Improved Japanese Purple Analog

In the era of electromechanical crypto machines, an analog is a device developed to "crack " a cipher.

The analog to crack the Japanese diplomatic cipher called PURPLE was first built in 1940 by the Army's Cryptoanalysis Branch. The machine deciphered Japanese Purple diplomatic communications after the daily settings had been determined.

The Army improved the analog in March 1944. Featured here,  is the improved version.

After much hard work and analysis, Rowlett and his team in the Army's Signals Intelligence Service were able to discern that six letters appeared with a different frequency than the others. Using the mamy messages and clues available to them, the group cracked "the sixes" via the use of 6 x 25 substitution chart. By using frequency counts of the cipher text letters, cryptanalysts figured out which letters made up the "sixes" for that particular day.

The discovery of "the sixes" was critical to breaking PURPLE but the additional work required to solve "the twenties" proved even more vexing. Rowlett's team spent almost a year pouring over massive amounts of message traffic but was unable to discover any consistent pattern. Finally in a flash intellectual insight, Genevieve Grotjan discerned that the patterns formed cycles. These cycles, twenty-five key positions long were produced not by a single scrambler , but by a series of three scramblers, each of which fed into the next. These critical breakthroughs closed the loop and eventually led to the construction of the PURPLE analog. 

The surviving portion of the Purple analog.
A photo of the entire Purple analog.
All photos in this table by Ralph Simpson

Contributors and Credits:

1) Ralph Simpson <ralphenator(at)>
2) Purple analog placard in display case at NCM.

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Apr 24/12