Protonmail uses PGP and not "home-brewn encryption" and encryption is on by default between Protonmail users.
Even when it does use PGP, it is meaningless, explanation:
I just created a spare protonmail account. It asked me to pick a username and password, and my account is created.
Next, I send there a message from my other account. Yes, on the receiving end it does write "End-to-end encrypted and signed message", but encrypted and signed by what exactly? I have never created a PGP keys and loaded the public key to Protonmail on either account (and never used my private key to decrypt anything. This can mean only one thing: even if there is some kind of encryption happening, Protonmail themselves generate keys, and uses it for encryption-decryption, never asking you for anything but your password. And if they can uses these keys to decrypt the messages for you, they can decrypt it for anybody.
Protonmail also gives a user an option to export his private keys. Yeah, right. Your private keys.
Verification is very simple: if you log in on a new device and see all your content while using only login and password to authenticate yourself, then the content stored on a server is NOT encrypted and is readable by server owner.
What about if the encryption key is derived from your password? This is common enough for "encrypt file with a password" services, I've personally implemented it in-browser as part of a small project.
Now, having your account password be the same as the email decryption password is also probably a bad idea, but we're far from the server owner being able to read your emails.
Protonmail keeps generated public and private keys on their servers.
Protonmail do not see your password and without it cannot decrypt the pub/private key pair.
> Your ProtonMail private key is generated in your browser. Before sending the private key to the server for storage, we encrypt it with your password (or mailbox password if you use two-password mode). This ensures that you and only you can use your private key.
So the only remaining question is whether ProtonMail has access to your password. If they do, they can decrypt your private key and then decrypt your emails. Often, passwords are sent in plaintext to a server for authentication. But ProtonMail uses the Secure Remote Password (SRP) protocol so they never see your password: https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco.... (source: https://protonmail.com/blog/encrypted_email_authentication/)
Of course, there are other threats to worry about, such as ProtonMail changing their client-side JavaScript to exfiltrate your password. But the system as they've documented it does not appear to have any way to decrypt your email server-side short of guessing your password.
Such agency can force PM to modify login process to derive password from submitted form, or to just switch private keys for non-encrypred ones, because the user won't even notice it.
Truly secure entity just wouldn't have private keys on a server at all. Users would have to go through an an uncomfortable process of generating and uploading keys to clients, but they would be truly safe.
To sum it up, you can't really have security and convenience at once. besides skipping a proper key management process, PM also mail skips such important steps as verification of email partner identify and key verification, so you have to trust PM that you are really talking to a person you think you are talking.
They don't. They have your encrypted private key, but there's no need to keep that secret. (The decryption key is derived from your password, so the password needs to be strong and secret.)
> Such agency can force PM to modify login process to derive password from submitted form, or to just switch private keys for non-encrypred ones, because the user won't even notice it.
Yes, definitely. It's hard to trust self-updating software (like JavaScript in the browser), particularly if you're concerned about targeted attacks. But creating your own private keys and then entering them in the browser wouldn't help you at all against that sort of attack. You would instead need a different type of client that could be trusted somehow not to leak your private key.
It's not uncommon for services like this to offer a downloadable version of the web client so you can pin a version and audit the code as needed. I think maybe https://github.com/ProtonMail/WebClient is that for ProtonMail? If so, you should be able to verify that code and then use that. The fact that an encrypted copy of your private key will live on ProtonMail's servers shouldn't bother you.
Your argument boils down to “govt can force them to change how they do that”, as opposed to a flaw in their approach.
True security is when the provider can't decrypt anything under all circumstances, even under coercion.