Zero-Knowledge Password Authentication

A login demo where the browser proves it knows your password using a zero-knowledge proof, so the password never reaches the server.

Why it exists

In a normal login, the password is sent to the server to be checked. This project tries a different approach: you register a hash of your password, and at login you prove you know a password that matches that hash without revealing it. I built it to learn how zero-knowledge circuits, proofs and verification fit together in a working web app.

How it works

The circuit (circuits/password_auth.circom) has one private input, the password, and one public input, the password hash. It checks that the Poseidon hash of the password equals the public hash.

1. Register. The browser hashes the password with Poseidon and sends only the email, name and hash to /register. The server stores them in backend/users.json.
2. Log in. The browser fetches the stored hash from /get-user, then builds a Groth16 proof in the browser with SnarkJS. The proof uses the compiled circuit (password_auth.wasm) and the proving key (password_auth_final.zkey).
3. Verify. The browser sends the proof to /verify. The server checks it against circuits/verification_key.json and replies with success or failure. The browser then confirms the proof's public hash matches the stored one.

API:

Method	Endpoint	What it does
POST	/register	Stores email, name and password hash
POST	/get-user	Returns the stored record for an email
POST	/verify	Verifies a Groth16 proof

Folders:

Path	Contents
circuits/	The Circom circuit, compiled files, keys and the verification key
backend/	Express server (port 3001)
frontend/	React 19 app with SCSS pages for register, login and dashboard
generate_input.js	Writes inputs.json for the sample password
inputs.json, witness.wtns, proof.json, public.json	Output from a sample run

How to run it

You need Node.js (.nvmrc says 18, and I tested with 20), Circom 2.0 or newer, and the SnarkJS command line (npm install -g snarkjs).

npm install   # installs circomlib and circomlibjs

Check the proof pipeline from the command line. This uses the circuit files already in the repo.

node generate_input.js
snarkjs wtns calculate circuits/password_auth_js/password_auth.wasm inputs.json witness.wtns
snarkjs groth16 prove circuits/password_auth_final.zkey witness.wtns proof.json public.json
snarkjs groth16 verify circuits/verification_key.json public.json proof.json

The last command prints OK! when the proof is valid.

Rebuild the circuit (optional).

circom circuits/password_auth.circom --r1cs --wasm --sym -o circuits -l node_modules/circomlib/circuits
snarkjs groth16 setup circuits/password_auth.r1cs circuits/pot12_final.ptau circuits/password_auth_0000.zkey
snarkjs zkey contribute circuits/password_auth_0000.zkey circuits/password_auth_final.zkey --name="first contribution" -e="type some random text here"
snarkjs zkey export verificationkey circuits/password_auth_final.zkey circuits/verification_key.json
cp circuits/password_auth_js/password_auth.wasm circuits/password_auth_final.zkey frontend/public/

The last line copies the new circuit and key to where the browser app loads them. Do not run generate_witness.js directly: the root package.json sets "type": "module", which breaks it. Use snarkjs wtns calculate as shown above.

Run the app.

cd backend && npm install && npm start    # http://localhost:3001

In a second terminal:

cd frontend && npm install && DISABLE_ESLINT_PLUGIN=true npm start    # http://localhost:3000

The DISABLE_ESLINT_PLUGIN=true part is needed because the repo has no lockfile, so a fresh install pulls newer lint packages that fail with a jest/globals error. Without it, the dev server reports one compile error. The frontend calls the backend at http://localhost:3001.

inputs.json uses a sample password (123456), and backend/users.json holds made-up test accounts. Both are sample data.

Results

I tested this on a clean copy of the repo:

• The command-line steps above work. The proof for the sample input verifies, and the backend's /verify returns {"success":true} for it.
• In the browser, I registered a test user, logged out and logged back in. The browser built a proof and the server verified it. A wrong password was rejected with "Login failed."

No performance measurements are included.

Known limitations (demo project)

• The server checks that a proof is valid, but not that it belongs to the account. The match against the stored hash is done in the browser, and the login state is kept in localStorage.
• /get-user returns the stored hash for any email.
• The stored value is an unsalted Poseidon hash, so a weak password could be guessed offline from it.
• Proofs are not tied to a one-time challenge, so a captured proof could be replayed.

Tech stack

Circom, SnarkJS (Groth16), Poseidon hash (circomlib), Node.js with Express, React 19 with SCSS.