Submission and verification

Once you have registered your application's verifying key(s) you are ready to submit application proofs to NEBRA UPA. In this section you will learn how to submit and verify proofs using our SDK.

Steps to submit and verify proofs

Proofs are submitted to UPA on-chain by calling the submit function in the NEBRA UPA contract.

function submit(
        bytes32[] calldata circuitIds,
        Groth16CompressedProof[] calldata proofs,
        uint256[][] calldata publicInputs
    ) external payable returns (bytes32 submissionId);

Each submission can contain one or more proofs. For convenience and type-safety, we recommend that you use our SDK to submit proofs instead of calling this function directly.

Step 1: Export proof data

SnarkJS

Let proofData be the output of snarkjs' fullProve function, i.e.

const proofData = await snarkjs.groth16.fullProve(
    inputs,
    circuitWasm,
    circuitZkey
  );

You may save the json serialization of proofData into a file snarkjs_proof.json if you intend to submit the proof with the upa tool. You can generate a UPA-compatible proof data file with the following command

$ upa convert proof-snarkjs \
    --snarkjs-proof snarkjs_proof.json \
    --proof-file proof.upa.json

Alternatively, if you want to submit via the typescript sdk, you can easily extract the UPA-compatible proof and inputs from proofData:

import { Groth16Proof } from "@nebrazkp/upa/sdk";

const proof = Groth16Proof.from_snarkjs(proofData.proof);
const inputs: bigint[] = proofData.publicSignals.map(BigInt);

Gnark

You can modify your gnark circuit code to export the proof and the inputs as follows:

import ("encoding/json")

proof, _ := groth16.prove(ccs, pk, witness, opt.proverOpts...)
proofJSON, _ := json.MarshalIndent(vk, "", "    ")
_ = os.WriteFile("gnark_proof.json", proofJSON, 0644)

pubWitness, _ := witness.Public()
publicWitnessJSON, _ := json.Marshal(pubWitness)
_ = os.WriteFile("gnark_inputs.json", publicWitnessJSON, 0644)

If you intend to submit the proof with the upa tool, you may generate a UPA-compatible proof data file with the following command

$ upa convert proof-gnark \
    --gnark-proof gnark_proof.json \
    --gnark-inputs gnark_inputs.json \
    --proof-file proof.upa.json

Alternatively, if you want to submit via the typescript sdk, you can convert the gnark proofs and inputs to the UPA-compatible format as follows

import { Groth16Proof } from "@nebrazkp/upa/sdk";

const proof = Groth16Proof.from_gnark(gnarkProof);
const inputs: bigint[] = gnarkInputs.map(BigInt);

where gnarkProof and gnarkInputs can be obtained from the gnark_proof.json and gnark_inputs.json files, respectively. For example

import type GnarkProof from "@nebrazkp/upa/sdk";
import type GnarkInputs from "@nebrazkp/upa/sdk";

const gnarkProof = JSON.parse(
    fs.readFileSync("path/to/gnark_proof.json", "ascii")
  ) as GnarkProof;
const gnarkInputs = JSON.parse(
    fs.readFileSync("path/to/gnark_inputs.json", "ascii")
  ).map(BigInt) as GnarkInputs;

Note on gnark proofs

For gnark proofs with a Pedersen commitment point, the UPA only supports those which have been generated with keccak256 as the hash to field function. In other words, you must run the prover with the following options:

import("golang.org/x/crypto/sha3")

groth16.prove(..., backend.WithProverHashToFieldFunction(sha3.NewLegacyKeccak256()))

Note gnark's default is the hash function RFC9380, which is not currently supported by NEBRA's UPA.

Step 2: Prepare proof data

Each proof is submitted along with its corresponding Circuit Id and public inputs as a CircuitIdProofsAndInputs, defined as the following type in the application module of the sdk.

type CircuitIdProofAndInputs = {
    circuitId: bigint;
    proof: Proof;
    inputs: BigNumberish[];
};

Prepare an array CircuitIdProofsAndInputs[] of the proofs you will submit.

