> When I see a hash that satisfies the difficulty, I know it must have taken time. The method by which the delay is accomplished is “work”, but the hash is a pr

> When I see a hash that satisfies the difficulty, I know it must have taken time. The method by which the delay is accomplished is “work”, but the hash is a proof of time, not work. I disagree with this characterization. Mostly because there actually is a more proper concept of Proof-of-Time, that doesn't depend on Bitcoin's dynamic difficulty adjustment to space puzzle solutions a certain time apart on average. It is what Bram Cohen is trying to use in conjunction with Proof-of-Space to make [1] a greener alternative to Bitcoin. One candidate problem for proving time is modular square roots. In a group of order 2^k-1, we can square element x (k-1) times to obtain x^(2^(k-1)), which is the square root of x^(2^k)=x. Since the k squarings cannot be parallellized, this takes time proportional to k. [1] https://chia.net/ https://chia.net/