Intelligence's modern legitimacy owes a lot to its increased openness: it is no longer a sealed Black Chamber permanently closed to public gaze. British initiatives have contributed substantially to this openness, particularly through three decisions in 1969–74 and their implementation in the rest of the decade. All were related to the Second World War successes at Bletchley Park in decrypting German messages enciphered in the ‘Enigma’ machine and other sophisticated enemy ciphers. The first decision, proposed in 1969 and taken in 1971, was to commission an official history that drew on these successes and became the multi-volume British Intelligence in the Second World War, known as the ‘Hinsley histories’ after Sir Harry Hinsley, their editor and principal author. The second decision, in 1974, was to acquiesce in the publication of the first British book-length account of the Enigma success, written by Group Captain Winterbotham and published as The Ultra Secret. The third decision, also in 1974, was for the wholesale release of these decrypted messages and intelligence reports based on them, with relatively few exceptions. For convenience they are all described here as ‘decrypts’ or ‘archives’.

Since then successive British governments have followed the Hinsley model, of commissioning independent scholars and providing full access to the archives, for other official intelligence histories: of the Security Service (2009), the Secret Intelligence Service (SIS) (2010), the Joint Intelligence Committee (JIC) (first volume 2014), and the Government Communication Headquarters (GCHQ) (2020). Australia has published official histories of its Security Service counterpart. A similar formula has been adopted for intelligence histories in Germany, the Netherlands, Norway and Denmark. Even a six-volume official history of Russian intelligence appeared between 1999 and 2006. As for the availability of intelligence archives, Britain's release of Enigma decrypts was followed by other pre-1945 intelligence records, a process which is now almost complete except for those of the SIS. Some British post-1945 documents have also become available, and more files are likely to be released following the publication of GCHQ's history. Elsewhere in Europe there have been extensive and important documentary releases on the Soviet-dominated intelligence organisations of the former Warsaw Pact countries, and some releases elsewhere, though the overall position on archival releases was assessed in 2015 as ‘a rather haphazard process, if it exists at all’.