It was a neat, compact set, Dieter thought admiringly; the kind of thing the Germans would have made, not at all what he would expect from the untidy British.
He already knew Helicopter’s times for transmission and reception. Now he had to learn the frequencies used and-most important-the code.
Helicopter plugged a lead into the power socket. Dieter said, “I thought it was battery-operated.”
“Battery or mains power. I believe the Gestapo’s favorite trick, when they’re trying to locate the source of an illicit radio transmission, is to switch off the town’s electricity block by block until the broadcast is cut off”
Dieter nodded.
“Well, with this set, if you lose the house current, you just have to reverse this plug, and it switches to battery operation.”
“Very good.” Dieter would pass that on to the Gestapo, in case they did not already know.
Helicopter plugged the power lead into an electrical outlet, then took the aerial wire and asked Stephanie to drape it over a tall cupboard. Dieter looked in the kitchen drawers and found a pencil and a scratch pad that Mademoiselle Lemas had probably used to make shopping lists. “You can use this to encode your message,” he said helpfully.
“First I’d better figure out what to say.” Helicopter scratched his head, then began to write in English:
ARRIVED OK STOP CRYPT RENDEZVOUS
UNSAFE STOP NABBED BY GESTAPO BUT
GOT AWAY OVER
“I suppose that’s it for now,” he said.
Dieter said, “We should give them a new rendezvous for future incomers. Say the Caf‚ de La Gare next to the railway station.”
Helicopter wrote it down.
He took from the case a silk handkerchief printed with a complex table showing letters in pairs. He also took out a pad of a dozen or so sheets of paper printed with five-letter nonsense words. Dieter recognized the makings of a one-time-pad encryption system. It was unbreakable-unless you had the pad.
Over the words of his message, Helicopter wrote the five-letter groups from the pad; then he used the letters he had written to select transpositions from the silk handkerchief Over the first five letters of ARRIVED he had written the first group from his one-time pad, which was
BGKRU. The first letter, B, told him which column to use from the grid on the silk handkerchief At the top of column B were the letters Ae. That told him to replace the A of ARRIVED with the letter e.
The code could not be broken in the usual way, because the next A would be represented not by e but by some other letter. In fact, any letter could stand for any other letter, and the only way to decrypt the message was by using the pad with the five-letter groups. Even if the code breakers could get hold of a coded message and its plain-language original, they could not use them to read another message, because the next message would be encoded with a different sheet from the pad- which was why it was called a “one-time” pad. Each sheet was used once, then burned.
When he had encrypted his message, Helicopter flicked the on/off switch and turned a knob marked in English “Crystal Selector.” Looking carefully, Dieter saw that the dial bore three faint markings in yellow wax crayon. Helicopter had mistrusted his memory and had marked his broadcast positions. The crystal he was using would be reserved for emergencies. Of the other two, one would be for transmission and the other for reception.
Finally he tuned in, and Dieter saw that the frequency dial was also marked with yellow crayon.
Before sending his message, he checked in with the receiving station by sending:
HLCP DXDX QTC1 QRK? K
Dieter frowned, figuring. The first group had to be the call sign “Helicopter.” The next one, “DXDX,” was a mystery. The number one at the end of “QTC1” suggested that this group meant something like: “I have one message to send you.” The question mark at the end of “QRK?” made him think this asked if he was being received loud and clear. “K” meant “Over,” he knew. That left the mysterious “DXDX.”
He tried a guess. “Don’t forget your security tag,” he said.