Computer Security and Cryptography

6.10 CRIBBING ENIGMA CIPHERTEXT

Example 6.2

Suppose that Enigma ciphertext begins with the suspected following plaintext:

This crib is examined for the presence of loops; E N I E is a loop consisting of E N (at position 29), N I (at position 12), and I E (at position 7). Three loops in the crib above are written as shown in Figure 6.12.

Welchman and Turing suggested testing plugboard connections and rotor positions by connecting double-ended Enigma's with the rotors set to the positions (j₀, j₁, j₂) (Fig. 6.13). The symbol for a double-ended Enigma is presented in Figure 6.14. To test the three loops, several double-ended Enigmas were interconnected. The three bombes shown in Figure 6.15 are set to test the plugboard connections E → A, P → A, and N → A. The figure shows the logical equivalents of parts of the bombe corresponding to the three loops rather than the actual bombe structure. The bombe cycles through the 26³ initial rotor offsets (j₀ + 7, j₁ + 9, j₂ + 4) to test the EPI loop. The bombe puts a voltage across the input port E; the current moves through the three double-ended rotors, returning to the Test Register. If

Figure 6.12 Loops in corresponding plainEx6.2 and cipherEx6.2.

• (j₀ + 7, j₁ + 9, j₂ + 4) is the correct initial rotor offset, and
• The plugboard connection ...