Editing Ultra (cryptography) (section)

==Postwar secrecy==
===Secrecy and initial silence (1945–1960s)===
Until the mid 1970s, the [[thirty year rule]] meant that there was no official mention of Bletchley Park.  This meant that although there were many operations where codes broken by Bletchley Park played an important role, this was not present in the history of those events.  Churchill's series [[The Second World War (book series)|The Second World War]] did mention Enigma but not that it had been broken.{{sfn|Deutsch|1977|p=16}}

While it is obvious why Britain and the U.S. went to considerable pains to keep Ultra a secret until the end of the war, it has been a matter of some conjecture why Ultra was kept officially secret for 29 years thereafter, until 1974. During that period, the important contributions to the war effort of a great many people remained unknown, and they were unable to share in the glory of what is now recognised as one of the chief reasons the Allies won the war – or, at least, as quickly as they did.

At least three explanations exist as to why Ultra was kept secret so long. Each has plausibility, and all may be true. First, as [[David Kahn (writer)|David Kahn]] pointed out in his 1974 ''New York Times'' review of Winterbotham's ''The Ultra Secret'', after the war, surplus Enigmas and Enigma-like machines were sold to [[Third World]] countries, which remained convinced of the security of the remarkable cipher machines. Their traffic was not as secure as they believed, however, which is one reason the British made the machines available.{{sfn|Kahn|1974|p=5}}{{better source needed|reason=can't tell from source if this is Kahn's conjecture or if he has facts|date=April 2016}}

By the 1970s, newer computer-based ciphers were becoming popular as the world increasingly turned to computerised communications, and the usefulness of Enigma copies (and rotor machines generally) rapidly decreased. Switzerland developed its own version of Enigma, known as [[NEMA (machine)|NEMA]], and used it into the late 1970s, while the United States [[National Security Agency]] (NSA) retired the last of its rotor-based encryption systems, the [[KL-7]] series, in the 1980s.

A second explanation relates to a misadventure of one of Churchill's predecessors, [[Stanley Baldwin]], between the World Wars, when he publicly disclosed information from decrypted Soviet communications about the [[General Strike]]. This had prompted the Soviets to change their ciphers, leading to a blackout.{{sfn|Aldrich|2010|p=18}}

The third explanation is given by Winterbotham, who recounts that two weeks after [[V-E Day]], on 25 May 1945, Churchill requested former recipients of Ultra intelligence not to divulge the source or the information that they had received from it, in order that there be neither damage to the future operations of the Secret Service nor any cause for the Axis to blame Ultra for their defeat.{{Sfn |Winterbotham|1974 |p =1 }}

===Partial disclosures===
In 1967, Polish military historian [[Władysław Kozaczuk]] in his book ''Bitwa o tajemnice'' ("Battle for Secrets") first revealed Enigma had been broken by Polish cryptologists before World War II.

Also published in 1967, [[David Kahn (writer)|David Kahn]]'s comprehensive chronicle of the history of cryptography, ''[[The Codebreakers]]'', does not mention Bletchley Park, although it does make the claim that Soviet forces were reading Enigma  messages by 1942.{{sfn|Deutsch|1977|p=16}} He also described the 1944 capture of a naval Enigma machine from {{GS|U-505||2}} and gave the first published hint about the scale, mechanisation and operational importance of the Anglo-American Enigma-breaking operation:

{{Quote|The Allies now read U-boat operational traffic. For they had, more than a year before the theft, succeeded in solving the difficult U-boat systems, and – in one of the finest cryptanalytic achievements of the war – managed to read the intercepts on a current basis. For this, the cryptanalysts needed the help of a mass of machinery that filled two buildings.{{sfn|Kahn|1967|p=506}}}}

