It's a very difficult problem to solve in a way that works for most people, i.e. secure, privacy preserving, easy to use, etc. It's easy to have one and not the

It's a very difficult problem to solve in a way that works for most people, i.e. secure, privacy preserving, easy to use, etc. It's easy to have one and not the other. It's not true that it can not be solved at all though. The guys from idena.io have a solution that definitely works, problem is it's not convenient and I highly doubt that it'll ever be adopted by the masses. But it is a sybil attack solution and it does work over the internet. Edit: Just noticed you said prove that it's always the same person. They can not do that. But they can prove that it's a unique human. Which seems good enough for online reviews imo, where it doesn't exactly matter who the person is.