Modern methods for breaking of Enigma ciphertexts

What is it about?

“Breaking German Army Ciphers” is the title of a Cryptologia article from 2005, describing the lucky survival of several hundred authentic Enigma messages of World War II, and an account of a ciphertext-only cryptanalysis of a large number of these messages, leaving only a few (mostly short messages) unbroken. After reviewing the work done, and investigating the reasons for both lucky breaks and close misses, the modern ciphertext-only attack on Enigma messages is improved, especially on genuine ones with short lengths and/or many garbles. The difficulties of a proper measure for the candidate’s closeness to a plaintext are clarified. The influence on the decryption process of an empty plugboard and one with only a few correct plugs is examined. The method is extended by a partial exhaustion of the plugboard combined with an optimized hillclimbing strategy. The newly designed software succeeds in breaking formerly unbroken messages.

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Why is it important?

The article adds new insight into the process of breaking Enigma ciphertexts with modern computer based search algorithms. Based on the new knowledge of why the existing methods sometimes fail to break Enigma ciphertexts, new and optimized strategies are developed with the aim of breaking short messages and messages with many garbles.