> what they did was to, albeit likely knowingly, calculate the hash collision probability /if each bit is a coin flip/, which comes out to pow(2, -k) for k bits

> what they did was to, albeit likely knowingly, calculate the hash collision probability /if each bit is a coin flip/, which comes out to pow(2, -k) for k bits. It's tiny. I doubt that's what they did. I think they ran tests on huge numbers of pictures, got an estimate, put in a safety factor, and determined the threshold to hit their target (and put in another safety buffer then). Naturally occurring collisions are not going to be an issue, and adversarial ones neither, I predict. Just as with current cloud providers.