make much of this unnecessary.