Definition
PayJoin — also called Pay-to-EndPoint (P2EP) and standardized in BIP-78 — is a collaborative payment in which both the sender and the receiver contribute inputs to the same transaction. The coordination is mediated by an endpoint the receiver runs, communicated through a BIP-21 payment URI. Unlike a CoinJoin, a PayJoin is an ordinary spend that simply happens to include one of the recipient's coins.
How the Privacy Works
Blockchain surveillance leans heavily on the assumption that every input in a transaction belongs to the same wallet. PayJoin quietly violates that assumption: because the recipient secretly supplies an input, an analyst who applies the common-input-ownership heuristic will incorrectly merge the sender's and receiver's coins into one cluster. The transaction looks identical to any other payment, so it cannot be filtered out on sight.
A Network-Wide Effect
The clever property is that PayJoin's benefit is collective. Because PayJoins are indistinguishable from normal payments, even a small fraction of them poisons the reliability of input-ownership clustering for all Bitcoin transactions — an analyst can no longer trust that combined inputs imply common ownership. BIP-78 defines the original synchronous HTTPS protocol; later work (BIP-77) adds an asynchronous, relay-based design suited to mobile wallets that are not always online.
PayJoin complements rather than replaces other tools. To go deeper, compare it with CoinJoin and read why the Common-Input-Ownership Heuristic is the assumption both techniques target.
In Simple Terms
PayJoin — also called Pay-to-EndPoint (P2EP) and standardized in BIP-78 — is a collaborative payment in which both the sender and the receiver contribute inputs…