Payjoin primary sources — 2026-05-03

Project status in this pass

• No separate standalone whitepaper or litepaper surfaced.
• The clearest current source of truth was the official project site, BIP 78, BIP 77, the project FAQ, and the Rust-Payjoin / Payjoin Dev Kit implementation README.
• The materials show Payjoin evolving from a simple synchronous payment protocol into a broader async coordination and wallet-integration stack.

Primary-source snapshots

1) Official project site

• URL: https://payjoin.org/
• Key excerpt:
  • The site says Payjoin preserves privacy by breaking the most common method used to spy on bitcoin users.
  • It also says Payjoin can settle many transactions at once to lower fees and higher throughput.
  • The site explicitly says Payjoin can augment Lightning by helping nodes fund and open channels in one transaction while maintaining privacy.

2) BIP 78 — A Simple Payjoin Proposal

• URL: https://raw.githubusercontent.com/bitcoin/bips/master/bip-0078.mediawiki
• Key excerpt:
  • BIP 78 defines Payjoin as a protocol for two parties to negotiate a coinjoin transaction during a payment.
  • It explains that the sender receives a BIP21 URI with a pj= endpoint, posts an original PSBT, gets a Payjoin proposal PSBT back, then verifies, re-signs, and broadcasts.
  • The motivation section explicitly targets common-input ownership, change script-type, and round-amount heuristics.

3) BIP 77 — Async Payjoin

• URL: https://raw.githubusercontent.com/bitcoin/bips/master/bip-0077.md
• Key excerpt:
  • BIP 77 says it is a backwards-compatible asynchronous version of Payjoin relative to BIP 78.
  • It replaces the need for a receiver to host a secure public endpoint with an untrusted directory server, asynchronous protocol flow, authenticated encrypted payloads, and OHTTP-based request privacy.
  • It explicitly says BIP 78’s public-facing-server and synchronous-communication requirements have been obstacles to adoption, especially for mobile and web wallets.

4) Project FAQ

• URL: https://raw.githubusercontent.com/payjoin/payjoin.org/master/docs/faq.md
• Key excerpt:
  • The FAQ says Payjoin works on top of Bitcoin without consensus changes.
  • It says limited wallet support has historically stemmed from lack of integration tooling and the old requirement for a receiver to be online behind an HTTPS server.
  • It positions Payjoin Dev Kit as the de-facto library and notes that newer async Payjoin development opens the protocol to more wallet types.

5) Rust-Payjoin / Payjoin Dev Kit implementation README

• URL: https://raw.githubusercontent.com/payjoin/rust-payjoin/master/README.md
• Key excerpt:
  • The README says the repository contains multiple crates implementing both BIP 77 Async Payjoin and BIP 78 Simple Payjoin.
  • It says the stack also includes associated OHTTP Relay and Payjoin Directory infrastructure.
  • It identifies the main payjoin crate, a no-frills CLI reference implementation, a combined mailroom/directory component, test utilities, and FFI bindings.

Working conclusion

Payjoin is best cataloged as Bitcoin payment-coordination and wallet-interoperability infrastructure: a privacy-preserving batching protocol whose current practical surface includes both simple and async specifications plus a reusable Dev Kit and directory/relay tooling that lower the barrier for wallet adoption.