A friend of mine has made something like this for a blockchain hackathon once, around 2 years ago. The technics he used where relatively simple. It stats with some Elliptic-curve cryptography math to split up a single main key into multiple keys. Every person would than have a full copy of the encrypted files and when enough people combine there keys on the blockchain they would get the main key to decrypt the files and from then on it would be public that the files have been opened and by who.

This app seams to use Shamir's Secret Sharing, this is something where I am not familiar with, but from how far I understand the Wikipedia article about it. it works roughly the same but it is more general.

I'm interested to see if people will actually use this. If anyone has some additional explanations about the differences between these algorithms then that would be very appreciated.