CoinJoin and mixers: privacy at the transaction level
is a Bitcoin technique where multiple parties combine their inputs and outputs into a single transaction, breaking the common-input-ownership heuristic: an observer can see the transaction but cannot tell which input funded which output. Wasabi and Samourai were the best-known CoinJoin coordinators; both have faced law-enforcement action (Wasabi's coordinator was shut down in 2024; Samourai's founders were indicted).
(Blender, Tornado Cash) are the smart-contract equivalent: deposit funds, receive a note, later withdraw from a different address. The mixer holds funds in a pool so deposits and withdrawals aren't directly linkable — until the user re-consolidates at a KYC'd exchange, which is the off-ramp link that often defeats the mix.
Two ways to break the link
CoinJoin and mixers break the direct on-chain link, but they don't break: (1) amount correlation (round numbers, exact-change patterns), (2) timing analysis (deposit-then-withdraw windows), (3) the off-ramp KYC link when funds re-consolidate, or (4) network-level metadata (IP addresses, node behavior). The Trace the Funds lab walks through exactly this: a CoinJoin that delays but doesn't prevent tracing, because the actor re-consolidates at an exchange.