Trout: Two-Round Threshold ECDSA from Class Groups

We present Trout (Two-ROUnd Threshold), the first distributed two-round ECDSA signing protocol for arbitrary thresholds. Trout has constant upload bandwidth per-party and processing time linear in the amount of participants. Moreover, Trout achieves the Identifiable Abort (IA) property, which means that if the protocol cannot terminate due to a failure, parties can attribute the failure to a specific party. We achieve this without a trusted setup. Our protocol relies on linear-homomorphic encryptions and commitments over class groups. To obtain our result, we leverage the recent construction of an exponent-VRF (Boneh et al., Euro-crypt 2025) and a novel protocol to multiply an encrypted value with a committed value and simultaneously decrypt it, which we call "scaled decryption". We believe that this protocol may be of independent interest. Our protocol has a very low communication cost of just 6.5 KB sent per party. Furthermore, we implemented our protocol in Rust and provide benchmarks for various configurations, showing its practicality even for 100 parties. Our implementation includes a constant-time variant which, to the best of our knowledge, is the first of its kind for class-group-based threshold ECDSA protocols.