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Computer Security and Cryptography by Alan G. Konheim

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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, …, uM − 1) is

image

TABLE 7.8 Normalized κ-Values for Example 7.4

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

image

To crib RED ciphertext for the plaintext u = (u0, u1, …, uM−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 uy[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 θV and θC 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) = (yi, yi + 1, …, yi + M−1) is the RED encipherment of the crib u = (u0, u1, …, uM−1), then Equations (7.11 ...

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