Circom ChaCha20

This is a circom implementation of the ChaCha20 stream cipher. It lets a user prove that they have the key to a ChaCha20 encrypted message without revealing the key.

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

Installation

``bash
npm install git+https://github.com/reclaimprotocol/circom-chacha20
`

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

`bash
npm install snarkjs
`

Usage

$3

`ts
import { generateProof, verifyProof, makeLocalSnarkJsZkOperator, toUint8Array } from '@reclaimprotocol/circom-chacha20'

const key = randomBytes(16)
const iv = randomBytes(12)
const data = 'Hello World!'
const ciphertext = chacha20(key, iv, data)

// the operator is the abstract interface for
// the snarkjs library to generate & verify the proof
const zkOperator = makeLocalSnarkJsZkOperator()
// 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(
// key, iv & counter are the private inputs to the circuit
{
key,
iv,
// this is the counter from which to start
// the chacha20 stream cipher. Read about
// the counter here: https://en.wikipedia.org/wiki/Stream_cipher
startCounter: 1
},
// the public ciphertext input to the circuit
{ ciphertext },
zkOperator
)

// you can check that the plaintext obtained from the circuit
// is the same as the plaintext obtained from the ciphertext
const plaintextBuffer = toUint8Array(plaintext)
// slice in case the plaintext was padded
.slice(0, data.length)
.toString()
console.log(plaintextBuffer) // "Hello World!"
`

$3

Continuing from the above example:

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

`

Development

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

$3

Run the tests via npm run test

Building the Circuit

$3

Official Ptau file for bn128 with 256k max constraints can be downloaded by running
`bash
npm run download:ptau
`

Build the circuit via npm run build:circuit

$3

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