CoinJoin

💡 The Plain-English Definition

CoinJoin is a privacy technique where multiple Bitcoin users combine their transactions into one, making it much harder for blockchain analysts to trace who sent what to whom. It’s the most widely used Bitcoin privacy tool available today.

🤔 But Why Though?

Standard Bitcoin transactions have a fundamental privacy weakness: the common input ownership heuristic — the assumption that all inputs (sources of Bitcoin) in a transaction belong to the same person. If Alice sends Bob 0.5 BTC using two UTXOs (discrete chunks of Bitcoin she owns), analysts assume both inputs belong to Alice. This assumption is correct the vast majority of the time and allows analysts to cluster addresses and trace funds across wallets.

CoinJoin breaks this assumption by combining inputs from multiple independent users into one transaction. If Alice, Bob, and Carol each contribute one input to a single transaction with three equal outputs of 0.5 BTC each, analysts can no longer determine which input funded which output. The link between sender and recipient is severed. All three outputs look identical, and any input could have funded any of them. The major implementations each approach this differently. Wasabi Wallet uses the WabiSabi protocol, where a coordinator organises CoinJoin rounds — users connect through Tor (software that anonymises internet traffic) and register inputs without the coordinator knowing which belongs to which user. Whirlpool (by Samourai Wallet) uses fixed denomination outputs, ensuring all outputs are identical for the strongest possible privacy. JoinMarket creates a marketplace where users who want privacy pay to participate alongside market makers. CoinJoin requires equal output amounts for maximum privacy — if outputs differ in size, analysts can sometimes infer which input funded which. Post-mix spending also matters: sending a freshly mixed output directly to a known address can undo much of the privacy gained.

🌍 The Real-World Analogy

Imagine ten people each depositing £100 into a pooled envelope, shaking it, and each withdrawing £100. An observer who saw who deposited can’t tell who withdrew what — all withdrawals are identical. CoinJoin is this shuffling process applied to Bitcoin, happening mathematically and automatically without anyone physically handling your coins.