Proton
Empowering you to choose a better internet where privacy is the default. Protect yourself online with Proton Mail, Proton VPN, Proton Calendar, Proton Drive. Proton Pass and SimpleLogin.
Proton Mail is the world's largest secure email provider. Swiss, end-to-end encrypted, private, and free.
Proton VPN is the world’s only open-source, publicly audited, unlimited and free VPN. Swiss-based, no-ads, and no-logs.
Proton Calendar is the world's first end-to-end encrypted calendar that allows you to keep your life private.
Proton Drive is a free end-to-end encrypted cloud storage that allows you to securely backup and share your files. It's open source, publicly audited, and Swiss-based.
Proton Pass Proton Pass is a free and open-source password manager which brings a higher level of security with rigorous end-to-end encryption of all data (including usernames, URLs, notes, and more) and email alias support.
SimpleLogin lets you send and receive emails anonymously via easily-generated unique email aliases.
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They're not bad, you're just misinformed at a fundamental level.
Proton Mail is like Bitwarden, it encrypts data client side and stores the encrypted blob server side, which is exactly what they're doing with your private key. Otherwise, you'd have to carry it around on a USB or do some other voodoo to be able to read your emails.
That paper is god awful bad. They're basically saying things like "it can't be secure because they rely on the client code to be delivered by TLS and you could have a MITM that results in different client code being sent!" and "proton allows you to set passwords that are weak, thereby not looking out for your best interest!
Their conclusion can be summarized as "Proton can't provide a secure web mail application, because nobody can." Their suggested remedy is also actually a thing now because there is a Proton Mail desktop application.
The whole thing is pretty ridiculous in any case because someone would have to have control over your DNS server, you'd have to go to a phishing instance of proton instead of the real one, you'd be logged out because the cookies wouldn't be decryptable by their server, so you'd then finally have to login handing over your password.
If you use Proton VPN (or some other trustworthy DNS) that situation can happen. For most people it's an extremely unlikely situation. It's not a Proton problem though, it's a web technology problem.
For most people this situation will never happen (but it would be nice if someone would solve the problem).
People are going to abuse services that allow anonymous signups... Proton does not claim to be an anonymous email service, merely a private email service.