SECRET
General Fertig had never seen one before. He found it fascinating.
It consisted of twenty-five aluminum disks. Each disk was about the size of a silver dollar and just a little thicker. The disks were stacked together and laid on their edges, so they could rotate independently on an axle. The stack of disks was about five inches long. On the outside of each disk there was printed an alphabet, sometimes A, B, C in proper sequence and sometimes with the characters in a random order.
"How does it work?" Fertig asked.
Captain Buchanan showed him.
Each of the disks was rotated until they all spelled out, horizontally on the "encrypt-decrypt line," the first twenty-five characters of the message they were to transmit. That left the other lines spelling out gibberish.
Cryptographic facilities were furnished a Top Secret document, known as the SOI (Signal Operating Instructions). Among other things, the SOI prescribed the use of another horizontal line, called the "genatrix," for use on a particular day. The gibberish on the genatrix line was what was sent over the air.
Actually, Buchanan explained, the SOI provided for a number of genatrix lines, for messages usually were far longer than twenty-five characters. The genatrix lines were selected at random. One day, for example. Lines 02,13,18, 21,07, and so on were selected, and Lines 24,04,16,09,09, and so on, the next.
When the message was received, all the decrypt operator had to do was consult his SOI for that day's genatrix lines. He would then set the first twenty five characters of the gibberish received on that genatrix line on his Device, Cryptographic, M94, and the decrypted message would appear on the encrypt decrypt line. He would then move to the next prescribed genatrix line and repeat the process until the entire message had been decrypted.
The forehead of the red-goa teed brigadier general creased thoughtfully.
Buchanan read his mind.
"In an emergency, Sir," Buchanan said, "in the absence of an SOI, there is an emergency procedure. A code block..."
"A what?" Fertig asked.
"A five-character group of letters, Sir," Buchanan explained, "is included as the third block of the five five-character blocks in the first twenty-five characters.
That alerts the decrypt operator to the absence of an SOI."
; "And then what?"
"First, there is a standard emergency genatrix line sequence. The message will then be decrypted. The receiving station will then attempt to determine the legitimacy of the sender by other means."
"Such as?"
"His name, for one thing. Then the maiden name of his wife's mother, the name of his high school principal, or his children. Personal data that would not be available to the enemy."
General Fertig nodded.
"You are a very clever fellow, Buchanan," Fertig said.
"You are herewith appointed cryptographic officer for United States forces in the Philippines."
That left two connected problems. The first was to get Gerardo Almendres's International Correspondence School transmitter-receiver up and running.
That would require electrical power, and that translated to mean a generator would be required.
Buchanan had no idea how that could be handled, but both he and It. Ball suggested that perhaps Master Sergeant George Withers might be of help.
Withers was the NCOIC (noncommissioned officer in charge) of the Explosive Ordnance Disposal Detachment on whose boat Buchanan had escaped
from Luzon. He was a competent fellow; master sergeants of the Regular
U.S.
Army are almost by definition highly knowledgeable and resourceful He had, after all, managed to acquire and hide the boat and bring his detachment safely to Mindanao on it.