Holm read the numbers back to himself and tried several simple substitution ciphers in his head. The first three tries were gobbledygook so he went across to his desk and grabbed a few sheets of paper and a couple of pencils. He handed a sheet and a pencil to Javed. ‘Let’s see what that brain of yours can do.’

Holm wrote the numbers out large. Then small. Then in a scrawl. Then neatly. He tried placing them in a clock face, moving round the dial in a random fashion. He multiplied the numbers together. Divided them. Added them.

‘You got something, boss?’ Javed said. He appeared to have a similar idea to Holm as he was using the calculator on his phone to perform some kind of complicated transformation. ‘Because it’s beyond me.’

‘Nothing, and we can’t very well give it to the bods at GCHQ. We have to work this out ourselves.’

‘They want us to get this,’ Javed said. ‘Else why bother?’

Holm read through the numbers once more. There were, of course, certain codes that couldn’t be deciphered. They were uncrackable and offered perfect secrecy even against the most powerful supercomputer.

‘I’ve got it,’ Holm said, realising what the cipher was. ‘These numbers are from a one-time pad.’

Javed nodded, understanding immediately. ‘So we’re stuffed?’

‘Unless we can find the key, yes.’

A code created with a one-time pad used a sequence of letters or numbers to encrypt the message. Without the source material, decryption was impossible.

Holm screwed up his piece of paper and lobbed it towards the filing cabinet where it bounced off the front and fell into the waste paper bin. He shook his head.

They want us to get this… Javed had said.

He stared at the filing cabinet for a few seconds and walked over and opened the top drawer. The drawer held various documents relating to the fake animal rights investigation, but Holm rummaged behind them and pulled out the index card he’d discovered when they’d first moved in to the office.

‘Christ.’ He read the name on the card. Robert Gerard Sands. The full name of Bobby Sands, the IRA hunger striker who’d died way back in the nineteen eighties. There were seventeen letters in the name and there were seventeen numbers in the tweet. He jabbed a finger at the name, not quite believing what he was seeing. ‘This is the key. This is the one-time pad.’

‘How the—?’

‘I’ve no bloody idea.’

‘A is zero, right?’ Javed was at his shoulder now, the lad’s face creased in concentration. ‘At least that’s what I was taught.’

‘Yes, let’s start with that.’ Holm began to do the calculations himself. He wrote out the full code sequence, below that the name, and below that an A to Z scale numbered zero to twenty-five.