Paper 2022/1643

End-to-End Secure Messaging with Traceability Only for Illegal Content

James Bartusek, University of California, Berkeley
Sanjam Garg, NTT Research, University of California, Berkeley
Abhishek Jain, Johns Hopkins University
Guru-Vamsi Policharla, University of California, Berkeley

As end-to-end encrypted messaging services become widely adopted, law enforcement agencies have increasingly expressed concern that such services interfere with their ability to maintain public safety. Indeed, there is a direct tension between preserving user privacy and enabling content moderation on these platforms. Recent research has begun to address this tension, proposing systems that purport to strike a balance between the privacy of ''honest'' users and traceability of ''malicious'' users. Unfortunately, these systems suffer from a lack of protection against malicious or coerced service providers. In this work, we address the privacy vs. content moderation question through the lens of pre-constrained cryptography [Ananth et al., ITCS 2022]. We introduce the notion of set pre-constrained (SPC) group signatures that guarantees security against malicious key generators. SPC group signatures offer the ability to trace users in messaging systems who originate pre-defined illegal content (such as child sexual abuse material), while providing security against malicious service providers. We construct concretely efficient protocols for SPC group signatures, and demonstrate the real-world feasibility of our approach via an implementation. The starting point for our solution is the recently introduced Apple PSI system, which we significantly modify to improve security and expand functionality.

Available format(s)
Cryptographic protocols
Publication info
group signatures end-to-end encrypted messaging tracing anonymity
Contact author(s)
jamesbartusek @ berkeley edu
sanjamg @ berkeley edu
abhishek @ cs jhu edu
guruvamsi policharla @ gmail com
2022-11-28: approved
2022-11-25: received
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Creative Commons Attribution


      author = {James Bartusek and Sanjam Garg and Abhishek Jain and Guru-Vamsi Policharla},
      title = {End-to-End Secure Messaging with Traceability Only for Illegal Content},
      howpublished = {Cryptology ePrint Archive, Paper 2022/1643},
      year = {2022},
      note = {\url{}},
      url = {}
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