Enigma (Greek word meaning mystery or something puzzling) was a German manufactured electro-mechanical rotor machine that was used to cipher and decipher messages. It was first used by the navy of the German armed forces. Polish cryptologists made the first attempts at breaking it, whose experiences were utilised by experts (including Alan Turing) at the legendary Bletchley Park in the UK.

Users of Enigma were absolutely convinced that messages produced by the machine could not be deciphered manually. However, they could not have known that the coded text generated by Enigma could be cracked with the help of another machine. The “Turing bombe”, an electro-mechanical computer, was developed at Bletchley Park to do this. Decoding navy signals proved to be the most difficult. A turning point came about in May 1941 when the U110 submarine was captured with an M3 Enigma on board along with cipher documents, a codebook, and substitution ciphers containing procedures for double letters relevant to key settings. However, M4 introduced on 1 February 1942 could not be cracked for ten months, which brought about German successes once again. It was decoded on 12 December 1942 using the cipher documents seized in the capture of another submarine, the U-559 in October the same year.

On 7 and 8 December the British Military Tournament, in collaboration with Bletchley Park, commemorated the seizure of the U-559. As part of the event held in Earls Court, London, the capture of the sub-marine was re-enacted, and visitors could write messages using an Enigma machine from World War Two, and send them to Bletchley Park. The two locations were connected via online video link, encrypted text was decrypted in real time with the help of a rebuilt Turing bombe, using the same methods as Turing and his colleagues at the time.