My principal concern in this case is a core problem with E2E web applications in general. For it to be E2E, your decryption needs to occur client-side. In the c

My principal concern in this case is a core problem with E2E web applications in general. For it to be E2E, your decryption needs to occur client-side. In the case of a web app, this would be JavaScript. An attacker could modify your decryption code to send the plaintext to an attacker with ease. More importantly, however, since the user won’t be storing their decryption key on their computer like they would with a real desktop app, you’ll be sending both the cipher text and the key to the user on every request, meaning that it’s not truly E2E since an attacker on the wire would have the ability to decrypt the message too. Your server would store both as well and could decrypt the messages if it were hacked. This is the fatal flaw of ProtonMail’s supposed E2E and I don’t think you could get around it without forcing the user to use a real app that stores the keys locally and not on your server.