Sia founder here. The hashing algorithm we use is blake2b. Definitely secure. We do probabilistic proofs, so we have the host provide us a small random samplin

Sia founder here. The hashing algorithm we use is blake2b. Definitely secure. We do probabilistic proofs, so we have the host provide us a small random sampling of actual data (so the host can't rely on precomputing), plus a proof that this actual data is what the contract says the host should be storing. See chapter 5: https://sia.tech/sia.pdf https://sia.tech/sia.pdf