Suppose that I have a document and I want to build a proof, for the year of 2020, that this document existed in 2017. I could, for one, use a public blockchain - that is the only method I know for accomplishing this. Is there, though, a more compact and self contained method to achieve the same?
I pick an inherently sequential hashing algorithm and build an ASIC for it that operates at provably near the thermodynamics limits. I keep "hashing" that document continuously, non-stop, until 2020. Then, at 2020, I publish the resulting
HASH^N(document) value. Someone with an equally powerful ASIC will be able to verify my document indeed existed in 2017. The only issue is that it will take him another 3 years to do so. Moreover, if I want to prove that in 2030, a verifier would need 13 years and so on.