This repository contains my project for the Citrea Origins Hacker House, PUBTC (Private, Unstoppable Bitcoin).
This was a research experiment to investigate the complexity of a system that would enable private transfers of a token balance on a public computer.
Projects like this already exist, like railgun and roman kyoto (what this is based on), but I wanted to see if I could build it in Noir (railgun has its circuits written in circom).
TLDR I thought it'd be easier than it was lol, and originally wanted to ship this with a report on how a protocol like this could implement compliance checks with differing levels of decentralisation, but that'll come later.
I've better documented the data structures at play in this presentation:
https://docs.google.com/presentation/d/1xvbygNVBei91_tnhPpN-txfTU43KPVPc51BcULWMqHY/edit?usp=sharing
Contains the note_verify circuit that powers this sorcery. This is a noir circuit, that complies to a solidity verifier contract.
Contains PUBTC.sol - a private, wrapped bitcoin token implementation. This deployment calls a verifier function generated from the circuits/ directory.
Contains utility scripts that covert the circuits to verifier contracts.
To build our solidity verifier contract:
nargo compile
bb write_vk -b ./target/note_verify.json
bb contractcopy the circuits/note_verify/circuits/contract.sol to contracts/verifiers/contract.sol.
To run tests:
bun install
bun hardhat test
On Citrea testnet, I was able to deposit 69,420 satoshis to the pool as alice:
Then, as Alice, I was able to privately send bob 420 satoshis:
Then, Bob was able to privately send Charlie 69 satoshis:
Then, Charlie was able to withdraw 24 satoshis to their address:
- Need to prove msg.value is what is in the note when inserted
- Need to move to noir v1
- Need to sum and check public inputs in solidity for withdrawals
- If I can migrate the zk from using a poseidon hash to using a keccak hash, it will make transfers/withdraws 75% cheaper in the EVM
- I need to bake the withdrawal address into the proof to avoid front running
- Need to make it so transfer/withdrawal can take multiple input notes
- add
useBureaucracy()hook
All code is unaudited, do not deploy in production.