coinjoin

coinjoin is a bitcoin technique where many strangers merge their payments into one big transaction, breaking the assumption that all inputs belong to one person.

bitcoin's ledger makes no attempt at anonymity - it just records addresses. chain analysts lean on a powerful assumption: if one transaction spends multiple inputs, those inputs probably belong to the same owner. coinjoin, proposed by gregory maxwell in 2013, weaponizes the exception: many users collaboratively build one transaction together, each contributing inputs and receiving outputs, with no input-output linkage existing at all.

done properly, the transaction is valid and signed by everyone jointly, yet an outside observer cannot tell which output belongs to which input. repeated over multiple rounds with equal-sized denominations, it degrades address clustering significantly - which is why wallets like wasabi and samourai whirlpool built automated coinjoin coordinators.

know the limits and the news: coinjoin does not erase history, equal-denomination rounds are identifiable as coinjoins, and analytics firms tag coins emerging from them - some exchanges treat heavy coinjoin history as grounds for frozen accounts or kyc interrogation. regulators have also taken direct action: samourai's founders were arrested in 2024, and wasabi shut down its own coinjoin coordinator the same year.

monero takes the opposite approach - hiding amounts and sender/receiver by default in the protocol rather than bolting mixing onto a transparent chain. our primer on monero vs bitcoin privacy covers that contrast in depth.