Svante,
I have had an idea about how AxCrypt can be made truly zero-knowledge. It integrates with the existing workflow and requires minimal development as all of the infrastructure is already in place.
The security increase by implementing this would be substantial and reduce the risk of AxCrypt being subject to legal process or rubber hose cryptanalysis because the user’s password would never be sent to the server and any evil modification would have to occur within the source code which is freely available and thus detectable.
It’s so simple that I wonder if I’m missing something obvious.
1 – Bob logs into the AxCrypt application using only his email address
2 – Server sends an authentication code to Bob’s email [1]
3 – Bob enters the code and the server sends the key-pair to Bob’s device [2]
4 – Server simultaneously sends any licence file plus a token [3]
4 – AxCrypt locally requests password which Bob
5 – Software now functions as it does currently
[1] Most email is sent over TLS which offers reasonable security. (As the user’s encryption password is currently sent over TLS to AxCrypt it can’t be disputed that an authentication code over TLS would reduce security.)
[2] Even if Mallory had access to Bob’s email the returned key-pair would be useless because the private key requires the password (known only to Bob) before it can be used to decrypt files.
[3] The token could be valid for 1-year. Unless Bob changed computers he wouldn’t be required to request another authentication code because the token would persist. Even upon restart, log off, shutdown, screensaver activation he’d only be asked to enter his password. The exception to this would be if he cleared all settings.
Password changes would only be allowed if the user was logged in to the application and online. The software would authenticate with the server (using the token) and upload any necessary files to the server but it wouldn’t need to transmit the password.
Eventually, to offer the greatest assurance, AxCrypt should implement fingerprint validation: if necessary behind an ‘Expert Mode’ button. Some users will be familiar with this as popular messaging apps like WhatsApp already provide this.
The only small issue I envisage is the free AxCrypt online Password Manager. I don’t know how popular this is but those users would have to use a different password for that. The side benefit is that if somebody forgot one password (e.g. the encryption password) they wouldn’t lose access to their passwords and vice versa. If you were really keen on one password for everything then you could send Bob an XML file of the passwords at step 4 above. The passwords would be useless without the encryption password (e.g. Mallory gained access to the authentication code). Hence whenever a user updated a password the encrypted XML would be sent back to the server and synchronised.
You could in theory get rid of the online account area as it’d be unnecessary although if you do keep it then you could simplify logging in by using the authentication token – the user clicks an ‘Account’ button within the software and that opens the online account with no further request for a password. Any upgrades to Premium would be done like this as well. You could introduce an option to ‘Disable Computer’ (i.e. revoke the authentication token) although this’d be largely unnecessary because the application prompts for the password at regular intervals so the risk from Mallory gaining access to the online account is negligible.
I’m posting this publicly so that any experts can comment on, criticise or improve this suggestion.
