Privacy Apps with Noir

A sound circuit is not a private app. These are the boundaries around it, where the failures actually happen.

Hash in-circuit with Poseidon, never keccak or SHA256. Poseidon costs ~600 gates against ~30,000 for a bit-oriented hash, and members prove in a browser. It is not in the stdlib — add the dependency and import it:

poseidon = { git = "https://github.com/noir-lang/poseidon", tag = "v0.3.0" }

use poseidon::poseidon::bn254::hash_2 — not std::hash::poseidon (removed). Whatever you pick, every layer must use the same algorithm and input order: commitment creation, Merkle parent hashing, the offchain mirror, the onchain tree. Poseidon and Poseidon2 are different functions with different outputs. Test one leaf hash and one parent hash across every layer before building on them.

Prove in-process with NoirJS, not by shelling out to the bb CLI. A script that spawns bb prove and reads the proof off disk works on your laptop and can never become the browser prover the app needs. The packages are @noir-lang/noir_js + @aztec/bb.js (not backend_barretenberg), and the class is UltraHonkBackend (not UltraPlonkBackend):

const backend = new UltraHonkBackend(circuit.bytecode, await Barretenberg.new());
const proof = await backend.generateProof(witness, { verifierTarget: "evm" });