Computer Security and Cryptography

7.5 GENERALIZED VOWELS AND CONSONANTS

Changes in RED were made after it was put into service; the letters were divided into two sets VOW with six elements and CON with 20 elements. A plugboard connected the VOW keyboard letters to the slip-ring vowels A, E, I, O, U, and Y, and the OCP vowels A, E, I, O, U, and Y were connected to the lamps in VOW. The same process was carried out with respect to the letters in CON.

The plugboard connections are part of the key and must be recovered. Fortunately, the process is quite simple; Table 7.24 lists the frequencies of occurrence of the (ciphertext) letters in cipherEx7.4. Note that the frequencies of the vowels A, E, I, O, U, and Y are in excess of 0.0615, and those of consonants are bounded above by 0.0427. Thus, simple frequency counts negate the effect of using generalized vowels/consonants.

TABLE 7.24 1-Gram Letter Counts and Frequencies in cipherEx7.4