Computer Security and Cryptography

7.9 CRIBBING PURPLE: FINDING THE V-STEPPER

We will illustrate a possible way to crib PURPLE ciphertext. We use English-language plaintext, the 1-gram English probabilities included in Chapter 3, and the PURPLE parameters in Example 7.7.

Even if a message indicator containing identifiers of the initial stepper settings was included in the clear in a message, the decipherment of intercepted PURPLE ciphertext depends on a large number of parameters which must be recovered.

VS: 25 × 6 entries in the vowel-stepper ciphertext alphabet;

CS: 3 × 25 × 20 entries in the consonant-stepper ciphertext alphabet;

PL: the plugboard connections.

In our analysis the initial settings are all 0; this is of no consequence in recovering the V-stepper. We indicate in Section 7.9.2 how the analysis of the C-steppers is effected and what changes must be made. We will sketch the ideas to find the V-stepper first and then illustrate them with an example. The plan of attack is a follows:

1. Make letter-counts and, as indicated in Section 7.5, determine the likely division of letters into vowels and consonants.
2. Construct crib tables whose entries are (u, v, i), consisting of

   (a) a crib u,

   (b) a corresponding ciphertext fragment v with the same vowel/consonant pattern as u,

   (c) the V-stepper position i at which u → v occurs, and

   (d) a score for entry.

3. Resolve contradictions of potential ciphertext fragments of cribs by a pruning algorithm and recover as much of the vowel-stepped ciphertext alphabet as possible. ...