protonmail provides encrypted "zero-access" encryption mailboxes. The way they explain "zero-access" is, at least for me, similar to zero-knowledge encryption. However protonmail has in its servers my private keys. They say that the keys are encrypted as well, but they also have in their servers my password for that encryption. Therefore, it seems to me that protonmail could at any time access my private keys and my mailbox.

Is this correct, or am I missing something? Is this the reason why they do not call it zero-knowledge encryption?


You are wrong in your assumption that protonmail stores the password used for the encryption of your private key.

Protonmail uses the Secure Remote Password Protocol (SRP on wikipedia - Protonmail blog post about SRP) and so they only store a verifier that is irreversibility related to your password.
When entering your password on the login form of protonmail, some javascript is executed client side and generates this verifier that it sends to protonmail.
Upon verification, protonmail returns the encrypted version of your private key which then gets decrypted client side using your password. This private key is then used to decrypt your emails.


Xavier59's answer is correct, in that protonmail uses SRP. Therefore (under normal circumstances) your password is never sent to protonmail's server.

However, the SRP is implemented in javascript, which is served by protonmail's server. If protonmail were to be compromised, or if protonmail were to 'go rogue', they could easily serve javascript that captures your password after you enter it, and sends the password back to their servers. Then, they would have everything necessary to decrypt your private key, and then decrypt your messages.

This is why javascript cryptography is a 'chicken-and-egg' problem. If you can't trust the server with your secrets, then how can you trust the server to deliver secure code? See https://www.nccgroup.trust/us/about-us/newsroom-and-events/blog/2011/august/javascript-cryptography-considered-harmful/ for some good reading on this subject.

  • educating reference, thank you very much... so it seems it is all wishful thinking... – JuanPi Nov 24 '19 at 22:55

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