[upofadown]

Keeping old messages around for all practical purposes negates forward secrecy in any messaging system. It isn't just an email issue. If they can get your secret key they can pretty much for sure get your old messages.

Most email users keep their messages in cloud storage (IMAP) so that changing computers is a non-issue. OpenPGP is an encrypt once scheme so that messages on an IMAP server are encrypted and stay encrypted.

[tptacek]

Systems that lack forward secrecy are by design incapable of preventing archives of eventually-plaintext messages. There's nothing you can do about it; every message you send is irrevocably a part of the adversary's record, and, because you rely on a single long-term key, you know eventually that record will be plaintext. That's why forward secrecy is such a big deal, and why every modern messaging cryptosystem uses ephemeral keys.

[reifyx]

But this only applies for messages that are deleted on your local device (either manually or through an automatic timer). Otherwise, whatever adversary stole your keys can steal your message archive too, they're on the same device. Now, assuming you aren't going to be deleting most of your mail, I don't see how forward secrecy is "such a big deal" in this scenario. It's certainly nice to have, but it definitely has drawbacks wrt the features I mentioned earlier.

Post-compromise security, on the other hand, makes more sense, since the future messages don't exist yet.

[tptacek]

You can't meaningfully delete messages in non-forward-secret systems, because part of the premise of all these systems is that your adversary is recording everything.

[reifyx]

I agree with you there. But is your point that any secure email system must critically have forward secrecy, or its insecure? Even though forward secrecy really only gives you any benefit for the messages that you delete, which most people don't in the context of email?

Just thinking, if people had the option between 1) deleting their mail and 2) email search, secure (unlike WhatsApp) and easy (unlike Signal) backups, ability to offload your email archive to the server (it's common to have gigabytes of mail, do you want to store all of it on a mobile phone forever? what happens if you drop it in a river?), and so on, don't you think people would go for option 2?

This is all disregarding the specifics of PGP-encrypted mail, for which I agree is not great.

[tptacek]

The point is that one of the basic properties of messaging encryption is forward secrecy. It's an argument about how messaging is different from backup, package signing, file encryption, distributed logs, file transfer, and secure transports (though some of these really want forward secrecy too), and how PGP advocates back-rationalize not needing forward secrecy so they can defend their weird archaic tool.

[reifyx]

Your argument is that forward secrecy is important in messaging because forward secrecy is important in messaging?

I'm not trying to be argumentative here, I actually don't understand what the reason it's so critical is, nor have I really found any explanations online. For text messaging where you don't really go back to read your old messages, sure, forward secrecy makes sense. Email seems to be a different story where user expectation is different and forward secrecy both precludes many desired features and also doesn't provide significantly more security, other than in very limited circumstances.

Also, I'm not an advocate of PGP at all. If people can use Signal for their usecase, great! They should do that. But Signal's model does not work for everyone's usecases. How do I send a Signal message to [e-mail] to report a vulnerability? Is the entire security team supposed to share a mobile phone with Signal on it? What about banks that need to send secure email to each other, but must retain all messages for compliance purposes? (Again, I'm not advocating that PGP should be used in this scenario either, just that there's room for a better solution here, possibly without forward secrecy by default).

[upofadown]

>Systems that lack forward secrecy are by design incapable of preventing archives of eventually-plaintext messages.

That is not what is being claimed here. Unless you add extra security in the form of something like a strong unique passphrase for the archived messages then an attack that gets the private key also gets the archived messages. In general, if you have a more secure method for protecting the archived messages you could of used it to protect the private key. It is effectively the same problem.