Training British Barristers in Interrogating Authenticated Digital Evidence
A mock trial at Inner Temple tests new standards for authenticating open-source material – and reveals what happens when cryptographic metadata meets centuries-old legal tradition.
"Never ask a question in court you don't know the answer to" is advice lawyers heed. On Saturday 19 July, 2025, I took part in a training session for British barristers built as a mock hearing. The objective: practicing cross-examining an expert witness about open-source intelligence (OSINT) as evidence.
OSINT is increasingly available to prosecutors thanks to the popularity of cell phone cameras and social media, but finding the photographer may prove difficult – if not impossible. Generative AI creates additional challenges to admissibility.
The fictional scenario we designed involved all the staples of open-source methodologies (geolocation, chronolocation, etc.) and was the first time C2PA metadata was being interrogated in a court-like setting for its reliability and probative weight.
The Case Itself
The fictional case derives from an alleged war crime in Yemen (in fact, an active investigation of Bellingcat). In our training scenario, the prosecution argues that a jet fighter pilot conducted a "double-tap" airstrike on June 12, 2023 at approximately 10:30 AM – first dropping one bomb, then returning to strike again and killing at least six civilians who were conducting rescue operations. Mr. Smith, the defendant, claims he conducted only a single, legitimate military strike at 9:00 AM targeting a military communications center when no civilians were present.
Ultimately, the case is about the second strike (which makes it a war crime). If its criminal nature can’t be established beyond a reasonable doubt, Smith won’t face consequences for an alleged strike against a possibly legitimate target.
The Evidence Presented
The mainstays of open-source investigations are relied upon to interrogate when the acts might have taken place:
- Firstly, chronolocation aims to reliably and accurately estimate the time of the incident.
- Secondly, contextual verification is put forward by the prosecution to corroborate a piece of open-source evidence with other open-source evidence. (The scenario deliberately doesn't include direct testimony, to focus the trainees on open-source investigations only.)
- Finally, and most relevantly for us at Starling Lab, some material includes C2PA metadata, a technical standard for provenance which is presently untested in judicial settings. We presented the trainees with a video including two Manifests: one with the media's authentication metadata produced straight from the C2PA-compatible Sony camera that captured it; the second added to the video after the fact by an investigative journalism collective, claiming to have verified the video, in a similar fashion to what BBC Verify have experimented with.
In addition to contributing to the scenario design and the case papers, I took part in the training as an expert witness interrogated and cross-examined by a group of six barristers (three on each side). I testified to the whole of the prosecution expert report as though the case was real and it were my own report. It was an intense four hours!
Impressions and takeaways
I found it fascinating to get an insight into what barristers – with no pre-existing familiarity with digital investigations methods – make of this bundle of evidence. It appeared quite corroborating, all-pointing-in-one-direction, but perhaps somewhat circular. In a trial, nothing is taken for granted.
As an expert witness the reliability of what you say is under assault – all answers contribute to your own credibility on the stand, and in your report.
The experience highlighted the paramount importance of intellectual honesty for an expert witness. Rather than serving the interests of the party who commissioned their report, the expert's primary responsibility lies with the court. This means providing unbiased, accurate, and complete information, even if certain nuances or findings might not directly support the commissioning party's case. The mock trial served as a practical exercise in upholding this principle, underscoring that the expert's credibility and the integrity of the judicial process depend on their unwavering commitment to the truth, irrespective of who is paying their fees.
The purpose of relying on expert witness testimony is to inform the court, but it comes with the responsibility of having to effectively communicate findings and opinions. Too long and you'll be cut out; too in the weeds and jargony and you'll lose the room. Analogies and relatable explanations are the name of the game there.
From a substance perspective, I found myself unable to get to some of the nuances I wanted to touch on. Because the exercise was quite focused on how much time was allocated to the metadata questions, we got somehow stuck on how C2PA data can be removed from an image, and how difficult it might be to acquire an X.509 certificate.
I had laid a few hooks for the defense to pick up on, and left a question or two unanswered. For example, the prosecution expert did not include a detailed verification report of the second organisation's cert (they really should have).
Prior to this mock trial, I took a week-long "Courtroom Testimony for Expert Witnesses" course from LEVA and Jonathan Hak KC. This gave me a hunch that I was trying to be too clever: as a training exercise it's not optimal to leave stones unturned, since no-one in the court is more of an expert than we are. In other words: The training aims to bring the lawyers to having some familiarity with the substance, and to practice the techniques – it is not for the defense to pick up on minute, detailed, very precise concerns from the report themselves.
Indeed, we did not stray far from the expert reports themselves due to lack of time. I'll be proposing a different, much more back-and-forth structure in a future training, so that we can derive must-dos and mainstays of _any_ C2PA verification.
I'm very grateful to the following people and organisations for giving me this opportunity to contribute to this exercise, and for inviting me to take part on the day. Walking the small alleyways of Temple, Blackfriars, and Fleet St is so humbling for this former law undergrad who spent a bit of time in various newsrooms, and in Hackney. So a huge thank you to:
- Professor Yvonne McDermott from Swansea University and the TRUE Project;
- Judge Korner KC, presently at the International Criminal Court (ICC)
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