‘It’s hard to know without seeing it. I guess it would depend on the quality of the lighting, the camera angles . . .’

‘But, in theory, is it possible that we could be completely convinced by a fake?’

Danny glanced over his shoulder to check no one was listening. ‘In theory, yes. Who you are talking about?’

‘The prime minister, say, or the US president.’

Danny nodded. ‘You could fake footage of either of them giving a speech they never gave saying things they never said – and people have.’

‘How?’

‘Well, they’ve given thousands of speeches, so you feed all those into a powerful piece of software called a neural network. You direct the software to learn the visual associations between particular words and their mouths as they say them. And if you want the final version to be particularly convincing, you’d get the software to compete with a copy of itself, one generating the imagery, the other trying to spot the fakes. They call them generative adversarial networks and it’s very effective. The computer goes on improving its work until it finds a way to beat the competing network that is trying to weed out the fakes, so you get pure computational hallucinations.’

‘What about a sex video?’

‘Same principle, though probably easier in reality. You just need to make the statistical connections between the individual you want to focus on and the aspect of his behaviour you wish to fake – in this case movement.’

‘Could you spot a fake?’

‘Probably. The GAN images have a creepy edge, though the software is improving all the time. The Russians might be ahead of us on this.’

‘Could I spot a fake?’

‘It all depends on the clarity of the image. If there is plenty of light and the visual and audio quality are good, you’d probably have a good sense of whether it’s real or not. But the lower the quality, the easier it might be to pass off as a fake.’

Kate touched Danny’s arm. ‘Thanks.’ She made a brief phone call to their liaison officer at GCHQ in Cheltenham to check whether they had any information on the claims Mikhail had made of a coup inside Moscow Centre against his father and Vasily Durov. She said they had heard nothing of the kind.

Kate walked up to the chief’s office on the fifth floor. C, otherwise known as Sir Alan Brabazon, was waiting for her, looking out at the lights of the House of Commons twinkling on the far side of the river. As he turned to face her, she thought how much he had aged these past six months, his thick curly hair now flecked with grey and his hooded eyelids locked under a permanent frown. His wife, Alice, had seen her cancer return – this time to the liver – and her life was now almost certainly measured in weeks rather than months. He walked to his desk and picked up the phone. ‘I’ll get Ian up here.’ He dialled and waited. ‘She’s here,’ he said, and replaced the receiver.