Abuse Reporting for Metadata-Hiding Communication Based on Secret Sharing

As interest in metadata-hiding communication grows in both research and practice, a need exists for stronger abuse reporting features on metadata-hiding platforms. While message franking has been deployed on major end-to-end encrypted platforms as a lightweight and effective abuse reporting feature, there is no comparable technique for metadata-hiding platforms. Existing efforts to support abuse reporting in this setting, such as asymmetric message franking or the Hecate scheme, require order of magnitude increases in client and server computation or fundamental changes to the architecture of messaging systems. As a result, while metadata-hiding communication inches closer to practice, critical content moderation concerns remain unaddressed. This paper demonstrates that, for broad classes of metadatahiding schemes, lightweight abuse reporting can be deployed with minimal changes to the overall architecture of the system. Our insight is that much of the structure needed to support abuse reporting already exists in these schemes. By taking a non-generic approach, we can reuse this structure to achieve abuse reporting with minimal overhead. In particular, we show how to modify schemes based on secret sharing user inputs to support a message franking-style protocol. Compared to prior work, our shared franking technique more than halves the time to prepare a franked message and gives order of magnitude reductions in server-side message processing times, as well as in the time to decrypt a message and verify a report.