All lessons11 min · Trainee Codebreaker
Cribs and Contradictions
Guess a word, then rule out where it cannot be.
Introduction
German operators were predictable: weather reports, WETTER, KEINEBESONDEREN. A guessed piece of plaintext is a crib.
Slide the crib along the intercept. Wherever a crib letter sits above the same cypher letter, that position is impossible — the machine could never do it. What survives is a menu for the Bombe.
Key ideaSince no letter encrypts to itself, a guessed word can be eliminated from most positions instantly.