groth16-compile

Compile Circom Circuit for zkVerify

Help the user compile their Circom circuit and run the trusted setup ceremony to prepare for proof generation.

Arguments

If invoked with an argument (e.g., /groth16-compile password_hash), use it as the circuit name.

If no argument provided, look for .circom files in circuits/ or ask the user which circuit to compile.

Scope

This skill compiles Circom circuits for Groth16 proof generation with snarkjs.

What this covers:

• Circom → R1CS compilation
• Groth16 trusted setup (Powers of Tau + phase 2 contribution)
• Proving key and verification key generation

Alternatives not covered:

• Noir (Aztec) → compiles to Ultraplonk/Ultrahonk, not Groth16
• Leo (Aleo) → targets Aleo's proof system
• Cairo (Starknet) → uses STARKs, not SNARKs

Pre-flight Check

Before compiling, verify tools are installed:

circom --version && npx snarkjs --version

Check	If Missing
circom	Auto-install (see below)
snarkjs	npm install snarkjs
circomlib	npm install circomlib (if circuit uses it)

IMPORTANT: Auto-Install Circom if Missing

If circom is not installed, DO NOT stop and ask the user to install it manually. Instead, automatically install it by running these commands:

# 1. Install Rust if not present
if ! command -v cargo &> /dev/null; then
    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
    source "$HOME/.cargo/env"
fi

# 2. Install build tools (Linux/Docker)
if [ -f /etc/debian_version ]; then
    apt-get update && apt-get install -y build-essential git || sudo apt-get update && sudo apt-get install -y build-essential git
fi

# 3. Clone and build circom
if [ ! -d /tmp/circom ]; then
    git clone https://github.com/iden3/circom.git /tmp/circom
fi
cd /tmp/circom && cargo build --release && cargo install --path circom

# 4. Ensure PATH is updated
source "$HOME/.cargo/env"

# 5. Verify
circom --version

Always proceed with installation automatically. Only stop if the installation itself fails after multiple attempts.

Compilation Steps

Step 1: Compile Circuit

mkdir -p build

circom circuits/<circuit_name>.circom \
    --r1cs \
    --wasm \
    --sym \
    -o build/ \
    -l node_modules

Note: The -l node_modules flag tells circom where to find circomlib.

Step 2: Check Constraint Count

npx snarkjs r1cs info build/<circuit_name>.r1cs

Note the constraint count to choose the right Powers of Tau size.

Step 3: Download Powers of Tau

# Choose size based on constraints (see reference.md for full table)
# pot12 = up to 4,096 constraints
curl -L https://storage.googleapis.com/zkevm/ptau/powersOfTau28_hez_final_12.ptau \
    -o build/pot12_final.ptau

Constraints	ptau	File
< 4,096	12	4.8 MB
< 16,384	14	19 MB
< 65,536	16	75 MB
< 262,144	18	300 MB

See reference.md for larger sizes.

Step 4: Generate Proving Key (zkey)

# Phase 2: Circuit-specific setup
npx snarkjs groth16 setup \
    build/<circuit_name>.r1cs \
    build/pot12_final.ptau \
    build/<circuit_name>_0000.zkey

# REQUIRED: Add contribution (DO NOT skip this!)
npx snarkjs zkey contribute \
    build/<circuit_name>_0000.zkey \
    build/<circuit_name>.zkey \
    --name="Circuit contribution" -v \
    -e="$(openssl rand -hex 32)"

WARNING: The _0000.zkey is NOT safe for production. The contribution step is mandatory.

Step 5: Export Verification Key

npx snarkjs zkey export verificationkey \
    build/<circuit_name>.zkey \
    build/verification_key.json

Step 6: Clean Up

rm -f build/<circuit_name>_0000.zkey

Expected Output

build/
├── <circuit_name>.r1cs           # Constraint system
├── <circuit_name>.zkey           # Proving key
├── <circuit_name>_js/
│   └── <circuit_name>.wasm       # Witness calculator
├── pot12_final.ptau              # Powers of Tau (reusable)
└── verification_key.json         # For zkVerify

After Compilation

1. Verify all files exist in build/
2. Run /groth16-prove to generate proofs
3. Keep .zkey file secure

Checklist

• Prerequisites installed (circom auto-installed if missing, snarkjs)
• Circuit compiled (r1cs + wasm generated)
• Powers of Tau downloaded (correct size for constraints)
• Phase 2 contribution completed (not using _0000.zkey!)
• Verification key exported
• Cleanup done (removed _0000.zkey)
• Reminded user of next steps

Automation Principles

DO NOT ask the user to do things manually. This skill should:

1. Auto-install circom if missing (using Rust/cargo)
2. Auto-install snarkjs if missing (npm install snarkjs)
3. Auto-download the correct Powers of Tau file
4. Run all compilation steps automatically
5. Only stop if there's an unrecoverable error

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Additional Resources

• installation.md - Install Rust, Circom, snarkjs, circomlib
• reference.md - Powers of Tau explanation, full ptau table, compiler flags
• troubleshooting.md - Common errors and fixes