Definition
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. 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, which is what preserves privacy across the round.
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 number of inputs is present. This thresholding ensures a transaction has enough participants to create a worthwhile anonymity set. Registering an input is a commitment: a participant who registers but then refuses to sign can stall the round, which is why protocols enforce consequences for non-cooperation.
Privacy hygiene during registration
To keep the coordinator from correlating identities, participants register inputs and outputs over separate network connections, commonly distinct Tor circuits. Sloppy coin selection at this stage, such as registering UTXOs that are already linked on-chain, can undermine the privacy the mix is meant to provide. Thoughtful input selection is therefore the first line of defense in any collaborative transaction.
The complementary step is output registration.
In Simple Terms
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…