Bletchley Park and Ultra

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Background

The Government Code and Cypher School (GC&CS) was established in November 1919 as a peacetime signals-intelligence successor to the Admiralty's wartime Room 40 and the War Office's MI1(b). It was placed under the administrative control of the Foreign Office in 1922 and, from 1923, under the operational direction of the Director of the Secret Intelligence Service (SIS), reflecting an institutional combination of foreign cryptanalysis with foreign human-intelligence collection that would persist until GC&CS was renamed the Government Communications Headquarters (GCHQ) in 1946. Throughout the inter-war period, GC&CS worked principally on diplomatic traffic and on the cipher systems of the Soviet Union, Italy, Japan, and — from the early 1930s onward — Nazi Germany.¹

The principal German cipher system of the period was the Enigma machine, an electromechanical rotor cipher device manufactured commercially from the 1920s and adopted in successive military variants by the Reichswehr (later the Wehrmacht), the Kriegsmarine, and the Luftwaffe across the 1930s. The German services had introduced operating procedures — daily key changes, message-specific session keys, plugboard configurations of substantially expanded size — that they assessed made the system cryptanalytically secure against the volume of intercepted traffic an opponent might collect.²

The Polish Cipher Bureau (Biuro Szyfrów) of the Polish General Staff, working under Lieutenant Colonel Gwido Langer and on the cryptanalytic side under the mathematicians Marian Rejewski, Jerzy Różycki, and Henryk Zygalski, achieved the first sustained breaks against military Enigma traffic from December 1932 onward. Rejewski's mathematical reconstruction of the Enigma rotor wirings, combined with the methods subsequently developed by the three for the recovery of daily keys, produced a Polish capability for routine reading of German military Enigma through the mid-1930s. The Polish electromechanical aid to key-recovery — the bomba kryptologiczna, designed by Rejewski in 1938 — was the direct precursor of the British Bombe of 1940.³

In the period of escalating crisis preceding the German invasion of Poland, the Polish Cipher Bureau elected to share the substance of its Enigma work with the British and French services. The transfer was effected at Pyry, near Warsaw, on 25–26 July 1939, where Polish Enigma replicas, the bomba design, and the body of cryptanalytic methodology developed by Rejewski, Różycki, and Zygalski were handed to the British representative Alastair Denniston (head of GC&CS) and the French representative Gustave Bertrand. The Pyry handover gave GC&CS a working position in German military Enigma several years in advance of what unaided British work would have produced.⁴

GC&CS relocated from its London premises at Broadway Buildings to Bletchley Park, an estate at Bletchley in Buckinghamshire, on 15 August 1939 — approximately three weeks before the British declaration of war on 3 September 1939. The site had been acquired in May 1938 by Admiral Sir Hugh Sinclair (Chief of SIS and Director of GC&CS) for the purpose of providing a wartime dispersal location at sufficient distance from London to be defensible against air attack but with main-line rail and trunk-telephone connectivity to the capital and the principal armed-service headquarters.⁵

The Programme

The cryptanalytic effort at Bletchley Park was organised functionally around the principal German cipher systems and the principal armed-service customers. Hut 6, under Gordon Welchman from 1940, was responsible for the Army and Air Force variants of the Enigma. Hut 8, under Alan Turing from 1939, was responsible for the substantially more difficult Kriegsmarine Enigma traffic — the operational reading of which was complicated by the introduction of a four-rotor variant on the U-boat circuits in February 1942. The decrypted traffic from each hut was passed for translation, evaluation, and onward intelligence reporting to a partnered hut: Hut 3 (working on Hut 6 product) and Hut 4 (working on Hut 8 product).⁶

The Bombe — the electromechanical machine that, given a presumed plaintext-ciphertext pairing of sufficient length (a "crib"), tested rotor settings to identify those consistent with the crib — was specified by Turing in late 1939 and refined by Welchman's introduction of the diagonal board in early 1940, an addition that materially reduced the number of false stops produced by a Bombe run. The first British Bombe, named Victory, entered service at Bletchley Park in March 1940; the improved Welchman-design Agnus Dei (later Agnes) followed in August 1940. By the end of the war the British Bombe inventory exceeded 200 machines, distributed across Bletchley Park and the outstations at Eastcote, Stanmore, Wavendon, Adstock, and Gayhurst.⁷

