Paper 2024/029

YouChoose: A Lightweight Anonymous Proof of Account Ownership

Abstract

Anonymous proofs of account ownership (anonymous PAOs) enable a prover to convince a verifier that the prover owns a valid account at a server without revealing their identity. Importantly, no server-side changes are required - the server does not even learn that such a proof is taking place. This functionality is particularly valuable in sensitive applications such as whistleblowing, or in settings where infrastructural changes at the server are impractical. Anonymous PAOs were first introduced by Wang et al. (IEEE S&P 2019), who realized them in the email setting via secure channel injection (SCI), a technique that requires carefully engineered multi-party computation (MPC) protocols. In this work, we propose YouChoose, a new approach to anonymous PAO that avoids MPC entirely, relying instead on the verifier to selectively forward TLS records. Compared to SCI, YouChoose is simpler, faster, and more adaptable across services. We also provide the first formal security definition for anonymous PAOs, establishing a rigorous framework for analysis and future constructions. Finally, we present a prototype implementation of YouChoose, evaluate it in the context of email and other social media accounts, showing its better performance than SCI with minimal latency overhead in practice.