Tornado Cash Hasher contract - circom

Question

In the Tornado Cash constructor, it takes an argument:

Ihasher _hasher;

where the Ihasher is:

interface IHasher {
   function MiMCSponge(uint256 in_xL, uint256 in_xR) external pure returns (uint256 xL, uint256 xR);
}

In the mainnet contract, they use the address: https://etherscan.io/address/0x83584f83f26af4edda9cbe8c730bc87c364b28fe

it's not verified, but apparently refers to a precompiled circom contract (my guess?).

Just want to know if anyone has information on:

• what is it exactly?
• what address to use if I want to deploy it to another EVM chain (say Polygon)?
• can I just deploy the bytecode on my own (will this work)?

Answer 1

Figured it out. They use the mimc_sponge hash function (https://byt3bit.github.io/primesym/) and it's all precompiled by the IDEN guys and just the bytecode is deployed on-chain (so no solidity).

Here is the script that tornado uses to deploy it:

https://github.com/tornadocash/tornado-core/blob/master/scripts/compileHasher.js

It goes to this file which has all the updated logic (tornado might be out of date): https://github.com/iden3/circomlibjs/blob/main/src/mimcsponge_gencontract.js

As you can see, they literally create the contract byte by byte. But it's just hashing, so you can deploy and redeploy pretty cheaply and it's fine.