How PURPLE Works

The PURPLE machine is a unique device for a couple of reasons. First, it is one of the few cryptographs to use stepping switches as the cryptographic element. Also, PURPLE divides the alphabet into one group of six letters and another group of 20 letters, and uses a different algorithm to encipher the two groups.

An 8-level, 25-position stepping switchThis stepping switch (or uniselector) connects an input terminal to one of 25 output terminals. An electro-magnet, attached to the switch, advances the switch to its next position (e.g. from output 1 to output 2) when an electrical pulse is applied. When wired properly in a cryptographic system, the result will be 25 unrelated cipher alphabets.

The PURPLE machine had input and output plugboards to ‘scramble’ the alphabets. However, inside the machine the letters were divided into two groups: the ‘sixes’ (which corresponded to the English vowels AEIOUY) and the ‘twenties’ (which corresponded to the consonants). Any input from the electric typewriter could be plugged into any input of the PURPLE machine. For example, if typewriter letter ‘E’ was plugged into input ‘O’ then it would be enciphered as one of the sixes. If ‘E’ was plugged to ‘C,’ however, it was enciphered as one of the twenties.

Below is a simplified schematic diagram of the PURPLE analog. Notice that the 26 leads from the input typewriter are divided into two groups, of six and 20 letters respectively.
The sixes letters are enciphered on a single 6-level, 25-position stepping switch. Each position of the sixes switch produces a new random alphabet. The sixes switch advances each time a letter is enciphered, however, so the sixes alphabets will repeat every 25 letters.
The twenties letters are enciphered on three cascaded stepping switches. Only one of the twenties switches advances with each enciphered letter, so the twenties alphabets will not repeat until 25x25x25, or 15,625, letters have been enciphered. Any of the twenties switches can be the ‘fast,’ ‘medium,’ or ‘slow’ stepping switch, which means that there are six possible switch motions.

Block Diagram of the PURPLE Analog

Created on ... February 8, 2003