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.
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.
Created on ... February 8, 2003