Private Message Franking with After Opening Privacy

Abstract

Grubbs et al. [11] initiated the formal study of message franking protocols. This new type of service launched by Facebook, allows the receiver in a secure messaging application to verifiably report to a third party an abusive message some sender has sent. A novel cryptographic primitive: committing AEAD has been initiated, whose functionality apart from confidentiality and authenticity asks for a compact commitment over the message, which is delivered to the receiver as part of the ciphertext. A new construction (Committing Encrypt and) has then been proposed, which is multi-opening secure and reduces the computational costs for the sender and the receiver. In this paper we provide a formal treatment of message franking protocols with minimum leakage whereby only the abusive blocks are opened, while the rest non-abusive blocks of the message remain private.