Huang Yiyi demonstrated how it worked as she talked. ‘This is the cryptographic bombe I’ve designed. As you can see, we have an upper plate in which there are many round holes in a honeycomb pattern, and on this gauge there are thirty-one notches to represent the thirty-one days of the month. The upper plate is mounted on rollers, so that it can move up and down of its own accord; again, it can be raised or lowered by thirty-one notches. At the top of the gauge there is a light source, and right here, in the hollow underneath, is where we put the radio intercepts. We can clip them in like this. This tray can also be moved sideways along its thirty-one notches, and again one notch represents one day. Now we must imagine that in addition to the upper plate moving up and down and the tray moving from side to side, our light source is also in constant motion. If the group of numbers illuminated by our light is the cipher key that allows us to read that day’s messages, then we can calculate how big the key must be: there are three hundred and sixty-five possibilities, which means that it is one whole year before the cipher key has to be repeated. Now if we want to make it more difficult for our light source to reveal the key, for example by adding another plate, then that gives us twice as many possible combinations; and extrapolating from that, the more plates you add, the more years it is before you have to repeat your cipher key. Now my first demonstration model has nine levels of plates …’

Chen got up and cut her short. ‘Comrade Huang, let me say this: if this bombe helps us crack the cipher then that is wonderful, but to my knowledge no cryptographic unit anywhere in the world uses a machine to generate cipher keys. Have any of you ever heard of such a thing?’

‘Have you ever heard of anyone daring to steal the portrait of Chairman Mao off the gate at Tiananmen?’ asked Huang Yiyi.

I laughed. ‘Only Sivincy.’

‘Exactly. Just as I told Deputy Director An, I am now more and more convinced that Sivincy’s plagiarism of the Enigma machine isn’t a straightforward rip-off; it is because she is so intelligent and so cunning that she is always up to nasty tricks – she enjoys breaking the rules.’

Comrade Chen said, ‘But have you considered, Comrade Huang, that the key to the cipher isn’t actually part of the cipher itself, it is an adjunct. Not having it will stop the novice but not the expert, so why would Sivincy go to so much trouble to hide it?’