Aztec Fee Payment Contracts

A Fee Payment Contract (FPC) for Aztec that enables private transaction fee sponsorship via FeeJuice.

Overview

Contract	Description	Auth model
PrivateFPC	Fully private. Users bridge FeeJuice from L1; the bridge claim converts to internal FJ balance for fee sponsorship.	Cryptographic bridge proof (no owner, no agent)

Project Structure

├── src/
│   ├── artifacts/                   # Generated contract bindings
│   ├── nr/                          # Noir smart contracts
│   │   ├── counter_contract/        # Test utility contract
│   │   └── private_contract/        # PrivateFPC
│   └── ts/                          # TypeScript package
│       ├── fee-payment-methods/     # Fee payment method classes
│       ├── utils/                   # Utilities (gas, deploy)
│       └── test/                    # Integration tests
├── target/                          # Compiled contract artifacts
├── benchmarks/                      # Performance benchmarks
└── docs/                            # Product requirements

Setup

Prerequisites

• Aztec CLI v4.2.0
• Node.js 22+
• Yarn 1.22+

Installation

yarn install

Compile Contracts

# Full rebuild: compile Noir + generate TS bindings
yarn ccc

# Or step by step
aztec compile
aztec codegen target --outdir src/artifacts

Testing

Start the Aztec sandbox:

aztec start --local-network

Run all tests:

yarn test        # Noir unit tests + JS integration tests
yarn test:nr     # Noir unit tests only
yarn test:js     # JS integration tests only

Deployment

PrivateFPC is a fully private contract — it has no public functions and no constructor. This means no on-chain deployment transaction is required. The contract address is computed deterministically from its class hash and a salt, and users interact with it privately by address.

Compute the address

1. Copy .env.example to .env and set PRIVATE_FPC_SALT:

   cp .env.example .env

2. Compile the contracts (required on first run):

   yarn ccc

3. Run the compute script:

   yarn compute

DANGER: The address is derived from compiled bytecode. A different Aztec version produces different bytecode and a different address. Sending funds to the wrong address means unrecoverable loss. Before using this address, verify the target network runs the same Aztec version as the one shown in the script output:

curl -s -X POST <NODE_URL> -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"node_getNodeInfo","id":1,"params":[]}' \
  | jq .result.nodeVersion

Usage

yarn add @defi-wonderland/aztec-fee-payment

See src/ts/README.md for detailed SDK documentation.

import {
  PrivateFPCContract,
  FPCFeePaymentMethod,
  PrivateMintAndPayFeePaymentMethod,
  registerPrivateContract,
} from '@defi-wonderland/aztec-fee-payment';

// Register the PrivateFPC — no deployment transaction needed (fully private contract)
const fpc = await registerPrivateContract(wallet, salt);

// --- L1: deposit to FeeJuicePortal with a claimer-bound secretHash ---
// secretHash = computeSecretHash(poseidon2([salt, claimerAddress], DOM_SEP))
// FeeJuicePortal.depositToAztecPublic(_to=fpc.address, _amount, secretHash)

// --- L2: two-step flow ---
// Step 1: claim FeeJuice on L2 (emits FeeJuice nullifier)
await feeJuice.methods.claim(fpc.address, amount, secret, leafIndex).send();

// Step 2: mint internal FJ balance by proving the bridge claim
await fpc.methods.mint(amount, salt, leafIndex).send();

// User sponsors a transaction from their internal balance
await myContract.methods.doSomething()
  .send({ fee: { paymentMethod: new FPCFeePaymentMethod(fpc.address) } });

// --- Cold-start: claim + mint + pay fee in one transaction ---
await myContract.methods.doSomething()
  .send({
    fee: {
      paymentMethod: new PrivateMintAndPayFeePaymentMethod(
        fpc.address, amount, secret, salt, leafIndex,
      ),
    },
  });

Benchmarks

yarn benchmark

Benchmarks are defined in Nargo.toml under [benchmark] and run against a live local network. Each contract has its own benchmark file in benchmarks/.

License

MIT