CoinJoin

A CoinJoin is a collaborative transaction: a Bitcoin transaction that is initiated and signed by multiple participants. Combining inputs from several parties breaks the common input ownership heuristic that chain-analysis firms rely on to link addresses to a single actor. The idea was first proposed on Bitcointalk in 2011 by a user going by hashcoin, and popularized in 2013 by Gregory Maxwell. A CoinJoin is always a batch transaction, and a dishonest coordinator cannot steal funds, only refuse to publish the final transaction.

CoinJoins fall into two broad types. Equal-output CoinJoins produce uniform output denominations, which lets participants hide in the crowd of other identical outputs. They are easy to spot on-chain because of the uniform structure. Unequal-output CoinJoins produce varied outputs and are harder to detect via chain analysis alone. Payjoin is a collaborative transaction between two parties that also qualifies as a CoinJoin under the broader definition.

Active implementations include JoinMarket, which uses a maker/taker market with fidelity bonds; Wasabi, which uses the WabiSabi protocol with a coordinator that cannot link inputs to outputs; and Ashigaru on Android, forked from Samourai's ZeroLink protocol. CoinJoin does not make coins anonymous. It moves chain analysis from quasi-deterministic linking to probabilistic guessing, and the privacy gained depends on anonymity set size and on how the user spends the coins afterward.

References

• hashcoin, Blind Bitcoin Transfers (2011)
• Gregory Maxwell, CoinJoin: Bitcoin privacy for the real world (2013)
• WabiSabi paper
• JoinMarket
• en.bitcoin.it: CoinJoin