‘Now we can be sure that she is psychologically abnormal, and, as we just discussed, such a person may want to conceal the fact but would not be able to. That means that when she set out to create RECOVERY, an exceptionally difficult cipher but one designed according to standard parameters, she might have had the intention but not the ability, because she found that her personality made it impossible to stick to the rules. She might have wanted to make DIFFICULT CENTURY a conventional if abstruse cipher, but she couldn’t.’

She said, feeling her way forward, ‘You mean RECOVERY cannot be an algorithmic cipher encrypted with another algorithmic cipher?’

I nodded my head.

She looked up at the ceiling and said, ‘If you’re right, normally there would be only one way to understand RECOVERY: it is an algorithmic cipher encrypted with a transposition cipher.’

‘Why can’t it be an algorithmic cipher encrypted with a substitution cipher?’ I asked.

‘Because that’s the route that Chen has been trying out, and he has got nowhere.’

‘So what are you going to do now?’

‘I don’t know.’

‘I thought you just said there was only one way?’

‘I said normally …’

I was listening carefully to what she was saying when suddenly she stopped speaking and indicated that I should pay attention to something outside. I could hear someone pacing up and down the corridor, and you could feel the impatience in the footsteps. I smiled. ‘It must be Chen. I dare say he wants to report some new development.’

She said, ‘I’ll go and call him in.’

‘Tell me what you think first.’

She cleared her throat. ‘I’m sure you remember the four cipher texts I gave you, which when combined gave you a four-word plain text: “I really love you”.’

I felt very uncomfortable. ‘Why are you bringing that up again?’

‘You’re scared, aren’t you? Well in that case I won’t carry on, and anyway there are people waiting to talk to you.’ She got up to go, but I stopped her and got her to carry on. She looked at me disdainfully. ‘Don’t worry, I will never talk to you about love again: that’s history and it’s all in the past. I want you to think about this sentence and what is special about it. I will read it aloud, and you can work out what is special about it. I really love you – Really I love you – You I really love – four words that you can put in any order without affecting the meaning.’

I looked at her curiously, for it was as if a fluttering and fast-moving beam of light had suddenly appeared in front of me, illuminating a strange and brilliant new world.