CoinJoin

CoinJoin is a Bitcoin transaction construction in which multiple parties combine their inputs and outputs into a single transaction such that an external chain observer cannot reliably correlate which input pays which output. The construction was described by Greg Maxwell in an August 2013 Bitcoin Talk post; operational implementations have proliferated since 2014 across coordinator-mediated services (Wasabi’s ZeroLink scheme, Samourai’s Whirlpool) and decentralised peer-discovery services (JoinMarket).

The privacy gained is at the input–output linkage level. Without CoinJoin, a Bitcoin transaction publishes its inputs and outputs to the chain in a way that lets an observer (with reasonable certainty) attribute each output to one of the inputs. Heuristics like common-input ownership — if a transaction has multiple inputs, they probably belong to the same wallet — and change-output detection are the basis of commercial chain analytics. A CoinJoin transaction with equal output amounts and many participants breaks the common-input ownership heuristic and increases the privacy set against the change-output heuristic.

The privacy properties of CoinJoin are not those of Monero’s RingCT-plus-ring-signature-plus-stealth-address construction. CoinJoin is opt-in (every participant has chosen to be in the mix), the privacy set is bounded by the participant count of a given mix, the amounts are visible on-chain (because Bitcoin does not have confidential transactions outside of opt-in protocol layers), and post-mix outputs can be deanonymised through subsequent on-chain behaviour. The technique is privacy-positive — measurably so against commercial chain analytics — but it is not default-private and it is not amount-hiding.

The legal posture on CoinJoin has shifted over the past several years. US federal prosecutors have pursued operators of CoinJoin coordinator services on money-transmission and unlicensed-money-services-business theories (the Samourai indictment in 2024 and the Wasabi response are the leading data points). The legal exposure attaches to the coordinator; the participant in a CoinJoin transaction is not, on US law as it has developed so far, in a meaningfully different legal position from a participant in any other Bitcoin transaction.

For an offshore-hosting operator’s payment routes, CoinJoin matters as an upstream operational choice the subscriber may have made before paying. The Bitcoin payment dossier describes acquisition paths that include CoinJoin and atomic swaps; the publication does not endorse any particular service.