He sits down on the other side of his desk, which is too big for the room. His office is on the second floor, and his window overlooks the courtyard we just walked through. A wi-fi router blinks in the corner. There’s a bookshelf, the books themselves hidden behind trophies and tchotchkes. A small, tacky statue of a fish with dull red scales looks particularly out of place on the top shelf.

Thistle and I sit on two squishy swivel chairs, possibly stolen from other departments. Mine has JOURNALISM scratched into one of the arms. The framed photos on Luxford’s desk—him with some skis, him getting a trophy in high school—are all facing outwards. They’re designed for visitors. No family in them. A Zippo lighter with a satin finish sits near a silver ashtray, despite the non-smoking signs all over the building and the smoke detector right above.

There’s a strange wooden device on his desk next to the printer. A horizontal rod with thirteen discs impaled on it, capital letters carved into the outside of the rims.

Luxford sees me looking.

‘You like that?’ He doesn’t wait for a response. ‘It’s called a wheel cipher. Thomas Jefferson invented it in 1795. Each disc has all twenty-six letters in a slightly different order, and a unique number. So you can spin the discs to make a row of letters which spells a message. Then you write down the letters from a different row, which will look like gibberish. Only someone else with the same discs in the same order will be able to decode it.’

‘Interesting,’ Thistle says convincingly. ‘Where’d you get it?’

‘A thankyou gift from the professor.’

‘Thanking you for what?’

He smiles modestly.

‘Seems like a lot of work,’ I say. ‘Couldn’t you do the same thing with a pen and paper? Just write down A equals F, B equals X, or whatever?’

‘That’s the clever part,’ Luxford says. ‘With a wheel cipher, A can equal a different letter each time it shows up. So you can’t just look for the most common letter and assume that it’s E. Although if you sent enough messages using the same sequence of discs, a computer could still use letter frequency to crack it.’

Now that I understand, I automatically scan the letters on the discs. Just nonsense, in its current configuration. I look away when I realise I’ve started memorising the rows. Wasting brain space.

‘Who has the other one?’ I ask.

‘No one,’ he says. ‘It’s decorative.’

‘Oh.’

The light fades from Luxford’s eyes as he realises we’re not going to ask any more questions about the cipher.

‘So,’ he asks, ‘any progress on the case?’

‘We’re still interviewing witnesses,’ Thistle says. ‘I don’t suppose you’ve remembered anything else about Professor Biggs’s movements on Tuesday?’

‘Sorry,’ Luxford says. ‘Like I said, I was helping students all day.’