International Association
for Cryptologic Research

As online group communication scenarios become more and more common these years, malicious or unpleasant messages are much easier to spread on the internet. Message franking is a crucial cryptographic mechanism designed for content moderation in online end-to-end messaging systems, allowing the receiver of a malicious message to report the message to the moderator. Unfortunately, the existing message franking schemes only consider 1-1 communication scenarios. In this paper, we systematically explore message franking in group communication scenarios. We introduce the notion of asymmetric group message franking (AGMF), and formalize its security requirements. Then, we provide a framework of constructing AGMF from a new primitive, called $\textup{HPS-KEM}^{\rm{\Sigma}}$. We also give a construction of $\textup{HPS-KEM}^{\rm{\Sigma}}$ based on the DDH assumption. Plugging the concrete $\textup{HPS-KEM}^{\rm{\Sigma}}$ scheme into our AGMF framework, we obtain a DDH-based AGMF scheme, which supports message franking in group communication scenarios.