I glared at her. ‘Clearly, I’m going to have to take you through the whole thing step by step again, so that you understand exactly who I am and what you are here to do.’

She laughed. ‘Don’t be cross. I’m sorry. I know. I really wasn’t able to attend this morning because I didn’t get to sleep until after four in the morning, because I was working on this.’ She took out a couple of pages and gave them to me.

I took them and asked, ‘What is this?’

‘It’s a letter I’ve written to Anderov in your name. Of course there are many things that I haven’t got right – you need to rewrite it so that it sounds like you – but the important things are there. I’m hoping you can get some personal information about Sivincy from Anderov – for example, the mathematician that she most admires, her habits, her family background, her marriage, and so on. None of that information is going to hurt our efforts to crack RECOVERY.’

‘Isn’t sending a letter a bit rash?’

‘Well, can you think of a better way? If you can, we’ll go for that.’

I put the letter in a drawer, and said coldly, ‘I’ll think about it. Right now, you’re coming with me.’ I got up to go, and deliberately did not tell her where we were going.

She ran after me and asked, ‘Where are you taking me?’

‘You’ll find out when we get there.’

I took her to Comrade Jin’s domain, the analysis room. The job of an analyst is to look for the cribs in every secret message, to see whether they can pick out any words or phrases. Some people called their work ‘autopsying a corpse’, because a radio intercept that has not yet been analysed is no different from a corpse, and what they were trying to do was in fact just the same as an autopsy – carrying out dissection and analysis of the body. To use Anderov’s words: the connection between analysis and cryptography is the same as that between the words and the text. If you want to write a text the first thing is that you need to know enough words. The analysts teach you the words, the cryptographers instruct you in the meaning. So you can understand how important analysis is to the work of cryptography.