Nethereum.ZkProofs.Snarkjs 7.1.0

Nethereum.ZkProofs.Snarkjs

A local Groth16 proof provider that runs snarkjs in a Node.js process, for desktop, server and CLI applications

Overview

Nethereum.ZkProofs.Snarkjs implements the IZkProofProvider abstraction from Nethereum.ZkProofs by shelling out to the snarkjs command line. It takes a Circom circuit (.wasm), its proving key (.zkey) and the input JSON, and returns a Groth16 proof with its public signals. Use it when your .NET process can launch Node.js (desktop apps, servers, CLIs, test runs). The package description says "Local snarkjs proof provider for desktop/server/CLI (Node.js process or native interop)".

The backend is behind an interface (ISnarkjsBackend), so SnarkjsProofProvider can be pointed at another way of running snarkjs; NodeJsSnarkjsBackend is the one implementation in this package. The package only proves with Groth16 (Scheme is always ZkProofScheme.Groth16).

Installation

dotnet add package Nethereum.ZkProofs.Snarkjs

Targets net8.0 and net10.0; depends on Nethereum.ZkProofs. To run proofs with the Node.js backend you need Node.js on the PATH (or pass its path) and snarkjs reachable either through npx snarkjs or through an explicit path to snarkjs's cli.js.

SnarkjsProofProvider

Namespace Nethereum.ZkProofs.Snarkjs.

public class SnarkjsProofProvider : IZkProofProvider
{
    public ZkProofScheme Scheme { get; }

    public SnarkjsProofProvider()
    public SnarkjsProofProvider(ISnarkjsBackend backend)

    public static SnarkjsProofProvider CreateNodeJs(string? nodePath = null, string? snarkjsPath = null)

    public async Task<ZkProofResult> FullProveAsync(ZkProofRequest request, CancellationToken cancellationToken = default)
}

public interface ISnarkjsBackend
{
    Task<(string proofJson, string publicJson)> FullProveAsync(
        string wasmPath, string zkeyPath, string inputJsonPath, CancellationToken cancellationToken = default);
}

• The parameterless constructor uses new NodeJsSnarkjsBackend(); CreateNodeJs(nodePath, snarkjsPath) uses new NodeJsSnarkjsBackend(nodePath, snarkjsPath).
• A null backend or a null request throws ArgumentNullException.
• FullProveAsync writes request.CircuitWasm, request.CircuitZkey and request.InputJson to circuit.wasm, circuit.zkey and input.json in a fresh temporary directory (nethereum_zkproofs_ plus a random suffix), calls the backend with those paths, builds the ZkProofResult from the returned JSON with ZkProofResult.BuildFromJson(ZkProofScheme.Groth16, proofJson, publicJson), and deletes the directory afterwards. Only CircuitWasm, CircuitZkey and InputJson of the request are used; WitnessBytes, CircuitGraph, CircuitName and Scheme are ignored by this provider.

ZkProofResult (from Nethereum.ZkProofs) carries Scheme, ProofJson, PublicSignalsJson and the parsed PublicSignals (BigInteger[]).

Prove through a backend

var backend = new RecordingBackend();
IZkProofProvider provider = new SnarkjsProofProvider(backend);

var result = await provider.FullProveAsync(new ZkProofRequest
{
    CircuitWasm = new byte[] { 1, 2, 3 },
    CircuitZkey = new byte[] { 4, 5, 6 },
    InputJson = "{\"a\":\"3\"}"
});

Here RecordingBackend is the test's own ISnarkjsBackend that records the three files it was given and returns ("{\"pi_a\":[\"1\",\"2\",\"1\"]}", "[\"7\",\"11\"]"); result.PublicSignals is [7, 11], result.Scheme is Groth16, and the temporary files are gone after the call. Backed by SnarkjsProofProviderTests.Given_ABackend_When_FullProveAsyncRuns_Then_TheCircuitFilesAreWrittenAndTheResultIsParsed (DocSection.Protocols, snarkjs-proof-provider, Order 1) in tests/Nethereum.ZkProofs.RapidSnark.Tests/SnarkjsProofProviderTests.cs.

NodeJsSnarkjsBackend

public class NodeJsSnarkjsBackend : ISnarkjsBackend
{
    public NodeJsSnarkjsBackend(string nodePath = null, string snarkjsPath = null)

    public async Task<(string proofJson, string publicJson)> FullProveAsync(
        string wasmPath, string zkeyPath, string inputJsonPath, CancellationToken cancellationToken = default)
}

• nodePath defaults to the first node found on the PATH (with .exe, .cmd, .bat extensions tried on Windows), or the bare name node.
• If snarkjsPath is given, snarkjs runs as node "<snarkjsPath>" <command>; if not, as npx snarkjs <command> (npx is also located on the PATH).
• FullProveAsync first checks that the wasm, zkey and input files exist (FileNotFoundException otherwise), then runs wtns calculate <wasm> <input> witness.wtns and groth16 prove <zkey> witness.wtns proof.json public.json, with the outputs written beside the input file, and returns the contents of proof.json and public.json. A non-zero exit code throws InvalidOperationException with the exit code, stderr and stdout.

The guard clauses are covered by SnarkjsProofProviderTests.Given_InvalidArguments_When_TheProviderOrBackendIsUsed_Then_ItRefuses (snarkjs-proof-provider, Order 2): a null backend or request throws ArgumentNullException, and a NodeJsSnarkjsBackend pointed at missing files throws FileNotFoundException without starting any process.

Using real snarkjs

var provider = SnarkjsProofProvider.CreateNodeJs(nodePath: "node", snarkjsPath: "snarkjs/cli.js");
var defaultProvider = SnarkjsProofProvider.CreateNodeJs();

CreateNodeJs only builds the provider; no process starts until FullProveAsync is called. Backed by SnarkjsProofProviderTests.Given_NodeAndSnarkjsPaths_When_CreateNodeJsRuns_Then_AGroth16ProviderIsReturnedWithoutStartingAnyProcess (snarkjs-proof-provider, Order 3). Calling FullProveAsync on it with a ZkProofRequest (CircuitWasm, CircuitZkey, InputJson) needs Node.js, snarkjs and real circuit artifacts, so that call is not run by any README example. The repository exercises it end to end in CrossProviderTests.CrossProvider_SnarkjsWitness_RapidSnarkProof_BothVerify (tests/Nethereum.ZkProofs.RapidSnark.Tests, trait Native-Integration), which proves the privacy-pool commitment circuit with snarkjs and again with rapidsnark and verifies both; it returns early with a skip message when the circuit artifacts are not present.

Related providers

The other IZkProofProvider implementations are listed in Nethereum.ZkProofs: for example RapidSnarkProofProvider (native) in Nethereum.ZkProofs.RapidSnark and SnarkjsBlazorProvider for the browser.