Paper 2019/565

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

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


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.

Available format(s)
Public-key cryptography
Publication info
A major revision of an IACR publication in CRYPTO 2019
message frankingdesignated verifier signaturesdeniabilityend-to-end encrypted messaging
Contact author(s)
nirvan tyagi @ gmail com
2019-05-27: received
Short URL
Creative Commons Attribution


      author = {Nirvan Tyagi and Paul Grubbs and Julia Len and Ian Miers and Thomas Ristenpart},
      title = {Asymmetric Message Franking: Content Moderation for Metadata-Private End-to-End Encryption},
      howpublished = {Cryptology ePrint Archive, Paper 2019/565},
      year = {2019},
      note = {\url{}},
      url = {}
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