I took the folder and saw that it contained a thick stack of papers, each page covered in formulae and the data for the calculations. The formulae were all very complex and there was a mass of figures to work through – so densely packed that as I read through them my eyes widened. I said, ‘Oh hell, this is going to be a huge exercise.’

‘Of course it is big,’ she said. ‘Each stage in the separation process is a living thing: when the top plate moves, the others all change; when the left figure moves, so does the one on the right. If you have to work through a whole stack of calculations even to get to the starting point, there is a limit to how small the exercise can possibly be.’

I handed the folder over to the manager of the Calculations Office, Comrade Jiang. ‘You have a look and estimate roughly how long this is going to take you to do.’

Comrade Jiang looked at it and said, ‘If we split our people into three teams and work round the clock, it will take us about a month.’

Huang Yiyi gave a sharp exclamation. ‘That long!’

‘We don’t have the facilities or the people to do it in less.’

‘If only we had computers.’

‘What happens if you’re wrong?’ asked Comrade Chen. ‘That is one hell of a waste of time!’

Comrade Chen’s word of warning startled everyone, and we all looked at each other, including Huang Yiyi; and then in the end all eyes turned to me, waiting for me to make the final decision. You know, right at that moment I found it very difficult to know what to do. It was a massive exercise which would involve an awful lot of time and effort, and what happened if the premise it was based on was wrong? That would be a disaster! But then I thought about it from another point of view: breaking a cipher is like looking for a needle in a haystack, so what is the chance of getting it right first time? But if you don’t go into the tiger’s den you can’t capture its cubs; if you don’t carry out the exercise how do you know whether the premise is wrong or not? So I was silent for a bit and then resolutely made my decision: ‘If you are right, we have made a massive step forward in cracking RECOVERY. For that, a month is worth it!’