pay-to-contract

A simple Go implementation of Pay To Contract

Overview

Pay to Contract is a scheme which extends the classic Bitcoin Pay-to-Public-Key model, providing a path to notable improvements to the privacy, security, and fungibility of Bitcoin payments. Most basically, Pay to Contract is useful as it enables us to encode more information in a single public key and then use this information to provide more "spend-time" flexibility. It achieves this through alteration of the public key which allows for the encoding of arbitrary data.

The ability to have the public key commit to arbitrary data at transaction "send-time" allows for costless commitment and selective revelation of sensitive/identifying Bitcoin script logic and forms the underpinnings of Taproot Bitcoin Improvement Proposal

Pay to Contract/Taproot together with Schnorr signatures and a collaborative signature scheme like MuSig, allows any and all multi-signature and complex script based transactions to hit the blockchain with the same form as a normal Pay to Public Key Hash (P2PKH). Smart contract spends with the approval of all contract participants, such as the opening and collaborative closing of channels on Lightning Network, will appear no differently than any other transaction.

Under BIP-Taproot the data is itself a commitment in form of a merkle root whose leaves are comprised of Bitcoin scripts specifying the various spending conditions for the transaction output (see M.A.S.T).

Usage

Install using:

go get -u github.com/calvinrzachman/pay-to-contract

In your code:

import "github.com/calvinrzachman/pay-to-contract"

// Compute tweaked public key
tPubX, tPubY := paytocontract.TweakAdd(publicKey, contract) 

// Compute tweaked secret to sign for Q 
tSecret := paytocontract.TweakSecretKey(privateKey, contract)

You can then pay to the tweaked public key using the classic P2PKH construction. Taproot/Pay to Contract outputs constructed as described can be spent in 2 ways:

• Key spend - provide a BIP-Schnorr signature for the tweaked public key
• Script spend - reveal the root of a merklized abstract syntax tree (MAST), a proof that the spending condition connects to the merkle root, and finally the script inputs

Note

This implementation is for learning purposes only. Code from this repository should not be used in a production environment.

Acknowledgements

Many thanks go out to Blockstream's Andrew Poelstra and Jonas Nick for documentation and presentations on the ideas implemented here. This repository could not have been completed without the viewing of several of their recorded talks on YouTube

Helpful Links

Taproot Is Coming: What It Is, and How It Will Benefit Bitcoin