CoinJoin
Also · Whirlpool · WabiSabi · JoinMarket
A CoinJoin is a single Bitcoin transaction built by several users together, with equal-sized outputs, so that an observer cannot tell which input paid which output.
A Bitcoin transaction can have many inputs and many outputs, and nothing requires them to belong to 1 person. A CoinJoin exploits this. Several participants each contribute an input and receive an output of identical size in the same transaction. Each signs only their own input. Looking at the chain, any input could have funded any of the equal outputs.
CoinJoin does not hide the transaction, it breaks the link between before and after. Its anonymity set is the number of equal outputs. Implementations differ in coordination, Whirlpool, JoinMarket and WabiSabi being the best known, and some coordinators have been shut down by law enforcement. What users do after the mix matters as much as the mix itself.
In French · CoinJoin
