PayJoin (P2EP)

PayJoin, also called pay-to-endpoint or P2EP, is a privacy technique defined in BIP 78 in which the receiver of a payment contributes one of their own inputs to the transaction, breaking the standard assumption that all inputs belong to the sender. The finished transaction looks like an ordinary payment, but its ownership structure quietly defies analysis.

Why it matters

Chain surveillance rests heavily on the common-input-ownership heuristic, the assumption that every input of a transaction is controlled by one entity. PayJoin poisons that heuristic: because the receiver's coin joins the sender's, clustering algorithms mislabel ownership, and even transactions that are not PayJoins gain deniability once the technique is plausible. Unlike large CoinJoins, PayJoin requires no coordinator, adds no distinctive on-chain fingerprint, and hides in plain sight among normal payments. Adoption has been the obstacle, since the receiver must be online to participate, though later protocol versions work toward asynchronous operation.

In the gold vs bitcoin debate

PayJoin represents bitcoin's incremental answer to a real deficiency against gold and cash: physical money changes hands without records, while bitcoin's default is a permanent public trail. Techniques that restore transactional privacy without custodians shape whether bitcoin can serve as private money rather than merely auditable money, and their legal treatment varies sharply by jurisdiction.