Cryptology ePrint Archive: Report 2019/565

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

Nirvan Tyagi and Paul Grubbs and Julia Len and Ian Miers and Thomas Ristenpart

Abstract: 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.

Category / Keywords: public-key cryptography / message franking, designated verifier signatures, deniability, end-to-end encrypted messaging

Original Publication (with major differences): IACR-CRYPTO-2019

Date: received 26 May 2019

Contact author: nirvan tyagi at gmail com

Available format(s): PDF | BibTeX Citation

Version: 20190527:092413 (All versions of this report)

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