Circom Symmetric Crypto

This library contains circom zero-knowledge proof circuits for symmetric crypto operations. The goal is to enable a user to prove that they have the key to a symmetric encrypted message without revealing the key.

The following algorithms are supported:

• chacha20
• aes-256-ctr
• aes-128-ctr
  • which includes any CTR implementation. For eg. aes-256-gcm
  • note: this is a WIP, and may be insecure (borrowed implementation from electron labs)

It uses the groth16 implementaion in snarkjs to generate the proof.

Installation

npm install git+https://gitlab.reclaimprotocol.org/Reclaim/zk-symmetric-crypto

If using on the browser, or nodejs, you will need to install snarkjs as well.

npm install snarkjs

Usage

Generating Proof

import { generateProof, verifyProof, makeLocalSnarkJsZkOperator } from '@reclaimprotocol/circom-symmetric-crypto'
import { createCipheriv, randomBytes } from 'crypto'

async function main() {
	const key = randomBytes(32)
	const iv = randomBytes(12)
	const algorithm = 'chacha20'
	const data = 'Hello World!'

	const cipher = createCipheriv('chacha20-poly1305', key, iv)
	const ciphertext = Buffer.concat([
		cipher.update(data),
		cipher.final()
	])

	// the operator is the abstract interface for
	// the snarkjs library to generate & verify the proof
	const operator = await makeLocalSnarkJsZkOperator(algorithm)
	// generate the proof that you have the key to the ciphertext
	const {
		// groth16-snarkjs proof as a JSON string
		proofJson,
		// the plaintext, obtained from the output of the circuit
		plaintext,
	} = await generateProof({
		algorithm,
		// key, iv & counter are the private inputs to the circuit
		privateInput: {
			key,
			iv,
			// this is the counter from which to start
			// the stream cipher. Read about
			// the counter here: https://en.wikipedia.org/wiki/Stream_cipher
			offset: 0
		},
		// the public ciphertext input to the circuit
		publicInput: { ciphertext },
		operator,
	})

	// you can check that the plaintext obtained from the circuit
	// is the same as the plaintext obtained from the ciphertext
	const plaintextBuffer = plaintext
		// slice in case the plaintext was padded
		.slice(0, data.length)
	// "Hello World!"
	console.log(Buffer.from(plaintextBuffer).toString())

	// you can verify the proof with the public inputs
	// and the proof JSON string
	await verifyProof({
		proof: {
			proofJson,
			plaintext,
			algorithm
		},
		// the public inputs to the circuit
		publicInput: { ciphertext },
		operator
	})
	console.log('Proof verified')
}

main()

Verifying Proof

Continuing from the above example:

// will assert the proof is valid,
// otherwise it will throw an error
await verifyProof(
	{ proofJson, plaintext, algorithm: 'chacha20' },
	{ ciphertext },
	zkOperator
)
console.log('proof verified')

Development

1. Clone the repository
2. Install dependencies via: npm i
3. Install circom

Running Tests

Run the tests via npm run test

Building the Circuit

Prerequisites

curl, jq

Official Ptau file for bn128 with 256k max constraints can be downloaded by running

npm run download:ptau

Build the circuits via ALG={alg} npm run build:circuit. For eg. ALG=chacha20 npm run build:circuit Note: ALG is the same as mentioned in the first section of this readme.

Regenerating the Verification Key

1. Generate bls12-381 parameters via npm run generate:ptau
2. Fix build-circuit.sh to use -p bls12381 parameter
   • note: we currently use BN-128 for our circuit, but plan to switch to BLs for greater security
   • zkey and ptau file verification is disabled right now due to a bug in the latest snarkJS version 0.7.0
3. TODO