The Signal protocol relies on a handshake protocol, formerly X3DH and now PQXDH, to set up secure conversations. One of its privacy properties, of value to Signal, is , allowing users to deny participation in communications. Prior analyses of deniability for these protocols, including post-quantum variants, use models highly tailored to the individual protocols and generally make ad-hoc adaptations to "standard" AKE definitions, obscuring the concrete deniability guarantees and complicating comparisons across protocols.Building on Hashimoto, Katsumata, and Wigger's abstraction for Signal handshake protocols (USENIX'25), we address this gap by presenting a unified framework for analyzing their deniability. We analyze Signal's classically secure X3DH and harvest-now-decrypt-later-secure PQXDH, and show the settings for which PQXDH is (un)deniable against harvest-now--later attacks, where a quantum judge retrospectively assesses the participation of classical users. We further analyze post-quantum alternatives like RingXKEM, whose deniability relies on ring signatures (RS). By introducing a novel metric inspired by differential privacy, we provide relaxed, pragmatic guarantees for deniability. Lastly, we also use this metric to define for RS, a relaxation of anonymity, allowing us to build an efficient RS from NIST-standardized Falcon (and MAYO), which is not anonymous, but is provably deniable. We believe this relaxation to have independent interest outside of the Signal handshake protocol.

Aug 13, 20250 citations

Practical Deniable Post-Quantum X3DH: A Lightweight Split-KEM for K-Waay

Guilhem Niot

The Signal Protocol, underpinning secure messaging for billions, faces the challenge of migrating to a post-quantum world while preserving critical properties like asynchrony and deniability. While Signal’s PQXDH key exchange protocol addresses post-quantum confidentiality, full migration of the X3DH protocol remains elusive. Relying on a split KEM (K-Waay, USENIX ’24) offers a promising migration path, but it has so far suffered from size limitations compared to concurrent works leveragingring signatures. This work introduces Sparrow-KEM and Sym-Sparrow-KEM, novel asymmetric and symmetric split KEMs respectively, i.e. for which keys can be used interchangeably for sending and receiving, or only in one direction. They are designed to optimize the K-Waay protocol for size efficiency. Leveraging the MLWE assumption, these constructions reduce by a factor 5.1× the communication of prior post-quantum X3DH based on split KEMs, plus provides a 40× speedup. Additionally, Sym-Sparrow-KEM is the first symmetric split-KEM to offer deniability, IND-1KCA, and IND-1BatchCCA security, capturing implicit authentication properties. We provide formal security proofs for both schemes, including deniability. Our results demonstrate the feasibility of a compact and deniable post-quantum X3DH protocol based on split KEMs.

ACM Asia Conference on Computer and Communications Security (ASIA CCS)Aug 13, 20255 citations