What is a payjoin (P2EP)?
A payment where the receiver adds an input too, breaking the assumption that every input belongs to the sender. It looks like an ordinary payment.
In a payjoin, sender and receiver build the transaction together: both contribute inputs, the receiver gets paid, and on chain the payment is indistinguishable from an ordinary one. Standardised as BIP-78, it's also called pay-to-endpoint, or P2EP.
Unlike a CoinJoinCoinJoinA payment made together by several people, which breaks the easy link between their old and new addresses. Nobody gives up their coins., amounts stay natural — there are no equal outputs to spot. Both sides must be online together, and the receiver's wallet must support it.
References
- Bitcoin Improvement Proposals, “BIP-78: A Simple Payjoin Proposal”.