[[Ladislas Farago]]'s 1971 best-seller ''The Game of the Foxes'' gave an early garbled version of the myth of the purloined Enigma. According to Farago, it was thanks to a "Polish-Swedish ring [that] the British obtained a working model of the 'Enigma' machine, which the Germans used to encipher their top-secret messages."{{sfn|Farago|1974|p=664}} "It was to pick up one of these machines that Commander Denniston went clandestinely to a secluded Polish castle [!] on the eve of the war. Dilly Knox later solved its keying, exposing all Abwehr signals encoded by this system."{{sfn|Farago|1974|p=674}} "In 1941 [t]he brilliant cryptologist Dillwyn Knox, working at the Government Code & Cypher School at the Bletchley centre of British code-cracking, solved the keying of the Abwehr's Enigma machine."{{sfn|Farago|1974|p=359}}

===1970s===
The 1973 public disclosure of Enigma decryption in the book ''Enigma'' by French intelligence officer [[Gustave Bertrand]]{{sfn|Bertrand|1973}} – which dealt mainly with the Polish and then Franco-Polish efforts before the [[Invasion of France]] and before the Ultra program{{sfn|Deutsch|1977|pp=16-17}} – generated pressure to discuss the rest of the Enigma–Ultra story.{{cn|date=May 2025}}

Since it was British and, later, American message-breaking which had been the most extensive, the importance of Enigma decrypts to the prosecution of the war remained unknown despite revelations by the Poles and the French of their early work on breaking the Enigma cipher. This work, which was carried out in the 1930s and continued into the early part of the war, was necessarily uninformed regarding further breakthroughs achieved by the Allies during the balance of the war.

The British ban was finally lifted in 1974, the year that a key participant on the distribution side of the Ultra project, [[F. W. Winterbotham]], published ''The Ultra Secret''.{{sfn |Winterbotham |1974 }} Winterbotham's book was written from memory and although officially allowed, there was no access to archives.{{sfn|Deutsch|1977|p=17}} Public discussion of Bletchley Park's work in the English speaking world finally became accepted, although some former staff considered themselves bound to silence forever.<ref>{{Citation |last=Withers-Green |first=Sheila |title=audiopause audio:<!--<<regularize this cite--> I made a promise that I wouldn't say anything |year=2010 |url=http://audioboo.fm/boos/176850-i-made-a-promise-that-i-wouldn-t-say-anything-sheila-withers-green-bpark2010?playlist_direction=forward |access-date=15 July 2011 |archive-url=https://web.archive.org/web/20111008045611/http://audioboo.fm/boos/176850-i-made-a-promise-that-i-wouldn-t-say-anything-sheila-withers-green-bpark2010?playlist_direction=forward |archive-date=8 October 2011 |url-status=live}}</ref>

Other books such as [[Anthony Cave Brown]]'s ''[[Bodyguard of Lies]]'' and [[William Stevenson (Canadian writer)|William Stevenson]]'s ''A Man called Intrepid'' were also being written at this time, and the military historian [[Harold C. Deutsch]] regards Winterbotham's revelations as only to have anticipated what were going to be a number of revelations.{{sfn|Deutsch|1977|p=28}}

===Public interest===
A succession of books by former participants and others followed. The official history of British intelligence in World War II was published in five volumes from 1979 to 1988, and included further details from official sources concerning the availability and employment of Ultra intelligence. It was chiefly edited by [[Harry Hinsley]], with one volume by [[Michael Howard (historian)|Michael Howard]]. There is also a one-volume collection of reminiscences by Ultra veterans, ''Codebreakers'' (1993), edited by Hinsley and Alan Stripp.

===Continued selective secrecy===
In 2012, [[Alan Turing]]'s last two papers on Enigma decryption were released to Britain's [[The National Archives (United Kingdom)|National Archives]].<ref>{{cite web|url=https://www.bbc.co.uk/news/technology-17771962|website=BBC|title=Alan Turing papers on code breaking released by GCHQ}}</ref>  The Departmental Historian at [[GCHQ]] stated that the seven decades' delay had been due to their "continuing sensitivity... It wouldn't have been safe to release [them earlier]."{{cn|date=May 2025}}