Turing–Welchman Bombe
A crib becomes a network of constraints; machinery searches for settings that survive.
Codebreaking machine; not an encryption system.
Why This Matters
The Bombe mechanised part of the attack on Enigma. Alan Turing developed the crib-based approach; Gordon Welchman’s diagonal board improved the search. British Tabulating Machine Company engineers built the machines. Operators and analysts remained essential.
How the Search Worked
A guessed plaintext passage, called a crib, was aligned with intercepted ciphertext. Its letter relationships formed a menu. The Bombe tested rotor arrangements against these constraints; contradictions eliminated candidates. A stop indicated a candidate to check, not a finished plaintext. Welchman’s diagonal board exploited the reciprocal connections of Enigma’s plugboard.
Read a Constraint
Suppose a proposed crib places A against ciphertext A. Enigma cannot encipher a letter as itself, so that alignment is impossible before a machine search begins. A different alignment may survive this first check and still fail the connected menu constraints. Surviving a filter is evidence to investigate, not proof of a correct key.
Historical Context and Limits
The first British Bombe, Victory, entered service in March 1940; the Turing–Welchman version followed in August. The earlier Polish bomba was an important predecessor with a different attack. This was an electromechanical codebreaking machine, not an encryption system or a general-purpose computer. It attacked Enigma, not Lorenz.