Proof-of-Spacetime

Proof-of-Spacetime (PoSt) is a cryptographic proof that a storage provider has continued to store particular data throughout a period, not just at a single moment. It is used in Filecoin, where it works together with Proof-of-Replication: the replication proof shows a unique copy was created, and spacetime proofs show the copy is still there. Providers are repeatedly challenged to produce proofs for randomly selected parts of their stored data within short deadlines, so they cannot discard data and rebuild it only when asked. Filecoin uses two main forms: frequent checks that every committed sector is still stored, with penalties for missed or faulty proofs, and a lottery-style check that helps decide which providers win the right to produce blocks, with chances weighted by storage power. Related ideas appear in other consensus mechanisms. Also see proof of time.