CoinJoin

CoinJoin is neither a new protocol nor a Bitcoin fork. It uses the native flexibility of transactions, which allow multiple inputs and outputs signed independently. Several users contribute their UTXOs to a single transaction and each one signs only their own inputs, so the coordinator never takes control of the funds. That sets it apart from a custodial mixer, which does assume custody. The technical goal is to defeat the common-input-ownership heuristic, the assumption that every input in a transaction belongs to the same holder.

Serious implementations use equal-value outputs, so they become indistinguishable from one another and form an anonymity set; the difference returns as change. Chaumian CoinJoin adds blind signatures and a central coordinator that authorizes outputs without being able to link them to specific inputs: it is trusted only for service availability, never for custody. WabiSabi, described in IACR ePrint 2021/206, replaces fixed denominations with keyed-verification anonymous credentials and supports variable amounts.

Proposed by Gregory Maxwell on 22 August 2013 on Bitcointalk, CoinJoin is the primary non-custodial on-chain route to recover privacy on a public ledger. It is also where financial privacy and traceability converge: chain-analysis firms distinguish these outputs and AML frameworks account for them. Implementations such as Wasabi (WabiSabi), Samourai (Whirlpool) and JoinMarket illustrate different coordination models. Good practice always pairs privacy with compliance and diligence on the source of funds.