Computer Security and Cryptography

7.4 CRIBBING RED CIPHERTEXT

We will describe the cryptanalysis of RED ciphertext using English-language text. The vowel/consonant pattern of a (plaintext) crib u = (u₀, u₁, …, u_{M − 1}) is

TABLE 7.8 Normalized κ-Values for Example 7.4

A necessary condition that the RED ciphertext fragment y_[i,i+M) ≡ (y_i,…, y_i+M−1) be the encipherment of the (plaintext) crib u = (u₀, u₁, …, u_M−1) is

To crib RED ciphertext for the plaintext u = (u₀, u₁, …, u_M−1), the RED ciphertext is searched for fragments y_[i,i+M) that have the same the vowel/consonant pattern as that of u.

Of course, Equation (7.17) is only a necessary condition that u → y_[i,i+M) and some fragments fail to correspond to plaintext crib. Additional constraints need to be imposed before concluding that y_[i,i+M) is the encipherment of the crib u.

7.4.1 Cribbing RED Cipherment: No Inactive Breakwheel Pins

If all pins on the breakwheel are active, then τ = 47 and P(i) = P(0) + i. As P(0) is unknown, the recovery of θ_V and θ_C by cryptanalysis assuming P(0) = 0 will then be related to Tables 7.5 and 7.6 by a shift in rows.

If y_[i,i+M) = (y_i, y_{i + 1}, …, y_{i + M−1}) is the RED encipherment of the crib u = (u₀, u₁, …, u_M−1), then Equations (7.11 ...