A cryptographic mechanism that allows a storage node to prove to the network that it is genuinely holding a specific file, without requiring the verifier to download the entire file.
Filecoin uses two complementary proofs: Proof of Replication (PoRep) proves that a unique copy of the data was stored, and Proof of Spacetime (PoSt) proves it is still being stored over time. Storj uses erasure coding and periodic data audits to achieve a similar guarantee.