"But Enoch Root in Manila, and Goto Dengo, could not have had such a thing. They would have to encrypt the message by hand—doing all of the calculations on sheets of scratch paper. Enoch already knew the algorithm , and could tell it to Goto Dengo, but you would have to agree on a key to put into that algorithm. The only time you could have decided on the key was while you were all together at the cemetery. And during your conversation there, I saw you pointing at Shaftoe’s headstone. So I figured that you were using that as a key—maybe his name, maybe his dates of birth and death, maybe his military serial number. It turned out to be the serial number."