Definition
A blame round is the recovery mechanism a CoinJoin protocol triggers when a transaction fails to finalize because one or more participants disrupted the signing phase, for example by registering inputs but never providing a signature. Rather than discarding the whole round, the coordinator restarts the necessary phases with only the cooperative participants, allowing an honest subset to still produce a valid mixed transaction. This keeps the system resilient against both accidental dropouts and deliberate sabotage.
Why disruption is a real threat
A CoinJoin transaction is only valid once every input is signed, so a single participant who refuses to sign can stall everyone. Without a remedy, an attacker could cheaply grief rounds, degrading the experience and shrinking the achievable anonymity set for everyone else. The blame round restores progress by isolating the misbehaving inputs and re-running the round among the rest.
Penalizing non-cooperation
To make disruption expensive, protocols temporarily blacklist the UTXOs of participants who failed to sign, so a would-be attacker must keep committing fresh coins to keep griefing. This denial-of-service protection is what lets an untrusted, permissionless mixing process stay usable in adversarial conditions. For sovereign users, it means a few bad actors cannot deny you access to fungibility-improving collaborative transactions.
Related: CoinJoin coordinator and input registration.
In Simple Terms
A blame round is the recovery mechanism a CoinJoin protocol triggers when a transaction fails to finalize because one or more participants disrupted the signing…