I was very excited and urged her to continue. Huang Yiyi said crossly, ‘I hadn’t realized you were so impatient. If you were equally excitable in emotional matters that would be wonderful.’ Since that was her attitude, it was obvious that she hadn’t given up hope in spite of all my efforts. She then made a request, saying that she would only continue talking if I sat down next to her. Really! But as it happened we had already walked into the wood and there was nobody else around, and I was tired, so there was no reason why I shouldn’t sit with her. I guessed that if I sat down that would be followed by a whole series of other requests, so I made my own demand: that after I sat down she would have to do what I said. She agreed to that, and then we both sat down and she carried on her explanation. She said that the current trend in cryptography was to make the key to the cipher highly complex, but that this process was limited by the constraints of wireless technology, particularly in cases where the communication had to be sent vast distances or to numerous places, so mostly the key was hidden in the message.

‘For example ULTRA, which was a high-level cipher – do you know what its key was?’

‘The first three sets of numbers on even-number days, the last three sets on odd-number days.’

‘Right. And it was concealed in the message text. Why did they want to make it part of the message?’

‘Because they were using a lot of different wirelesses and it was during wartime, so the sets were moving around a lot and there was a high turnover in personnel. If they hadn’t done it that way – if, for example, they had used a key from a one-time pad, if the person with the pad got killed, communications would be crippled.’

‘That’s the point. RECOVERY is the cipher that Sivincy designed for the American military, and since the end of the Second World War the US army has been involved in many theatres, with bases all over the world. With their bases so spread out, and their communications networks so numerous, we can be sure that it is impossible for RECOVERY to have its own one-time pad.’

‘Mmm. A one-time pad also wouldn’t suit the uses that the KMT are putting it to now, letting their special agents use it.’

‘Right. The KMT are using RECOVERY as the cipher through which Taiwan communicates with their special agents on the Mainland, and so we can be sure that the key is part of the message. The special agents have to split up – they need to be able to work in the widest field possible – so if the key isn’t part of the message, the whole system could easily break down.’

‘Mmm.’