BIP 77: Payjoin Version 2: Serverless Payjoin

[DanGould]

This document proposes a backwards-compatible second version of the payjoin protocol described in BIP 78, allowing complete payjoin receiver functionality including payment output substitution without requiring them to host a secure public endpoint. This requirement is replaced with an untrusted third-party relay and streaming clients which communicate using an asynchronous protocol and authenticated, encrypted payloads. It was originally proposed to the mailing list here: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-January/021364.html

The protocol has received many rounds of review and an earlier version of this proposal has also been sent to the mailing list: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-August/021868.html

Feedback from that list post has been incorporated into this draft.

Proposing this as an Standards Track BIP to ensure wallets across the ecosystem can come to rough consensus on a single serverless payjoin standard and correctly implement it.

A few decisions have yet to reach rough consensus:

• How should the receiver pubkeys be encoded in the BIP 21 URI? Both base64uri and Blockchain Commons UR have been proposed, with the former having more available libraries and the latter better suited for QR encoding.

The bip does contain some reference implementations based on earlier versions of this spec, but they must evolve based on the decisions we come to here.

[luke-jr]

Let's call this BIP 77