Back at the People’s Police headquarters, before she could even brief Tilsner on the strange Interhotel meeting, she found herself having to deal with a slightly breathless Jonas Schmidt.

‘Ah, Comrade Müller. I’m glad I’ve found you. I’ve finally had a chance to study those files of encoded data in detail, and I think I’ve cracked the code and worked out what it all means.’

‘Go on.’

Schmidt looked pointedly at Tilsner at the adjoining desk. ‘I think it would be easier if you came down to the lab and we went through them there, Comrade Hauptmann. Some of the information is a little sensitive.’

Tilsner laughed. ‘That’s OK, both of you. Keep me out of the loop, why don’t you? I’m obviously utterly untrustworthy. I might as well go out to the nearest bar and start cracking open the beers.’

‘Don’t be like that, Werner,’ said Müller. ‘I’m sure that’s not what Jonas meant.’ Despite what she said aloud, privately she knew it was exactly what her forensic scientist was implying. ‘I’ll fill you in on everything in a few minutes’ time.’

*

As they descended to the forensic laboratory in the elevator, Müller could tell Schmidt still wasn’t going to talk publicly about his findings until they were actually in the sanctuary of his work environment. But even there, the Kriminaltechniker was taking no chances. As he’d done on previous cases in Berlin, he ushered his commanding officer into the lab’s dark room, switched on the eerie red safety light, and closed the door.

‘Can’t you just put the normal light on, Jonas?’

‘The bulb’s gone, I’m afraid, Comrade Hauptmann, and I haven’t got round to fixing it yet. You should be able to see all you need with this.’

Müller sighed, and gave a small nod. ‘Come on then. What have you found?’

‘Well I’ve managed to decipher the documents. They’re written in a form of what’s known as a Caesar cipher – a very early version of coding.’

‘What does that mean?’ asked Müller.

‘Well, for example, each letter might be transposed by three spaces in the alphabet. So the letter “a” becomes the letter “d”, a “b” is an “e” and so on.’

Müller frowned. ‘That doesn’t sound as though it would be a terribly difficult code to crack. I might even manage it.’

‘Precisely, Comrade Hauptmann. But it can be made almost indecipherable by using a Vigenère cipher.’

‘What’s that when it’s at home?’

‘It’s just a way of – if you like – disrupting the Caesar cipher in a particular order, that can only be solved using a special alphabetical table – a tabula recta. The table shifts the alphabet along, and which position you use depends on a repeating keyword.’