Input Registration

Input registration is the opening phase of a coordinated CoinJoin round. Each participant selects which of their unspent outputs (UTXOs) they want to mix and registers them with the CoinJoin coordinator, proving ownership by signing with the keys that control each input. In return, modern protocols like WabiSabi issue cryptographically blinded credentials representing the registered value. These credentials let the participant later claim outputs without the coordinator being able to link the input to the output — the blinding is what makes the coordinator a facilitator rather than a surveillance point, and it is the property the round's privacy ultimately rests on.

How the phase is timed and gated

A round typically begins once the number of registered inputs reaches a maximum, or after a fixed input-registration window closes, provided a minimum participant count is met. This thresholding ensures the eventual transaction has enough participants to create a worthwhile anonymity set — a mix of three people protects little; a mix of a hundred protects everyone in it. Registration is also a commitment with teeth: a participant who registers inputs but then disappears or refuses to sign the final transaction stalls the round for everyone, so protocols enforce consequences for non-cooperation, typically banning the offending UTXOs from future rounds for a period. This is also where denial-of-service and Sybil defenses live — fees and proof-of-ownership requirements make it expensive for an attacker to flood a round with their own inputs in hopes of shrinking everyone else's effective anonymity set.

Privacy hygiene during registration

The mechanics protect you only if your own behavior does not betray you first. To keep the coordinator from correlating registrations by network identity, each input is registered over a separate network connection — in practice, distinct Tor circuits — so nothing at the transport layer ties your inputs to each other or to your later output claims. Coin selection is the second discipline: registering UTXOs that are already publicly linked on-chain (say, several outputs from one KYC withdrawal) can be reasonable, since they are already clustered, but mixing linked and unlinked coins carelessly, or registering a UTXO whose history you have not checked with coin control, can leak more than the round gains you. Thoughtful input selection is the first line of defense in any collaborative transaction — the protocol can hide the mapping between inputs and outputs, but it cannot un-link what your inputs already said about each other.

Where it sits in the round

Input registration is phase one of a pipeline: once the input set is locked, participants reconnect under fresh identities to perform output registration, claiming equal-denomination outputs against their blinded credentials, and the round concludes with signing, where every participant verifies the assembled transaction pays them correctly before contributing signatures. Nothing is custodial at any point — coins move only if the final CoinJoin transaction, checked by every participant, gets every required signature. For the sovereign user, the phase is worth understanding because it is where most practical privacy is won or lost: the coordinator sees what you register, the network sees how you connect, and both are under your control.

What can go wrong, and how protocols cope

Input registration is also the phase where rounds fail, so its error handling is instructive. Participants drop offline, wallets crash mid-round, and adversaries probe with inputs they intend to abandon; a well-designed coordinator treats all three identically, since it cannot distinguish malice from misfortune. Rounds therefore run on strict timeouts — miss your phase and the round proceeds without you or restarts — and repeat offenders' UTXOs accumulate escalating cooldowns. From the user's side, the reliability lesson is mundane: run your mixing wallet on a machine that stays awake and connected, because every failed round costs the whole cohort time. And keep perspective on what the coordinator learns even when everything works — it sees which UTXOs entered mixing and when, which is why registering over Tor and letting rounds proceed at their own pace is part of the protocol's design, not optional polish.