Breaking Tunny: The Lorenz Cipher, Bill Tutte and Colossus

Enigma gets the movies, but the Lorenz SZ40/42 cipher, nicknamed Tunny at Bletchley Park, carried Hitler's strategic communications with his top generals. This episode traces how British Y stations picked up strange radio teletype signals they called new music, how the Vernam cipher and XOR logic underpinned the system, and how twelve co-prime wheels with intermittent stepping produced roughly 16 billion billion starting combinations. We also revisit the eBay find that opened the story: a piece of Tunny equipment bought from a garden shed in Southend-on-Sea for nine pounds fifty.

The break came from human error. On August 30, 1941, a German operator in Athens retransmitted a 4,000-character message using the same wheel settings, giving Brigadier John Tiltman a depth from which he extracted the keystream. Bill Tutte then reverse-engineered the entire machine by hand without ever seeing one, and Tommy Flowers built Colossus, the first large-scale electronic digital computer, to industrialise the attack. We close with Churchill's order to smash the machines and the decades of secrecy that followed.

• How the Vernam cipher and the stencil analogy explain XOR encryption and its reciprocity
• The chi, psi and motor wheels of the Lorenz machine and why co-prime cam counts mattered
• The Athens operator's fatal retransmission and Tiltman's manual recovery of the keystream
• Bill Tutte's row-length analysis that isolated the 41-cam chi wheel and deduced the whole architecture
• Why Tommy Flowers partly funded Colossus himself and why leaving the valves switched on kept it reliable