In time, Lt. Francis A. Raven discovered a pattern being used by the Japanese in their daily keys. He noticed that each month was broken into three ten day segments in which a pattern was discerned. With the final touches made to the puzzle by Lt. Raven, the Purple cipher was effectively broken and Japanese secrets were exposed.
https:// io9.gizmodo.com/how-the-u-s-cracked-japans-purple-encryption-machine-458385664
Idk if this is relevant.