Advanced Placement (AP) Computer Science

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In context of code breaking, which element is crucial for the British Bombe to work effectively?

Knowledge of the entire message
Knowledge of at least 20 characters of the message
Rapid calculation of prime numbers
Access to multiple key sheets

The correct answer is: Knowledge of at least 20 characters of the message

The British Bombe was specifically designed to break the Enigma codes used by German forces during World War II. For the Bombe to operate effectively, it required knowledge of at least a certain number of characters from the encrypted message. This is tied to the notion of "cribs," which are known plaintext segments that could correspond to parts of the ciphertext. By having knowledge of these character sequences, the Bombe could narrow down potential settings of the Enigma machine, allowing it to test various configurations and ultimately decipher the coded messages. The effectiveness of the Bombe hinged on the fact that even a small sample of known messages can significantly reduce the number of possible combinations the machine has to consider. In this context, having at least 20 characters aids in creating a more fruitful environment for the Bombe to test its hypotheses about the machine's operations.