This is already the case with Proton Mail. Your encryption key is stored encrypted so that we have no access to it. It is decrypted when you enter your password, which we have no access to. The article above confirms that, in fact, and all of these cases are proof that our encryption cannot be bypassed by legal means. All the data stored encrypted on our servers (email content, calendars, attachments, etc.) is inaccessible to us: https://proton.me/blog/zero-access-encryption. All we can provide is the unencrypted metadata which we need to have access to in order for the services to work properly.