Nethereum.CircomWitnessCalc 6.1.0

Nethereum.CircomWitnessCalc

Native circom witness generation for .NET using iden3/circom-witnesscalc. Computes witnesses from circom circuit graphs without JavaScript or Node.js — works on desktop, server, and mobile.

Companion to Nethereum.ZkProofs.RapidSnark for a fully native ZK proof pipeline.

How It Works

Circom circuits are normally executed via JavaScript (snarkjs + Node.js). This package replaces that with a native C library:

1. Compile your .circom circuit into a binary graph (.graph.bin) using build-circuit (one-time)
2. At runtime, pass the graph + JSON inputs to WitnessCalculator.CalculateWitness() which returns the witness bytes (.wtns format)
3. Feed the witness to Nethereum.ZkProofs.RapidSnark for fast native proof generation

                          ONE-TIME (build step)
    circuit.circom  ──>  build-circuit --O1  ──>  circuit.graph.bin
                                                       |
                          PER-REQUEST (runtime)        |
    input JSON  ──>  WitnessCalculator  ──>  witness.wtns  ──>  RapidSnark  ──>  proof.json
                     (this package)                             (ZkProofs.RapidSnark)

No Node.js, no JavaScript, no WASM runtime. Pure native code on all platforms.

Usage

Direct witness calculation

byte[] graphData = File.ReadAllBytes("circuit.graph.bin");
string inputsJson = """{"nullifier":"123","secret":"456","value":"1000","label":"1"}""";

byte[] witnessBytes = WitnessCalculator.CalculateWitness(graphData, inputsJson);
// witnessBytes is standard .wtns format, compatible with rapidsnark and snarkjs

Combined with RapidSnark (full native pipeline)

// NativeProofProvider handles both witness generation and proving
var provider = new NativeProofProvider();

var result = await provider.FullProveAsync(new ZkProofRequest
{
    CircuitZkey = File.ReadAllBytes("circuit.zkey"),
    CircuitGraph = File.ReadAllBytes("circuit.graph.bin"),
    InputJson = inputsJson,
});
// result.ProofJson, result.PublicSignalsJson

With embedded circuit artifacts (e.g. Privacy Pools)

// PrivacyPoolCircuitSource embeds .zkey + .graph.bin as resources
var circuitSource = new PrivacyPoolCircuitSource();
var proofProvider = new PrivacyPoolProofProvider(new NativeProofProvider(), circuitSource);

var result = await proofProvider.GenerateRagequitProofAsync(witnessInput);

Supported Platforms

Pre-built native binaries are included for all platforms (built via Nethereum/circom-witnesscalc CI):

Generating Circuit Graphs

Circuit graphs are compiled from .circom source using the build-circuit tool from iden3/circom-witnesscalc.

Install build-circuit

git clone https://github.com/iden3/circom-witnesscalc.git
cd circom-witnesscalc
cargo build --release -p build-circuit
# Output: target/release/build-circuit (.exe on Windows)

Build dependencies: Rust toolchain, protoc, LLVM/clang. See Building from source below.

Compile a circuit

build-circuit circuit.circom circuit.graph.bin -l node_modules/ --O1

Important: use --O1 optimization. The default --O2 over-optimizes and reduces signal count below what the zkey expects. --O1 matches circom's default optimization used during trusted setup.

If your circuit uses circomkit (no inline component main), create a wrapper:

pragma circom 2.2.0;
include "myCircuit.circom";
component main {public [input1, input2]} = MyTemplate();

Verify signal count matches zkey

circom circuit_main.circom --r1cs -l node_modules/
snarkjs r1cs info circuit_main.r1cs
# Check "# of Wires" matches what your zkey expects

If mismatched, rapidsnark will error: Invalid witness length. Circuit: <expected>, witness: <actual>.

Updating Native Binaries

Native binaries are built by the CI workflow at Nethereum/circom-witnesscalc:

1. Push or create a release on the fork
2. Download artifacts from the workflow run
3. Copy shared libraries to runtimes/<rid>/native/

# Download all artifacts from a CI run
gh run download <run-id> -R Nethereum/circom-witnesscalc

# Copy to package runtimes
cp circom-witnesscalc-win-x64/circom_witnesscalc.dll     runtimes/win-x64/native/
cp circom-witnesscalc-linux-x64/libcircom_witnesscalc.so  runtimes/linux-x64/native/
cp circom-witnesscalc-linux-arm64/libcircom_witnesscalc.so runtimes/linux-arm64/native/
cp circom-witnesscalc-osx-arm64/libcircom_witnesscalc.dylib runtimes/osx-arm64/native/
cp circom-witnesscalc-osx-x64/libcircom_witnesscalc.dylib runtimes/osx-x64/native/
cp circom-witnesscalc-android-arm64/libcircom_witnesscalc.so runtimes/android-arm64/native/

Building from Source

All platforms (CI)

The Nethereum/circom-witnesscalc fork has a GitHub Actions workflow that builds for Windows, Linux (x64/arm64), macOS (x64/arm64), Android (arm64/x64), and iOS (arm64/sim).

Windows (local)

Prerequisites: Rust toolchain, protoc, LLVM/clang.

# Install protoc
choco install protoc
# or download: https://github.com/protocolbuffers/protobuf/releases

# Install LLVM (for libclang)
winget install LLVM.LLVM
# or choco install llvm

# Clone and build
git clone https://github.com/iden3/circom-witnesscalc.git
cd circom-witnesscalc
set LIBCLANG_PATH=C:\Program Files\LLVM\bin
cargo build --release

Output: target/release/circom_witnesscalc.dll (512 KB). Depends only on standard Windows system DLLs.

Linux

sudo apt-get install -y protobuf-compiler libclang-dev
git clone https://github.com/iden3/circom-witnesscalc.git
cd circom-witnesscalc
cargo build --release

macOS

brew install protobuf
git clone https://github.com/iden3/circom-witnesscalc.git
cd circom-witnesscalc
cargo build --release

C API

The library exports a single function (include/graph_witness.h):

typedef struct {
  int code;           // 0 = OK, 1 = ERROR
  char *error_msg;    // error message (caller must free)
} gw_status_t;

int gw_calc_witness(
    const char *inputs,              // JSON string of circuit inputs
    const void *graph_data,          // binary circuit graph bytes
    const size_t graph_data_len,     // graph length
    void **wtns_data,                // OUT: witness .wtns bytes (caller must free)
    size_t *wtns_len,                // OUT: witness length
    const gw_status_t *status        // OUT: error status
);

Credits

• iden3/circom-witnesscalc — Native witness calculator (Rust with C FFI)
• iden3/circom — Circom circuit compiler
• iden3/rapidsnark — Native Groth16 prover