Crypto glossary
CoinJoin
CoinJoin is a Bitcoin privacy technique in which many users combine their payments into one transaction with equal outputs, breaking links between addresses.
Proposed by Bitcoin developer Gregory Maxwell in 2013, CoinJoin exploits the fact that a transaction can have inputs from different owners as long as each signs their own. Participants contribute inputs and receive outputs of identical value at fresh addresses. Because the outputs look the same, an observer cannot tell which input paid which output, undermining the common assumption that all inputs of a transaction share an owner.
Unlike custodial mixers, CoinJoin is non-custodial: no coordinator can steal funds, since each user only signs a transaction that pays them correctly. Implementations such as Wasabi Wallet, JoinMarket and the former Samourai Whirlpool offered it to ordinary users, and analytics firms developed heuristics to flag CoinJoin transactions.
Privacy depends on the number of participants, careful handling of change outputs and not merging mixed and unmixed coins afterwards. Regulators treat CoinJoin warily: in 2024 US authorities charged the developers of Samourai Wallet, a major coordinator shut down, and some exchanges have frozen deposits traced to CoinJoins. Legal treatment varies by country.
Example
Fifty users each contribute about 0.1 BTC to a CoinJoin and receive 0.1 BTC outputs at new addresses; chain analysts can see the transaction but not which new output belongs to which participant.