[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).

[tptacek]

I'm not really sure what you're trying to say.

The premise of cryptographically secure messaging is that you have an adversary recording all your message traffic.

Lack of forward secrecy implies, logically, that if your long-term secret is ever compromised, every message you've ever sent is recoverable from the adversary's archive.

The point of forward secrecy is to break that attack, so that your adversary needs your long-term secret at the time it was used to send a message; having it after the fact doesn't help.

I'm sometimes in the mood to write long posts and comments explaining this stuff, but today, on the bottom of this old thread, if you're trying to make a point about PGP vs. Signal and don't know how forward secrecy works, I'm probably the wrong person to have this conversation with.

[reifyx]

>The premise of cryptographically secure messaging is that you have an adversary recording all your message traffic.

Agreed.

>Lack of forward secrecy implies, logically, that if your long-term secret is ever compromised, every message you've ever sent is recoverable from the adversary's archive.

Also agreed. I am trying to say that this only gives you better security for messages that you have deleted on your device, because if you haven't, regardless of whether your protocol is forward-secret or not, the adversary that has the power to compromise your device will get access to the message the plaintext of which is on the device, even if the keys aren't. Thus, the scope is significantly limited, unless you have a policy to regularly delete old messages on your device, and most people do not want this for email.

I can assure you I understand the cryptographic properties of forward secrecy. I don't understand your claim that it is a strict requirement for every secure messaging system, including an email-like usecase.

>I'm sometimes in the mood to write long posts and comments explaining this stuff, but today, on the bottom of this old thread, if you're trying to make a point about PGP vs. Signal...

I already said several times I don't care about PGP. I feel like you're not really reading or responding to any of my arguments about why forward secrecy doesn't really help you much in most users' threat models or why it precludes various desirable features (of course, I could be wrong here, which is what I'm asking about). Thanks for your time anyway.

[tptacek]

Again: without forward secrecy, you can't delete old messages, because your adversary has already recorded them. The point is that a lack of forward secrecy creates a subtle limit to the security that can be achieved.

[reifyx]

Yes, that's exactly what we both agreed on 6 comments ago. The question is, is the ability to delete messages critical enough to require in any possible secure messaging solution, at the expense of features like email search, archiving, backup, and transfer-to-new-device?

[tptacek]

Yes, it obviously is.