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What is a CoinJoin?
A CoinJoin is a single Bitcoin transaction built jointly by several people, each putting in their own coins and receiving outputs of the same value. Because those outputs look alike, an outside observer cannot easily tell which input paid which output, breaking the usual trail between someone's old and new coins.
What it means for you
In the US, FinCEN proposed in 2023 that covered financial institutions report transactions they know or suspect involve mixing in or involving jurisdictions outside the US; its definition of mixing includes pooling coins from several people and software that coordinates a transaction's structure. Research has also found that tracing coins before and after a CoinJoin can narrow the privacy it provides.
How it works
Participants combine their inputs and outputs into one transaction, with the order shuffled. When each participant's main output has the same value, the denomination, an observer cannot tell without extra information which input funded which output; change outputs, with odd amounts, often remain linkable. Building the transaction can be coordinated by one participant or by a third-party coordinator. CoinJoin breaks the common-input-ownership heuristic, the assumption that all inputs to a transaction belong to one owner, which much blockchain analysis relies on.
Sources: Heuristics for Detecting CoinJoin Transactions on the Bitcoin Blockchain (Schnoering and Vazirgiannis, arXiv), Adoption and Actual Privacy of Decentralized CoinJoin Implementations in Bitcoin (Stuetz et al., arXiv), FinCEN proposed rule: CVC mixing as a class of transactions of primary money laundering concern · checked 4 October 2026
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