Tornado Cash Privacy Solution

Tornado Cash is a non-custodial Ethereum and ERC20 privacy solution based on zkSNARKs. It improves transaction privacy by breaking the on-chain link between recipient and destination addresses. It uses a smart contract that accepts ETH deposits that can be withdrawn by a different address. Whenever ETH is withdrawn by the new address, there is no way to link the withdrawal to the deposit, ensuring complete privacy.

To make a deposit user generates a secret and sends its hash (called a commitment) along with the deposit amount to the Tornado smart contract. The contract accepts the deposit and adds the commitment to its list of deposits.

Later, the user decides to make a withdrawal. In order to do that, the user should provide a proof that he or she possesses a secret to an unspent commitment from the smart contract’s list of deposits. zkSnark technology allows that to happen without revealing which exact deposit corresponds to this secret. The smart contract will check the proof, and transfer deposited funds to the address specified for withdrawal. An external observer will be unable to determine which deposit this withdrawal came from.

You can read more about it in this medium article

Specs

• Deposit gas const: 1088354 (43381 + 50859 * tree_depth)
• Withdraw gas cost: 301233
• Circuit Constraints = 28271 (1869 + 1325 * tree_depth)
• Circuit Proof time = 10213ms (1071 + 347 * tree_depth)
• Serverless

Whitepaper

https://tornado.cash/Tornado.cash_whitepaper_v1.4.pdf

Was it audited?

Tornado.cash protocols, circuits, and smart contracts were audited by a group of experts from ABDK Consulting, specializing in zero knowledge, cryptography, and smart contracts.

During the audit no critical issues were found and all outstanding issues were fixed. The results can be found here:

• Cryptographic review https://tornado.cash/Tornado_cryptographic_review.pdf
• Smart contract audit https://tornado.cash/Tornado_solidity_audit.pdf
• Zk-SNARK circuits audit https://tornado.cash/Tornado_circuit_audit.pdf

Underlying circomlib dependency is currently being audited, and the team already published most of the fixes for found issues

Requirements

1. node v11.15.0
2. npm install -g npx

Usage

You can see example usage in cli.js, it works both in console and in browser.

1. npm install
2. cp .env.example .env
3. npm run build - this may take 10 minutes or more
4. npx ganache-cli
5. npm run test - optionally runs tests. It may fail on the first try, just run it again.

Use browser version on Kovan:

1. vi .env - add your Kovan private key to deploy contracts
2. npm run migrate
3. npx http-server - serve current dir, you can use any other static http server
4. Open localhost:8080

Use with command line version. Works for Ganache, Kovan and Mainnet:

Initialization

1. cp .env.example .env
2. npm run download
3. npm run build:contract

Ganache

1. make sure you complete steps from Initialization
2. ganache-cli -i 1337
3. npm run migrate:dev
4. ./cli.js test
5. ./cli.js --help

Kovan, Mainnet

1. make sure you complete steps from Initialization
2. Add PRIVATE_KEY to .env file
3. ./cli.js --help

Example:

./cli.js deposit ETH 0.1 --rpc https://kovan.infura.io/v3/27a9649f826b4e31a83e07ae09a87448

Your note: tornado-eth-0.1-42-0xf73dd6833ccbcc046c44228c8e2aa312bf49e08389dadc7c65e6a73239867b7ef49c705c4db227e2fadd8489a494b6880bdcb6016047e019d1abec1c7652 Tornado ETH balance is 8.9 Sender account ETH balance is 1004873.470619891361352542 Submitting deposit transaction Tornado ETH balance is 9 Sender account ETH balance is 1004873.361652048361352542

./cli.js withdraw tornado-eth-0.1-42-0xf73dd6833ccbcc046c44228c8e2aa312bf49e08389dadc7c65e6a73239867b7ef49c705c4db227e2fadd8489a494b6880bdcb6016047e019d1abec1c7652 0x8589427373D6D84E98730D7795D8f6f8731FDA16 --rpc https://kovan.infura.io/v3/27a9649f826b4e31a83e07ae09a87448 --relayer https://kovan-frelay.duckdns.org

Relay address: 0x6A31736e7490AbE5D5676be059DFf064AB4aC754 Getting current state from tornado contract Generating SNARK proof Proof time: 9117.051ms Sending withdraw transaction through relay Transaction submitted through the relay. View transaction on etherscan https://kovan.etherscan.io/tx/0xcb21ae8cad723818c6bc7273e83e00c8393fcdbe74802ce5d562acad691a2a7b Transaction mined in block 17036120 Done

Deploy ETH Tornado Cash

1. cp .env.example .env
2. Tune all necessary params
3. npx truffle migrate --network kovan --reset --f 2 --to 4

Deploy ERC20 Tornado Cash

1. cp .env.example .env
2. Tune all necessary params
3. npx truffle migrate --network kovan --reset --f 2 --to 3
4. npx truffle migrate --network kovan --reset --f 5

Note. If you want to reuse the same verifier for all the instances, then after you deployed one of the instances you should only run 4th or 5th migration for ETH or ERC20 contracts respectively (--f 4 --to 4 or --f 5).

How to resolve ENS name to DNS name for a relayer

1. Visit https://etherscan.io/enslookup and put relayer ENS name to the form.
2. Copy the namehash (1) and click on the Resolver link (2)
3. Go to Contract tab. Click on Read Contract and scrolldown to the 5. text method.
4. Put the values:
5. Click Query and you will get the DNS name. Just add https:// to it and use it as relayer url

Credits

Special thanks to @barryWhiteHat and @kobigurk for valuable input, and to @jbaylina for awesome Circom & Websnark framework

Minimal demo example

1. npm i
2. ganache-cli -d
3. npm run download
4. npm run build:contract
5. cp .env.example .env
6. npm run migrate:dev
7. node minimal-demo.js

Emulate MPC trusted setup ceremony

cargo install zkutil
npx circom circuits/withdraw.circom -o build/circuits/withdraw.json
zkutil setup -c build/circuits/withdraw.json -p build/circuits/withdraw.params
zkutil export-keys -c build/circuits/withdraw.json -p build/circuits/withdraw.params -r build/circuits/withdraw_proving_key.json -v build/circuits/withdraw_verification_key.json
zkutil generate-verifier -p build/circuits/withdraw.params -v build/circuits/Verifier.sol
sed -i -e 's/pragma solidity \^0.6.0/pragma solidity 0.5.17/g' ./build/circuits/Verifier.sol