Cryptanalysis of the Enigma Bletchley Park to describe the network that used the same Enigma setting sheets. Initially these were recorded using coloured pencils and were given the names "red", "light blue" etc., and later the names of birds such as "kestrel". During World War II the settings for most networks lasted for 24 hours, although towards the end of the war, some were changed more frequently. The sheets had columns specifying, for each day of the month, the rotors to be used and their positions, the ring positions and the plugboard connections. For security, the dates were in reverse chronological order down the
The World That Never Was as "intriguing, provocative and written with a novelist’s eye for detail ... an engrossing journey". In "The Guardian", anarchist writer Stuart Christie noted the relevance of the book's themes to contemporary tactics in policing and the Metropolitan Police Service's reluctance to make public files from the 1890s. Francis Wheen, writing in the "Financial Times", wrote that the book was "exhilarating" and questioned whether "the world that never was – one ignoring borders, and divisions of class and religion – [might] resemble the world as it might one day be"; while Peter Preston of "The Observer" described Butterworth's story as one