Whirlpool

Whirlpool is a Chaumian CoinJoin implementation built on the ZeroLink framework, originally developed by the Samourai Wallet team. Its goal is to maximize the ambiguity of which input maps to which output by mixing coins in fixed denominations and uniform transaction shapes, so an outside analyst cannot follow a coin through a mix.

Equal pools, equal outputs

Whirlpool organizes mixing into pools of fixed sizes, with each round structured as exactly five inputs and five identical outputs. Because every output in a round is the same amount, there is no way to distinguish them by value, and a single mix yields many plausible input-to-output interpretations. Repeated rounds compound this ambiguity, growing the effective anonymity set of a coin over time. A coordinator schedules rounds, but because the design is a Chaumian CoinJoin it remains non-custodial — the coordinator never holds user funds.

Design philosophy and context

Whirlpool's equal-output discipline is the source of its strength and also a usability constraint: users mix in set denominations rather than arbitrary amounts. Independent research has analyzed how anonymity-set quality varies across pools and over time, and the coordinator software has since been continued by other groups. As with any privacy tool, effectiveness depends on careful post-mix coin handling to avoid re-linking outputs.

Whirlpool exemplifies the coordinator-run, equal-output branch of Bitcoin privacy, complementing the market-based JoinMarket approach. Both build on the core CoinJoin primitive.