The high-level German strategic cipher was the Lorenz SZ40/SZ42 teleprinter cipher attachment ("Tunny" in Bletchley designation), which carried communications between the German High Command in Berlin and the field-army commanders in the European theatres. Lorenz traffic was substantially more difficult to attack than Enigma traffic, the cipher being a Vernam-style XOR system driven by the output of twelve rotors. The mathematical reconstruction of the Lorenz machine — performed by Bill Tutte in 1941–1942 from a single intercepted message in depth — was followed by the construction of the Heath Robinson cryptanalytic machine in 1943 and, from December 1943, the Colossus electronic computer designed by Tommy Flowers of the Post Office Research Station at Dollis Hill. Colossus was the first programmable electronic digital computer; ten Mark II machines were operational at Bletchley Park by the end of the war, providing routine reading of Lorenz traffic from spring 1944 through the German surrender.⁸

The intelligence product from these operations was distributed to a tightly compartmented list of Allied military commanders under the security designation Ultra, with operational rules — devised principally to protect the source from inadvertent disclosure through over-precise tactical exploitation — that required the apparent attribution of the underlying intelligence to a notional well-placed agent inside the German command structure. The peak personnel strength at Bletchley Park, including the outstations and the Y Service intercept stations that supplied the raw material, exceeded 10,000 by the end of 1944.⁹

Disclosure

The Bletchley Park operation was held under the strictest wartime British security classification and remained classified, in the United Kingdom and in the United States, for nearly three decades after the German surrender. The post-war retention of the Bombe machinery and of operational methods was governed by the recognition that Enigma-class rotor cipher systems remained in use in a number of states — many of them captured German equipment — and by the consequent assessment that public disclosure of the wartime breaks would substantially compromise continuing exploitation. The Cabinet Office position from 1945 onward was that the wartime operation was not to be acknowledged in any official capacity.¹⁰

The seal on the Ultra secret was lifted by the publication, in 1974, of The Ultra Secret by Group Captain F. W. Winterbotham — the SIS officer who, during the war, had administered the security and onward distribution of the Ultra product to the operational commands. Winterbotham's account, written from memory and without access to the operational records, was followed across the late 1970s and the 1980s by a substantial body of memoir literature from former Bletchley personnel (Welchman, Calvocoressi, Cairncross, Hinsley) and by the publication of the multi-volume official history British Intelligence in the Second World War, prepared under the editorship of F. H. Hinsley between 1979 and 1990 and based on the underlying GC&CS, SIS, and operational records.¹¹

The progressive declassification of the underlying records — the HW series at the National Archives at Kew (HW 1 for the Prime Minister's daily Ultra summary, HW 11 for selected Naval Section translations, HW 14 and HW 25 for the principal cryptographic and policy files, and the broader HW series across approximately twenty further classes) — has continued from the 1990s to the present and provides the primary-source basis for the contemporary scholarly account.¹²

Legacy

The official assessment of Ultra's contribution to the Allied war effort was made by F. H. Hinsley in successive volumes of the official history and in his 1993 lecture The Influence of Ultra in the Second World War, in which Hinsley judged that Ultra had shortened the war by "not less than two years and probably by four." That assessment — heavily qualified by Hinsley with respect to specific operational episodes and individual command decisions — has been broadly accepted as the responsible upper bound by subsequent scholarship, with consensus opinion in the literature settling at a shortening of approximately two years across the European theatre.¹³

Bletchley Park is the immediate institutional ancestor of the Government Communications Headquarters. The post-war GC&CS, redesignated GCHQ in 1946, retained the personnel, the methods, and (in modified form) the cryptanalytic equipment of the wartime operation, and conducted the transfer of capability that was institutionalised in the BRUSA Agreement of 17 May 1943 between GC&CS and the United States Army (forerunner of the post-war NSA) and in the bilateral Five Eyes / UKUSA Agreement of 5 March 1946 between GCHQ and the US Signals Intelligence Service — a bilateral UK–US accord to which Canada acceded in 1948 and Australia and New Zealand in 1956.¹⁴

