Here’s an in-depth analysis of Proton Mail’s security architecture as of 2021:
https://eprint.iacr.org/2018/1121.pdf
It’s highly technical, but here’s the headline: “As it stands, ProtonMail does not meet its self-professed security goals when these are subjected to analysis.”
Maybe they’ve fixed things since 2021 — but fundamentally, Proton Mail is trying to make a pig fly here; email protocol just weren’t designed for E2E encryption. There will always be leaks, slips, gaps.
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