I'm not an expert at all on cryptography so I can't comment on that, however when looking for info about thorn I found a ftm page where a uni researcher acknowledges it's not possible to do it yet. It should be either this https://www.ftm.eu/articles/ashton-kutchers-non-profit-start... or this one, I can't remember at the moment https://www.ftm.eu/articles/ashton-kutcher-s-anti-childabuse...

Edit "possible" as in very computationally expensive to do it on a mass scale