Definition
Pay-to-EndPoint (P2EP) is the conceptual ancestor of PayJoin. It describes a payment in which the receiver contributes one or more of their own inputs to the same transaction that the sender is funding. Because the resulting transaction now contains inputs owned by two different parties, it directly poisons one of blockchain analysis's most relied-upon assumptions. The idea circulated in the Bitcoin privacy community before being refined and standardized as BIP78.
From concept to standard
The "endpoint" in the name refers to a communication endpoint, typically an HTTP server the receiver runs, that the sender's wallet contacts to negotiate the joint transaction. Early proposals such as Bustapay (BIP79) explored the same territory. The modern, widely-implemented version is the BIP78 PayJoin protocol, which formalizes the message flow using partially-signed Bitcoin transactions (PSBTs). In practice the terms P2EP and PayJoin are often used interchangeably, though PayJoin is the name that stuck.
Why receiver inputs matter
Surveillance firms typically assume every input in a transaction belongs to one entity. When the receiver silently adds an input, that assumption produces a wrong cluster, and the recorded payment amount no longer reflects the true amount sent. This is a rare privacy technique that also damages the data quality of the entire chain-analysis industry, not just the privacy of the two participants. For merchants, it doubles as a way to consolidate small UTXOs while accepting payment.
The standardized successor is covered in our PayJoin entry, and the heuristic it defeats is detailed under Output Linking and the Common-Input-Ownership Heuristic.
In Simple Terms
Pay-to-EndPoint (P2EP) is the conceptual ancestor of PayJoin. It describes a payment in which the receiver contributes one or more of their own inputs to…