International Association for Cryptologic Research

International Association
for Cryptologic Research


End to End Secure Messaging with Traceability Only for Illegal Content

James Bartusek , UC Berkeley
Sanjam Garg , UC Berkeley
Abhishek Jain , Johns Hopkins University
Guru-Vamsi Policharla , UC Berkeley
DOI: 10.1007/978-3-031-30589-4_2 (login may be required)
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Presentation: Slides
Conference: EUROCRYPT 2023
Abstract: 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 {\em set pre-constrained} (SPC) {\em group signatures} that guarantees security against \emph{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.
  title={End to End Secure Messaging with Traceability Only for Illegal Content},
  author={James Bartusek and Sanjam Garg and Abhishek Jain and Guru-Vamsi Policharla},