Alternatively, proofs may be submitted alongside a verifying key instead of a circuit Id, as an array of AppVkProofInputs:

export class AppVkProofInputs<VK = Groth16VerifyingKey, PROOF = Groth16Proof> {
  constructor(
    public readonly vk: VK,
    public readonly proof: PROOF,
    public readonly inputs: bigint[]
  ) {}
...
}

Step 3: Submit proofs

Option A: On-chain submission

Using your UpaClient (see setup), submit your array CircuitIdProofsAndInputs[].

const submissionHandle = await upaClient.submitProofs(circuitIdProofAndInputs);

Be sure to keep the returned submissionHandle as it contains information used by your application contract to check whether the proof has been verified by NEBRA UPA. It contains a Submission object that stores the proof Ids for each submitted proof and a submission Id for the entire submission. See Single and multi-proof submissions for more details.

Fee estimation (optional)

NEBRA UPA charges a nominal fee for each proof submission. Your UpaClient can estimate this fee.

const value = await upaClient.estimateFee(submissionSize);

This fee amount value can then be passed as a PayableOverrides option into upaClient.submitProofs. If no value is specified then the fee is computed automatically.

const submissionHandle = await upaClient.submitProofs(
  circuitIdProofAndInputs,
  { value }
  );

Proof submission via the upa tool

If you have a json file with UPA-compatible proof data such as proof.upa.json generated in the previous step.

First, you need to create a file with the circuitId, the proof and the inputs. You can do that e.g. using jq:

$ jq '. | {circuitId: ${cid}, proof: .proof, inputs: .inputs}' proof.upa.json > cid_proof.upa.json

where cid is the actual value of the circuit Id computed at vk registration time.

To submit the proof(s), run the following command

$ upa submit-proofs --proofs-file cid_proof.upa.json --proof-ids-file proof-id.json --submission-file submission-data.json

The option --proof-ids-file produces an output file with the proof id(s). In the case of multi-proof submissions, the option--submission-file saves the submission data to a file.

If more convenient, it is also possible to submit a file with the verifying key (instead of the circuitId), the proof and the inputs. You can generate such a file with the following command:

$ jq --argfile vk vk.upa.json '. | {vk: $vk, proof: .proof, inputs: .inputs}' proof.upa.json > vk_proof.upa.json

and then submit the file as before

$ upa submit-proofs --proofs-file vk_proof.upa.json --proof-ids-file proof-id.json --submission-file submission-data.json

Option B: Off-chain submission (not yet released)

Before sending an off-chain submission, you need to deposit ether to the off-chain aggregator's deposit contract. You can make a deposit using the command

upa off-chain deposit --deposits-contract <CONTRACT_ADDRESS> --amount-eth <AMOUNT_ETH>

Then prepare a JSON file containing an array of AppVkProofInputs objects. Note that for off-chain submissions, you must submit a verifying key along with the proofs, not a circuit Id.

To submit a proof(s) file named proof.upa.json, run the following command

$ upa off-chain submit proof.upa.json --deposits-contract <CONTRACT_ADDRESS> 

If the aggregator agrees to aggregate your submission, then this command outputs a signed response from the aggregator, which you may use to refund the aggregation fee if your submission has not been aggregated by a certain expiration block. (See the command upa off-chain refund-fee)

Otherwise, the aggregator rejects your submission then it will respond with an error message.

Step 4: Wait for proofs to be verified on NEBRA UPA

Using your submission's submissionId, wait for NEBRA UPA to verify your submission by awaiting waitForSubmissionVerified.

const submitProofTxReceipt = await waitForSubmissionVerified(
  upaInstance,
  submissionId
);

Once your submission has been verified, you can send a request to your application contract with inputs corresponding to your submission. This request uses the same inputs as before, but you will no longer need to pass in a proof when using NEBRA UPA. Your application contract will use NEBRA UPA to check the verification status of these inputs before executing the request.

Step 5: Application contract checks verification status

Your app smart contract will call isProofVerified from the NEBRA UPA contracts to check whether a proof has been verified or not.

// For single-proof submissions
function isProofVerified(
    bytes32 circuitId,
    uint256[] calldata publicInputs
) external view returns (bool);

// Verify a single proof in a multi-proof 
// submission
function isProofVerified(
    bytes32 circuitId,
    uint256[] calldata publicInputs,
    ProofReference calldata proofReference
) external view returns (bool);

For single-proof submissions, your smart contract calls isProofVerified as follows.

