What is Halo 2?

Halo2 is a newer alternative proving system to Filecoin’s existing Groth16 proving system.

• https://zcash.github.io/halo2/index.html

Goals:

• Improved flexibility for on-chain proofs aggregation
  • TODO: (CryptoNetLab): how to utilize these aggregation capabilities within Filecoin correctly and securely
  • This may include the ability to use Halo2’s recursion
• No more trusted setups for new circuits (making it faster to roll out future proofs innovations)
• (TBD) Possibly faster proving and verification

Halo 2 Dev Timeline

Milestones

1. Pasta field hashers and vanilla proofs
2. GPU Pasta field arithmetic
   • [x] Fork pasta_curves and implement ec_gpu::GpuField for Pasta fields
     • [ ] Get it merged upstream: https://github.com/zcash/pasta_curves/pull/31 [Preferred; PR approved]
       • [ ] Alternative: Move vmx’s fork to the filecoin-project Github
   • [ ] Make neptune's GPU code generic over any field
     • [x] Code finished
     • [ ] https://github.com/filecoin-project/neptune/pull/135 merged
     • [ ] Release new version of neptune which removes futhark and integrates vmx’s GPU changes
   • [ ] Make rust-fil-proofs GPU code compatible with multiple fields
     • [x] Code finished
     • [ ] https://github.com/filecoin-project/rust-fil-proofs/pull/1560 merged
     • Requires pasta_curves and neptune releases
3. Poseidon circuit in neptune and rust-fil-proofs
   • Replace ECC’s poseidon::Spec with neptune::PoseidonConstants
   • Replace const generics with typenum
4. SHA256 hash function in rust-fil-proofs
   • Create wrapper around halo2_gadgets::Sha256 which uses typenum to specify preimage lengths (measured in number of u32s)
5. Hasher circuit interface in rust-fil-proofs that’s capable of computing and verifying digests
6. Merkle proof circuit with configurable hash function and arity (using typenum)
   • This may require separate Merkle proof gadgets for each hasher (Sha256 and Poseidon)
7. Proofs circuit utils
   • Binary decomposition circuit
   • 32-bit uints
   • Pick and insert circuits
8. Proofs circuits (PoRep, PoSt, ESU)
9. rust-fil-proofs Halo2 APIs
   • TBD: API version handling in existing APIs vs new versioned APIs
   • TBD: Parameter loading, caching and runtime validation against manifest (if required)
   • Create Halo2 and Groth16 circuits for a vanilla proof
   • Halo2 compatible CompoundProof interface: generate SNARK from vanilla proof, SNARK verification
   • Halo2 keygen, reading, and writing