JoinMarket

JoinMarket is open-source software for building CoinJoin transactions, with an incentive structure designed to make collaborative privacy practical. It creates a marketplace for CoinJoin liquidity: participants who are willing to wait offer their coins for mixing, and participants who want to mix immediately pay a small fee for the privilege. Throughout, private keys never leave the user's wallet, so the system is fully non-custodial — no coordinator ever holds funds, and no account or registration exists to censor.

Makers and takers

JoinMarket runs on an order-book model with two roles. Market makers are patient participants who post standing offers — their fee, how much they will contribute to miner fees, and the size range they will mix — and earn fees whenever a mix includes them. Market takers are time-sensitive participants who pick from those offers and pay to assemble a CoinJoin on demand, choosing how many counterparties to include. This fee market keeps a standing pool of liquidity available, so a taker can usually mix within minutes rather than waiting for enough volunteers to coincide. The taker also gains a subtle privacy edge: only the taker knows the full mapping of the transaction it assembled, while each maker sees just its own inputs and outputs.

Why the marketplace matters

Earlier CoinJoin attempts struggled with a coordination problem: they needed several users wanting to mix similar amounts at the same moment. By paying makers to provide liquidity, JoinMarket decouples the taker's timing from other users' schedules and rewards people for contributing to a larger anonymity set — privacy as a market good rather than an act of charity. Coordination has historically happened over redundant messaging transports such as IRC and directory servers, so no single infrastructure operator can switch the market off. The design also added fidelity bonds — makers can timelock bitcoin to make Sybil attacks expensive, since flooding the order book with fake makers would require locking real capital for each identity.

Trade-offs and place in the toolbox

JoinMarket's honest costs are operational: it is software for people comfortable running a node-backed wallet and, ideally, leaving a machine online — makers earn passive fees precisely because they provide always-on liquidity, and a home server or repurposed mining-adjacent box does the job. Amounts are not forced into fixed denominations, which preserves flexibility but puts more responsibility on the user for careful coin control and avoiding address reuse afterward; sloppy post-mix handling can undo what the mix achieved. JoinMarket represents the decentralized, market-driven branch of Bitcoin CoinJoin, contrasting with coordinator-run equal-output systems like Whirlpool — no central coordinator to shut down, at the price of more user skill. For the underlying technique and its on-chain footprint, see CoinJoin.

Running a maker as infrastructure

The maker role deserves a special mention for self-hosters, because it inverts the usual privacy economics: instead of paying for privacy, an always-on maker earns a modest yield in fees while passively improving the anonymity set for everyone. The requirements are familiar territory for anyone already running a node — a Bitcoin Core backend, a machine that stays up, and coins you are comfortable keeping in a hot-but-non-custodial wallet, sized to what you would keep in any operational wallet rather than your savings. Yields are honest but small; the real return is participating in privacy infrastructure that has no company to subpoena and no server to seize. It is the same instinct that puts a miner in the garage instead of a hosting contract: contribute the resource yourself, and the network owes its properties to no one. If that instinct resonates, start small: a modest maker bond on hardware you already run teaches the mechanics with limited exposure, and the order book will still be there when you are ready to scale it.