The history of what is still as I write this an ongoing scandal: perhaps the greatest miscarriage of justice in UK history, when the Post Office turned on and prosecuted its own operators for fraud on the basis of flawed evidence from its Horizon IT system.

The scandal came from a series of interlocking arrangements. The Post Office could mount prosecutions in its own right (something that its management now claims to have been unaware of), with its own investigators able to interview under caution but seemingly not required to record interviews or allow legal representation. The contracts under which postmasters operated were mis-represented to turn any discrepancies into the postmasters’ fault, while simultaneously denying them the data they’d need to investigate themselves unless they were exceptionally careful and persistent – and the few who were were prosecuted and driven out anyway.

As a computer scientist I have to say that the most ridiculous part of the story is the presumption in law that a computer system works properly absent explicit to the contrary, that – when coupled with an asymmetry of who can access to software and its logs – makes it impossible to fight against computer-generated evidence. No system of this size ever works flawlessly, and a careful designer would add features on that assumption, such as paper-trails or detailed logs that can then be forensically analysed. Equally, no such system would ever be constructed without remote access to terminal endpoints to allow problems to be remotely fixed, and again a careful designer would ensure that such interventions were logged so as to be auditable. This is so self-evident that I find it hard to believe that Horizon wasn’t designed that way, which makes the repeated denials even more suspicious.

But there’s a social commentary here too. The justice system, juries, and the communities of the prosecuted postmasters were almost eager to believe their guilt. Part of that was the lack of proper media scrutiny to bring the extent of prosecutions into the public eye, and part due to obfuscation by the Post Office itself – but it surely there should be systemic oversight of such things, given the massive consequences. And surely communities should have been less ready to believe accusations made against their own members that went against the evidence of their own eyes.

4/5. Finished Sunday 7 April, 2024.

(Originally published on Goodreads.)