Cryptology ePrint Archive: Report 2021/1686

Hecate: Abuse Reporting in Secure Messengers with Sealed Sender

Rawane Issa and Nicolas Alhaddad and Mayank Varia

Abstract: End-to-end encryption provides strong privacy protections to billions of people, but it also complicates efforts to moderate content that can seriously harm people. To address this concern, Tyagi et al. [CRYPTO 2019] introduced the concept of asymmetric message franking (AMF), which allows people to report abusive content to a moderator, while otherwise retaining end-to-end privacy by default and even compatibility with anonymous communication systems like Signal’s sealed sender.

In this work, we provide a new construction for asymmetric message franking called Hecate that is faster, more secure, and introduces additional functionality compared to Tyagi et al. First, our construction uses fewer invocations of standardized crypto primitives and operates in the plain model. Second, on top of AMF’s accountability and deniability requirements, we also add forward and backward secrecy. Third, we combine AMF with source tracing, another approach to content moderation that has previously been considered only in the setting of non-anonymous networks. Source tracing allows for messages to be forwarded, and a report only identifies the original source who created a message. To provide anonymity for senders and forwarders, we introduce a model of "AMF with preprocessing" whereby every client authenticates with the moderator out-of-band to receive a token that they later consume when sending a message anonymously.

Category / Keywords: applications / end-to-end encrypted messaging, anonymity, tracing, abuse reporting, message franking

Original Publication (with major differences): USENIX Security 2022

Date: received 22 Dec 2021, last revised 11 May 2022

Contact author: ra1issa at bu edu

Available format(s): PDF | BibTeX Citation

Version: 20220511:011717 (All versions of this report)

Short URL: ia.cr/2021/1686


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