/bɒm/
noun — "an electromechanical device designed to help decrypt Enigma-encrypted messages."
Bombe was an electromechanical machine developed during World War II to expedite the decryption of messages encoded by the German Enigma cipher. Designed to test multiple possible Enigma rotor and plugboard settings systematically, the Bombe reduced the vast number of potential key combinations to manageable levels, enabling Allied cryptanalysts to read enemy communications in near real-time.
Technically, the Bombe operated by simulating multiple Enigma machines in parallel. It used rotating drums to represent Enigma rotors and an array of electrical circuits to detect contradictions in letter substitutions. By applying known or guessed plaintext segments, known as "cribs," the machine could eliminate impossible settings and isolate potential keys. Once a viable setting was identified, human operators would use actual Enigma machines to fully decrypt the intercepted messages.
# conceptual example of Bombe logic
# given a crib: "ATTACKATDAWN"
# simulate Enigma with all rotor/plugboard settings
# if encrypted crib matches ciphertext with no contradictions:
# candidate key found
# else:
# discard settings
Conceptually, the Bombe was a tool for constraint-solving at scale. Each drum rotation and electrical test represented a partial evaluation of the Enigma’s possible keyspace. By eliminating combinations that violated logical constraints, the machine narrowed the search efficiently, turning what would have been an astronomically large brute-force task into a solvable problem within hours.
In practice, the Bombe’s design and operation required a combination of mechanical engineering, electrical engineering, and mathematical insight. Alan Turing, Gordon Welchman, and their colleagues at Bletchley Park pioneered the device, contributing key innovations such as the "diagonal board," which significantly improved the machine's efficiency. The Bombe was instrumental in providing timely intelligence, affecting the outcome of critical military operations during the war.
See Enigma, Cipher, Cryptanalysis, Codebreaking.