Editing Ultra (cryptography) (section)

===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|1974a |p =1 }}