CoinJoin
A Bitcoin privacy technique combining multiple users' payments into one transaction to obscure who paid whom.
CoinJoin is a collaborative Bitcoin transaction where multiple users combine their payments into a single transaction with many inputs and outputs, making it difficult for an outside observer to determine which input funded which output. It improves privacy on Bitcoin’s otherwise fully-transparent ledger without changing the protocol.
CoinJoin sits at the center of the privacy-versus-surveillance debate: it’s a legitimate tool for financial privacy, but its use has drawn regulatory scrutiny and even prosecutions of tool developers, blurring lines between building privacy software and facilitating money laundering. It illustrates a genuine dilemma — the same technique that protects an ordinary user’s financial privacy can also obscure illicit flows, and where to draw legal lines remains fiercely contested.