Non-interactive Designated Verifier Proofs and Undeniable Signatures

Abstract. Non-interactive designated verifier (NIDV) proofs were first introduced by Jakobsson et al. and have widely been used as confirmation and denial proofs for undeniable signature schemes. There appears to be no formal security modelling for NIDV undeniable signatures or for NIDV proofs in general. Indeed, recent work by Wang has shown the original NIDV undeniable signature scheme of Jakobsson et al. to be flawed. We argue that NIDV proofs may have applications outside of the context of undeniable signatures and are therefore of independent interest. We therefore present two security models, one for general NIDV proof systems, and one specifically for NIDV undeniable signatures. We go on to repair the NIDV proofs of Jakobsson et al., producing secure NIDV proofs suited to combination with Chaum’s original undeniable signature scheme resulting in a secure and efficient concrete NIDV undeniable signature scheme.

Asymmetric Message Franking: Content Moderation for Metadata-Private End-to-End Encryption

Content moderation is crucial for stopping abuse and harassment via messaging on online platforms. Existing moderation mechanisms, such as message franking, require platform providers to see user identifiers on encrypted traffic. These mechanisms cannot be used in messaging systems in which users can hide their identities, such as Signal. The key technical challenge preventing moderation is in simultaneously achieving cryptographic accountability while preserving deniability. In this work, we resolve this tension with a new cryptographic primitive: asymmetric message franking schemes (AMFs). We define strong security notions for AMFs, including the first formal treatment of deniability in moderation settings. We then construct, analyze, and implement an AMF scheme that is fast enough for deployment. We detail how to use AMFs to build content moderation for metadata-private messaging