Cryptographic Analysis of Delta Chat

We analyse the cryptographic protocols underlying Delta Chat, a decentralised messaging application which uses email infrastructure for message delivery. It provides end-toend encryption by implementing the Autocrypt standard and the SecureJoin protocols, both making use of the OpenPGP standard. Delta Chat's adoption by categories of high-risk users such as journalists and activists, but also more generally users in regions affected by Internet censorship, makes it a target for powerful adversaries. Yet, the security of its protocols has not been studied to date. We describe five new attacks on Delta Chat in its own threat model, exploiting cross-protocol interactions between its implementation of SecureJoin and Autocrypt, as well as bugs in rPGP, its OpenPGP library. The findings have been disclosed to the Delta Chat team, who implemented fixes.