How did British cryptanalysts break the Lorenz cipher?

During World War II, British cryptanalysts at Bletchley Park successfully decrypted high-level German strategic messages enciphered by the Lorenz teleprinter stream cipher attachments. Dubbed Tunny by the British, the system was initially diagnosed through German operational errors rather than possessing a physical machine. This decryption effort drove the development of early computing machinery, including the programmable Colossus computers.

What we know

• The British Government Code and Cypher School at Bletchley Park decrypted high-level German army messages sent via Lorenz SZ teleprinter attachments, providing crucial Ultra intelligence. [3, 4]
• German operational shortcomings allowed British cryptographers to make their initial diagnosis of the system and find a way into decryption without having a physical machine for the first three years. [3, 4]
• The immense decryption challenges posed by Tunny messages led directly to the creation of Colossus, recognized as the world's first electronic, programmable digital computer. [3, 4]
• By the end of World War II, roughly 90 percent of selected Tunny messages were routinely decrypted at Bletchley Park. [3]

Where to go next

• how it workedWhat was the role of the Colossus computer in breaking ciphers?
• compared withHow did Bletchley Park decode Enigma messages?
• whereWhere were the intercepted German radio transmissions picked up?
• what followedWhat was the impact of Ultra intelligence on World War II?
• whoWho was Albert W. Small and what did he report about Bletchley Park?
• an unexpected connectionHow did diplomatic codes in Japan share methodology with military intercept work?Both Allied decryption efforts on complex Axis ciphers like the Japanese Type B machine and the German Lorenz cipher relied heavily on identifying structural patterns and operational errors.

Sources

• [1]cryptanalysis (Wikidata Q897511) · CC0 1.0
• [2]Cryptanalysis · CC BY-SA 4.0
• [3]Cryptanalysis of the Lorenz cipher · CC BY-SA 4.0
• [4]Lorenz cipher · CC BY-SA 4.0
• [5]Type B Cipher Machine · CC BY-SA 4.0