Computer Security and Cryptography

7.7 COMPONENTS OF THE PURPLE MACHINE

PURPLE had a typewriter input, lamp output, a plugboard, and an internal switch implementing polyalphabetic substitutions. The rotor in the RED system was replaced by 25-position stepping switches or steppers, which were used as components in the automatic dial telephone system in the United States in the 1930s. A stepper allows any input line to be connected to any output line. The top and side views of a PURPLE stepper are depicted in Figure 7.5. The wiper or (blade) moves horizontally; passing between a pair of compressed contacts creates an electrical path from the input to output lines.

7.7.1 Encipherment of Letters in VOW

PURPLE continued the paradigm used in RED to encipher vowels to vowels and consonants to consonants. The wipers on all levels pointed in each level to the same output position and moved in unison, rotating (or stepping) one position for each letter enciphered. The PURPLE vowel-stepper implemented 25 (different) permutations of the vowels VOW = {A, E, I, O, U, Y} (Fig. 7.6).

To allow encipherment of generalized vowels as in RED, a plugboard connected

• The VOW keyboard letters to the six input contacts on the six levels, and
• From each of the 25 letter outputs on each level to the VOW output lamps.

The PURPLE vowel-stepper implemented a periodic polyphabetic substitution with period 25. The vowel x is enciphered to the vowel y as a result of Three transformations

Transformation #1

Figure 7.5 Side and top view of stepping ...