In this work, we provide the first independent analysis of ProtonMail's cryptographic architecture. We find that for the majority of ProtonMail users, no end-to-end encryption guarantees have ever been provided by the ProtonMail service and that the "Zero-Knowledge Password Proofs" are negated by the service itself. We also find and document weaknesses in ProtonMail's "Encrypt-to-Outside" feature.

https://eprint.iacr.org/2018/1121