"It’s just a brute force statistics problem," Waterhouse says. "Suppose that Tokyo sent the Azure message to Rabaul on October 15th, 1943. Now, suppose I take all of the messages that were sent out from Rabaul on October 14th and I index them in various ways: what destinations they were transmitted to, how long they were, and, if we were able to decrypt them, what their subject matter was. Were they orders for troop movements? Supply shipments? Changes in tactics or procedures? Then, I take all of the messages that were sent out from Rabaul on October 16th—the day after the Azure message came in from Tokyo—and I run exactly the same statistical analysis on them."