Paper 2026/1455

Trout++: Robust Asynchronous Two-Round ECDSA for Arbitrary Thresholds

Abstract

We present Trout++, a complete threshold signing suite for ECDSA signatures. Trout++ descends from the recent Trout protocol (Dahari-Garbian, Nof, and Parker, ACM CCS 2025) and inherits its transparent setup, two-round structure, and strong security guarantees, while introducing several significant improvements. Unlike Trout, Trout++ offers pre-signing, where the first round is key-, signing-set-, and message- independent. This property is not only important in its own right but also enables us to apply the ROAST transformation (Ruffing, Ronge, Jin, Schneider-Bensch, and Schröder, ACM CCS 2022) to our protocol, yielding the \textit{first} arbitrary-threshold (including with a dishonest majority), robust, asynchronous signing protocol for ECDSA signatures. Furthermore, we introduce several optimizations to the building blocks in Trout that reduce both bandwidth and computation. Our benchmark results show that our implementation of Trout++ is approximately twice as fast \emph{and} twice as small compared to the prior implementation of Trout. We also present a constant-time implementation with running time that is \textit{ten times faster} than Trout's constant-time implementation. Finally, we show how Trout++ can be composed with an account derivation scheme that does not require additional setups per derivation, enabling more practical solutions for managing key material. In total, we achieve complexities and functionality comparable to, and closing the gap with, leading solutions for Schnorr signatures (such as FROST).