Cryptology ePrint Archive: Report 2019/016

Fast Message Franking: From Invisible Salamanders to Encryptment

Yevgeniy Dodis and Paul Grubbs and Thomas Ristenpart and Joanne Woodage

Abstract: Message franking enables cryptographically verifiable reporting of abusive content in end-to-end encrypted messaging. Grubbs, Lu, and Ristenpart recently formalized the needed underlying primitive, what they call compactly committing authenticated encryption (AE), and analyzed the security of a number of approaches. But all known secure schemes are still slow compared to the fastest standard AE schemes. For this reason Facebook Messenger uses AES-GCM for franking of attachments such as images or videos. We show how to break Facebook’s attachment franking scheme: a malicious user can send an objectionable image to a recipient but that recipient cannot report it as abuse. The core problem stems from use of fast but non-committing AE, and so we build the fastest compactly committing AE schemes to date. To do so we introduce a new primitive, called encryptment, which captures the essential properties needed. We prove that, unfortunately, schemes with performance profile similar to AES-GCM won’t work. Instead, we show how to efficiently transform Merkle-Damgärd-style hash functions into secure encryptments, and how to efficiently build compactly committing AE from encryptment. Ultimately our main construction allows franking using just a single computation of SHA-256 or SHA-3. Encryptment proves useful for a variety of other applications, such as remotely keyed AE and concealments, and our results imply the first single-pass schemes in these settings as well.

Category / Keywords: secret-key cryptography / authenticated encryption, encrypted messaging

Original Publication (with major differences): IACR-CRYPTO-2018

Date: received 6 Jan 2019, last revised 14 Jan 2019

Contact author: joanne woodage 2014 at live rhul ac uk, pag225 at cornell edu

Available format(s): PDF | BibTeX Citation

Note: Full version. Fixed broken cites

Version: 20190115:013417 (All versions of this report)

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