Payjoin

Payjoin is a protocol that lets the sender and receiver of a Bitcoin transaction cooperate to build a single transaction with inputs from both parties. The resulting transaction breaks the common-input-ownership heuristic that chain analysis tools rely on, and at the same time batches two payments into one, which saves on fees. Used widely, Payjoin makes the common-input heuristic useless across the whole network, not only for the people using it.

The Payjoin Foundation is the 501(c)(3) non-profit stewarding this work, incorporated in Delaware in 2025 and led by Dan Gould. Its main project is the Payjoin Dev Kit: a Rust implementation (rust-payjoin) with bindings for Python, Dart, Swift, Kotlin, JavaScript, and C#. The dev kit powers BIP 77, Async Payjoin, which uses Oblivious HTTP so sender and receiver can exchange messages without both being online at once. That removed the biggest practical barrier to adoption: Payjoin v1 needed the receiver to run a public server, often behind Tor and TLS, which very few wallets ever did.

Why fund it?

Bitcoin privacy on-chain needs widespread adoption of tools that actively break surveillance heuristics. A clean, production-ready library gives wallet and service developers a drop-in way to add this behavior by default, without each project building its own implementation.

OpenSats started funding this work as the Payjoin Dev Kit in the first wave of Bitcoin grants in July 2023, and renewed the grant in the eleventh wave and again in the seventeenth wave, now directly awarded to the Payjoin Foundation. For a longer write-up, see the impact reports on Async Payjoin and the Payjoin Dev Kit.

What's next?

Bull Bitcoin shipped Payjoin v2 in December 2024 and Cake Wallet followed in May 2025. BDK-CLI shipped v3.0.0 with Payjoin support in early 2026. Pull requests are open for Liana, LDK-Node, Boltz, and LEXE, and a BTCPayServer plugin is in development.

The payjoin crate has moved from version 0.11 to 1.0-rc.2, adding persistence baked into the core state machine, a session-event log so apps can show transaction history, and stable FFI for every binding target. payjoin-mailroom bundles the directory, OHTTP relay, and metrics into a single binary that BOB Space, Cake Wallet, Vinteum, Achow101, and others now run as public infrastructure.

Alongside the dev kit, the Foundation runs a research track led by Armin Sabouri on wallet fingerprinting (tx-indexer, btsim, and an automated RAG pipeline for wallet code), publishes open weekly dev-call notes and a study club, and is drafting a multiparty Payjoin v3 protocol with multiple receivers.