Definition
CoinJoin is a privacy technique, first proposed by Bitcoin Core contributor Gregory Maxwell in 2013, in which several independent participants combine their payments into a single transaction. Each user contributes inputs and receives outputs, but because everyone signs only their own inputs, no participant ever takes custody of anyone else's coins. The result is one large transaction whose internal input-to-output mapping is ambiguous to an outside observer.
Why Equal Outputs Matter
The privacy gain depends on using equal-value (uniform) output amounts. If everyone receives, say, an identical 0.01 BTC output, a blockchain analyst cannot tell which input funded which output by matching amounts. With mismatched values, the mapping is often trivially reconstructable. Equal-value CoinJoins turn deterministic tracing into a probabilistic guess, lowering the confidence of any clustering attempt and weakening the common-input-ownership heuristic across the whole network.
Tradeoffs to Understand
CoinJoins are conspicuous: their structured equal outputs are easy to flag on-chain, and some exchanges treat post-mix coins with extra scrutiny. They also cost coordination and miner fees, and any leftover change can re-link coins if spent carelessly. CoinJoin improves anonymity-set size rather than guaranteeing anonymity — privacy still depends on disciplined coin control afterward. As a self-custody practice, it pairs naturally with running your own node so you are not leaking your UTXO set to a third-party server.
For related concepts, see our entries on PayJoin and the Common-Input-Ownership Heuristic that CoinJoin is designed to defeat.
In Simple Terms
CoinJoin is a privacy technique, first proposed by Bitcoin Core contributor Gregory Maxwell in 2013, in which several independent participants combine their payments into a…