Anonymity set
Also · Anonset · Entropy
An anonymity set is the number of plausible owners for a coin or output. Privacy on Bitcoin is measured in these odds, not in absolutes.
Privacy on a public ledger is never total. The useful question is how many candidates an observer must consider. After a CoinJoin with 5 equal outputs, each output could belong to any of 5 participants. Its anonymity set is 5. Chain analysis then has a 1 in 5 guess rather than a certainty. Larger sets and repeated rounds push the odds further out.
The number only holds if the user does not undo it afterwards. Spending 2 mixed outputs together, sending them straight to a KYC exchange or reusing an address collapses the set back to 1. Metrics such as entropy try to score a transaction’s ambiguity more finely. All of them describe probabilities, which is why Bitcoin privacy should be discussed as a gradient rather than a switch.
In French · Ensemble d’anonymat