Technical Report on Threshold ECDSA in the Preprocessing Setup

Threshold ECDSA (Elliptic Curve Digital Signature Algorithm) has garnered significant
attention from both industry and research, primarily due to its blockchain applications, especially in the context of threshold wallets. Existing solutions often assume that the client is either an integral part of the signers, as evident in self-custodial or non-custodial wallets, or that they entirely delegate the signing procedure to an external network, as observed in custodial wallets. The former allows clients to defend against collusion attacks from other signers, while the latter enables more efficient online phases through the use of preprocessing material. In this report, we explore the development of a threshold signature scheme that keeps some of the key benefits from both custodial and non-custodial settings. Specifically, we present a threshold ECDSA signature scheme in the client-server model that combines the optimal online round complexity from the custodial setting with resilience against collusion among all signer nodes during the online phase, as observed in non-custodial wallets. The scheme is also secure against user impersonation attacks, a key concern with custodial wallets.