Stacked DRG - Offline PoRep Circuit Spec #


Stacked DRG Overview #

Stacked DRG PoRep is based on layering DRG graphs LAYERS times. The data represented in each DRG layer is a labeling based on previously labeled nodes. The final labeled layer is the SDR key, and the ‘final layer’ of replication the replica, an encoding of the original data using the generated key.

  • ReplicaId is a unique replica identifier (see the Filecoin Proofs spec for details).
  • CommD is the Merkle tree root hash of the input data to the first layer.
  • CommC is the Merkle tree root hash of the SDR column commitments.
  • CommRLast is the Merkle tree root hash of the replica.
  • CommR is the on-chain commitment to the replica, dervied as the hash of the concatenation of CommC and CommRLast.

The (offline) proof size in SDR is too large for blockchain usage (~3MB). We use SNARKs to generate a proof of knowledge of a correct SDR proof. In other words, we implement the SDR proof verification algorithm in an arithmetic circuit and use SNARKs to prove that it was evaluated correctly.

This circuit proves that given a Merkle root CommD, CommRLast, and commRStar, that the prover knew the correct replicated data at each layer.

Spec notation #

  • Fr: Field element of BLS12-381
  • UInt: Unsigned integer
  • {0..x}: From 0 (included) to x (not included) (e.g. [0,x) )
  • Check:
    • If there is an equality, create a constraint
    • otherwise, execute the function
  • Inclusion path: Binary representation of the Merkle tree path that must be proven packed into a single Fr element.

Offline PoRep circuit #

Public Parameters #

Parameters that are embeded in the circuits or used to generate the circuit

  • LAYERS : UInt: Number of DRG layers.
  • LAYER_CHALLENGES : [LAYERS]UInt: Number of challenges per layer.
  • EXPANSION_DEGREE: UInt: Degree of each bipartite expander graph to extend dependencies between layers.
  • BASE_DEGREE: UInt: Degree of each Depth Robust Graph.
  • TREE_DEPTH: UInt: Depth of the Merkle tree. Note, this is (log_2(Size of original data in bytes/32 bytes per leaf)).
  • PARENT_COUNT : UInt: Defined as EXPANSION_DEGREE+BASE_DEGREE.

Public Inputs #

Inputs that the prover uses to generate a SNARK proof and that the verifier uses to verify it

  • ReplicaId : Fr: A unique identifier for the replica.
  • CommD : Fr: the Merkle tree root hash of the original data (input to the first layer).
  • CommR : Fr: The Merkle tree root hash of the final replica (output of the last layer).
  • InclusionPath : [LAYERS][]Fr: Inclusion path for the challenged data and replica leaf.
  • ParentInclusionPath : [LAYERS][][PARENT_COUNT]Fr: Inclusion path for the parents of the corresponding InclusionPath[l][c].

Design notes:

  • CommRLast is a private input used during during Proof-of-Spacetime. To enable this, the prover must store CommC and use it to prove that CommRLast is included in CommR [TODO: define ‘included’ language.]
  • InclusionPath and ParentInclusionPath: Each layer l has LAYER_CHALLENGES[l] inclusion paths.

Private Inputs #

Inputs that the prover uses to generate a SNARK proof, these are not needed by the verifier to verify the proof

  • CommR : [LAYERS-1]Fr: Commitment of the encoded data at each layer.

    Note: Size is LAYERS-1 since the commitment to the last layer is CommRLast

  • DataProof : [LAYERS][][TREE_DEPTH]Fr: Merkle tree inclusion proof for the current layer unencoded challenged leaf.

  • ReplicaProof : [LAYERS][][TREE_DEPTH]Fr: Merkle tree inclusion proof for the current layer encoded challenged leaves.

  • ParentProof : [LAYERS][][PARENT_COUNT][TREE_DEPTH]Fr: Pedersen hashes of the Merkle inclusion proofs of the parent leaves for each challenged leaf at layer l.

  • DataValue : [LAYERS][]Fr: Value of the unencoded challenged leaves at layer l.

  • ReplicaValue : [LAYERS][]Fr: Value of the encoded leaves for each challenged leaf at layer l.

  • ParentValue : [LAYERS][][PARENT_COUNT]Fr: Value of the parent leaves for each challenged leaf at layer l.

Circuit #

In high level, we do 4 checks:

  1. ReplicaId Check: Check the binary representation of the ReplicaId
  2. Inclusion Proofs Checks: Check the inclusion proofs
  3. Encoding Checks: Check that the data has been correctly encoding into a replica
  4. CommRStar Check: Check that CommRStar has been generated correctly

Detailed

// 1: ReplicaId Check - Check ReplicaId is equal to its bit representation
let ReplicaIdBits : [255]Fr = Fr_to_bits(ReplicaId)
assert(Packed(replica_id_bits) == ReplicaId)

let DataRoot, ReplicaRoot Fr

for l in range LAYERS {

  if l == 0 {
    DataRoot = CommD
  } else {
    DataRoot = CommR[l-1]
  }

  if l == LAYERS-1 {
    ReplicaRoot = CommRLast
  } else {
    ReplicaRoot = CommR[l]
  }

  for c in range LAYERS_CHALLENGES[l] {
    // 2: Inclusion Proofs Checks
    // 2.1: Check inclusion proofs for data leaves are correct
    assert(MerkleTreeVerify(DataRoot, InclusionPath[l][c], DataProof[l][c], DataValue[l][c]))
    // 2.2: Check inclusion proofs for replica leaves are correct
    assert(MerkleTreeVerify(ReplicaRoot, InclusionPath[l][c], ReplicaProof[l][c], ReplicaValue[l][c]))
    // 2.3: Check inclusion proofs for parent leaves are correct
    for p in range PARENT_COUNT {
      assert(MerkleTreeVerify(ReplicaRoot, ParentInclusionPath[l][c][p], ParentProof[l][c][p]))
    }

    // 3: Encoding checks - Check that replica leaves have been correctly encoded
    let ParentBits [PARENT_COUNT][255]Fr
    for p in range PARENT_COUNT {
      // 3.1: Check that each ParentValue is equal to its bit representation
      let parent = ParentValue[l][c][p]
      ParentBits[p] = Fr_to_bits(parent)
      assert(Packed(ParentBits[p]) == parent)
    }

    // 3.2: KDF check - Check that each key has generated correctly
    // PreImage = ReplicaIdBits || ParentBits[1] .. ParentBits[PARENT_NODES]
    let PreImage = ReplicaIdBits
    for parentbits in ParentBits {
      PreImage.Append(parentbits)
    }
    let key Fr = SHA256(PreImage)
    assert(SHA256(PreImage) == key)

    // 3.3: Check that the data has been encoded to a replica with the right key
    assert(ReplicaValue[l][c] == DataValue[l][c] + key)
  }

  // 4: CommRStar check - Check that the CommRStar constructed correctly
  let hash = ReplicaId
  for l in range LAYERS-1 {
    hash.Append(CommR[l])
  }
  hash.Append(CommRLast)

  assert(CommRStar == PedersenHash(hash))
  // TODO check if we need to do packing/unpacking
}

Verification of offline porep proof #

  • SNARK proof check: Check that given the SNARK proof and the public inputs, the SNARK verification outputs true
  • Parent checks: For each leaf = InclusionPath[l][c]:
    • Check that all ParentsInclusionPaths_[l][c][0..PARENT_COUNT} are the correct parent leaves of leaf in the DRG graph, if a leaf has less than PARENT_COUNT, repeat the leaf with the highest label in the graph.
    • Check that the parent leaves are in ascending numerical order.