Paper 2021/596

Mutual Accountability Layer: Accountable Anonymity within Accountable Trust

Vanesa Daza, Abida Haque, Alessandra Scafuro, Alexandros Zacharakis, and Arantxa Zapico

Abstract

Anonymous cryptographic primitives reduce the traces left by the users when interacting over a digital platform. However, they also prevent a platform owner to hold users accountable in case of malicious behaviour. Revocable anonymity offers a compromise by allowing only the manager (and not the other users) of the digital platform to de-anonymize user's activities when necessary. However, such de-anonymization power can be abused too, as a misbehaving manager can de-anonymize all the activities without user's awareness. Previous work propose to mitigate this issue by distributing the de-anonymization power across several entities. However, there is no comprehensive and formal treatment where both accountability and non-frameability (i.e., the inability to falsely accuse a party of misbehavior) for both the user and the manager are explicitly defined and provably achieved. In this paper we formally define mutual accountability: a user can be held accountable for her otherwise anonymous digital actions and a manager is held accountable for every de-anonymization attempt; plus, no honest party can be framed -- regardless of what malicious parties do. Instead of distributing the de-anonymization power across entities, instead, we decouple the power of de-anonymization from the power of monitoring de-anonymization attempts. This allows for greater flexibility, particularly in the choice of the monitoring entities. We show that our framework can be instantiated generically from threshold encryption schemes and succinct non-interactive zero-knowledge. We also show that the highly-efficient threshold group signature scheme by Camenisch et al.(SCN'20) can be modified and extended to instantiate our framework.