To celebrate the circulation of the new £50 note depicting Alan Turing, GCHQ — the successor organization to his Bletchley Park codebreaking team — recently released what it described as its “toughest puzzle ever”. It seemed apt.

Regarded as a founder of computer science, Turing cracked Germany’s Enigma machine during the Second World War, but the puzzling circumstances surrounding his death have never been fully resolved.

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