// `upaVerifier` is an instance of the `IUpaVerifier` contract interface
bool isProofVerified = upaVerifier.isProofVerified(circuitId, publicInputs);

For multi-proof submissions, your application contract will also need to provide a ProofReference to identify a specific proof in the submission (see Proof references).

bool isProofVerified = upaVerifier.isProofVerified(
  circuitId,
  publicInputs,
  proofReference
);

If you used our typescript SDK for a multi-proof submission, your SubmissionHandle can compute this proof reference which can then be passed to your application contract as part of your request.

// Gets the proof reference of the j-th proof in this submission.
const proofReference = submissionHandle.submission.computeProofReference(j);

What is a Proof Id?

UPA assigns a $\mathsf{proofId}$ to each proof it receives. This $\mathsf{proofId}$ is calculated as the Keccak hash of the proof's circuit id and public inputs:

$$\mathsf{proofId} = \mathsf{keccak}(\mathsf{circuitId}, \mathsf{PI})$$

Verifying a Proof Id directly

The IUpaVerifier interface contract also provides the following variations of isProofVerified:

function isProofVerified(bytes32 proofId) external view returns (bool);

function isProofVerified(
    bytes32 proofId,
    ProofReference calldata proofReference
) external view returns (bool);

In some cases, computing the proof Id internally in the application contract and calling isProofVerified with that proof Id instead of the circuit Id and the public inputs will translate into further gas savings. In that case, we provide the following library function to call from your application contract internally:

// [...]
import "@nebrazkp/upa/contracts/UpaLib.sol";

contract YourApp is Groth16Verifier {
    // [...]
    function submitTransaction(
        uint256[] calldata publicInputs,
    ) public {
        bytes32 proofId = UpaLib.computeProofId(circuitId, publicInputs);
        bool isProofCorrect = upaVerifier.isVerified(proofId);
        require(isProofCorrect, "Proof was not correct.");

        // Proceed with app's business logic 
        // ...
    }
}

We recommend application developers to always measure the gas costs of both variants before choosing an implementation.

Single and multi-proof submissions

The majority of the cost of single-proof submissions comes from storing metadata about each proof such as its $\mathsf{proofId}$. This storage cost may be significantly reduced by taking advantage of multi-proof submissions.

• Multi-proof submissions store their corresponding $\mathsf{proofId}$s in a Merkle tree.

  • The $\mathsf{submissionId}$ of a multi-proof submission is the Merkle root of the $\mathsf{proofId}$s.

  • A single-proof submission's $\mathsf{submissionId}$ is the $\mathsf{proofId}$ of its single proof.

• A submission's proofs are either all accepted if all of them are valid, or they are all rejected if any proof is invalid.

• An aggregated batch can contain proofs from different submissions.

• A submission may span multiple batches.

Proof references

To check the verification status of the j-th proof of a multi-proof submission identified by $\mathsf{submissionId}$, you must provide its ProofReference in addition to its $\mathsf{proofId}$. A ProofReference is a Merkle proof that this $\mathsf{proofId}$ is indeed the j-th leaf of a Merkle tree with root $\mathsf{submissionId}$.

Atomic verification of multi-proof submissions

In the case of multi-proof submissions, you can save even more gas at verification time by calling isSubmissionVerified instead of repeated calls to isProofVerified. The IUpaVerifier interfaces provides the following functions

// General case
function isSubmissionVerified(
    bytes32[] calldata circuitIds,
    uint256[][] memory publicInputsArray
) external view returns (bool);

// Special case when all circuit Ids are the same
function isSubmissionVerified(
    bytes32 circuitId,
    uint256[][] memory publicInputsArray
) external view returns (bool)

which attests to whether every proof in a submission has been verified or not.

As with isProofVerified, we provide the following variant which only takes a submission Id

function isSubmissionVerified(
    bytes32 submissionId
) external view returns (bool);

and the following UpaLib library functions to compute the submission Id in your application contract:

// general case
function computeSubmissionId(
        bytes32[] calldata circuitIds,
        uint256[][] memory publicInputsArray
) internal pure returns (bytes32)

// special case, all circuit ids are the same
function computeSubmissionId(
        bytes32 circuitId,
        uint256[][] memory publicInputsArray
) internal pure returns (bytes32)

// general case, the caller knows the proof Ids
// in the submission
function computeSubmissionId(
        bytes32[] memory proofIds
) internal pure returns (bytes32)