Paper 2024/1920

An Extended Hierarchy of Security Notions for Threshold Signature Schemes and Automated Analysis of Protocols That Use Them

Abstract

Despite decades of work on threshold signature schemes, there is still limited agreement on their desired properties and threat models. In this work we significantly extend and repair previous work to give a unified syntax for threshold signature schemes and a new hierarchy of security notions for them. Moreover, our new hierarchy allows us to develop an automated analysis approach for protocols that use threshold signatures, which can discover attacks on protocols that exploit the details of the security notion offered by the used scheme, which can help choose the correct security notion (and scheme that fulfills it) that is required for a specific protocol. Unlike prior work, our syntax for threshold signatures covers both non-interactive and interactive signature schemes with any number of key generation and signing rounds, and our hierarchy of security notions additionally includes elements such as various types of corruption and malicious key generation. We show the applicability of our hierarchy by selecting representative threshold signature schemes from the literature, extracting their core security features, and categorizing them according to our hierarchy. As a side effect of our work, we show through a counterexample that a previous attempt at building a unified hierarchy of unforgeability notions does not meet its claimed ordering, and show how to repair it without further restricting the scope of the definitions. Based on our syntax and hierarchy, we develop the first systematic, automated analysis method for higher-level protocols that use threshold signatures. We use a symbolic analysis framework to abstractly model threshold signature schemes that meet security notions in our hierarchy, and implement this in the Tamarin prover. Given a higher-level protocol that uses threshold signatures, and a security notion from our hierarchy, our automated approach can find attacks on such protocols that exploit the subtle differences between elements of our hierarchy. Our approach can be used to formally analyze the security implications of implementing different threshold signature schemes in higher-level protocols.