I remembered I had brought a little present back from Moscow specially for her, so I asked her to come to my office. It was a pretty Russian doll, and she loved it the moment she set eyes on it. ‘This is a pair to the one I have at home: a prince and a princess.’

‘I noticed the prince in your room, so I bought this one for you specially to make a pair.’

She praised the princess’s prettiness for a bit, and then she raised her head and said, ‘Why are you being so good to me?’

‘What do you mean, so good? It was easy, and the princess wasn’t expensive.’

She looked at me, and then as if something had disappointed her, she said to herself, ‘I don’t understand you, you are … too mysterious for me.’

I said cheerfully, ‘It doesn’t matter if you don’t understand me: all we want is for you to understand RECOVERY.’ I asked her if she thought I was right, that Sivincy’s experience of being raped as a girl would have had a great effect in perverting her character. She said of course, it was quite sufficient to explain her abnormal psychology.

‘Would it be possible for someone psychologically abnormal to conceal that fact?’

‘I don’t think so. Even if she wanted to, she wouldn’t be able to hide every trace. Say I myself wanted to pretend to be reserved, I might take everyone in for a bit but I wouldn’t be able to keep it up for a lifetime. Now pretty much everyone in the unit looks at me askance. Why? Because the fox can’t hide its tail for ever. The same goes for you. Mountains may crumble and rivers change their course, but human nature is hard to change.’

‘You ought to remember that when you gave me the choice of picking which principles Sivincy would have used to construct RECOVERY, I chose a mathematical cipher created by encrypting an algorithmic cipher with a second algorithmic cipher. Do you know why I picked that? It was because Sivincy, to use your own words, had already pulled a very nasty trick on us, toying with the cryptographers trying to break her cipher. So I guessed that when she was thinking about her cipher she would do her very best to create an exceptionally difficult one, which on the one hand would prove her abilities and on the other would demonstrate that her plagiarism didn’t arise from stupidity but was in fact entirely intentional, a joke that she was playing on other cryptographers.’

She looked at me a bit startled, and then asked me to carry on talking.