The Colossus computer, dismantled at the end of the war on the order of the Cabinet Office and not publicly disclosed until the 1970s, has been retrospectively recognised as the first programmable electronic digital computer. The constraints imposed by post-war British classification on public discussion of Colossus prevented the wartime achievement from contributing directly to the public computing literature of the 1940s and early 1950s; a working reconstruction of Colossus by Tony Sale was completed in 2007 and is on display at the National Museum of Computing at Bletchley Park. The Bletchley Park site itself was preserved by the Bletchley Park Trust from 1992 onward and operates as a museum.¹⁵

Related dossiers and agencies

This dossier is the foundational episode for the post-war Five Eyes / UKUSA Agreement and the institutional ancestor of GCHQ (the post-1946 redesignation of GC&CS) and — through the wartime BRUSA Agreement — of the National Security Agency. The wartime customer-relationship is with the Secret Intelligence Service, under whose Director GC&CS administratively sat. The country-level context is on the pages for the United Kingdom and the United States.

Sources & Further Reading

1. Christopher Andrew, The Defence of the Realm: The Authorized History of MI5 (Allen Lane, 2009), ch. 1; GCHQ historical overview; National Archives, HW 3 (GC&CS administrative records).
2. David Kahn, Seizing the Enigma: The Race to Break the German U-Boat Codes, 1939–1943 (Houghton Mifflin, 1991); Hugh Sebag-Montefiore, Enigma: The Battle for the Code (Weidenfeld & Nicolson, 2000), chs. 1–3.
3. Marian Rejewski, "How Polish Mathematicians Deciphered the Enigma," Annals of the History of Computing 3, no. 3 (1981), pp. 213–234; Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two (University Publications of America, 1984).
4. F. H. Hinsley et al., British Intelligence in the Second World War (HMSO, 1979–1990), vol. III, app. 30; Kozaczuk, Enigma, ch. 5; National Archives, HW 3/32 (Denniston's account of the Pyry conference).
5. Sinclair McKay, The Secret Life of Bletchley Park (Aurum Press, 2010), ch. 1; Bletchley Park Trust historical materials.
6. Gordon Welchman, The Hut Six Story: Breaking the Enigma Codes (McGraw-Hill, 1982); F. H. Hinsley and Alan Stripp (eds.), Codebreakers: The Inside Story of Bletchley Park (Oxford University Press, 1993), chs. 8–13.
7. B. Jack Copeland (ed.), Alan Turing's Automatic Computing Engine (Oxford University Press, 2005); Welchman, The Hut Six Story, chs. 4–6; National Archives, HW 14 (cryptographic policy).
8. B. Jack Copeland (ed.), Colossus: The Secrets of Bletchley Park's Codebreaking Computers (Oxford University Press, 2006); Hinsley and Stripp (eds.), Codebreakers, chs. 14–17.
9. F. W. Winterbotham, The Ultra Secret (Weidenfeld & Nicolson, 1974); Ralph Bennett, Behind the Battle: Intelligence in the War with Germany 1939–1945 (Sinclair-Stevenson, 1994).
10. F. H. Hinsley, British Intelligence in the Second World War (HMSO, 1979–1990), vol. I, preface; Christopher Andrew, The Defence of the Realm, ch. 7.
11. F. W. Winterbotham, The Ultra Secret; Welchman, The Hut Six Story; Peter Calvocoressi, Top Secret Ultra (Cassell, 1980); F. H. Hinsley et al., British Intelligence in the Second World War (HMSO, 1979–1990), 5 vols.
12. <a href="https://discovery.nationalarchives.gov.uk/results/r?_q=HW&_sd=1939&_ed=1946">National Archives, HW series (Government Code and Cypher School records); Richard J. Aldrich, GCHQ: The Uncensored Story of Britain's Most Secret Intelligence Agency (HarperPress, 2010).
13. F. H. Hinsley, "The Influence of Ultra in the Second World War," lecture delivered at Cambridge University, 19 October 1993, reprinted in Hinsley and Stripp (eds.), Codebreakers; Bennett, Behind the Battle.
14. Aldrich, GCHQ, chs. 1–2; NSA, UKUSA declassified records release; GCHQ, UKUSA Agreement release.
15. B. Jack Copeland (ed.), Colossus; National Museum of Computing — Colossus reconstruction; Bletchley Park Trust.