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 t ...