[upofadown]

Forward secrecy does not provide any value against cryptography compromise. Quite the opposite as it depends on the security of the cryptography over the long term to insure old messages stay inaccessible after the key is forgotten.

Forward secrecy addresses this specific attack:

* Someone builds a archive of your encrypted messages, possibly without your knowledge or consent.

* That someone then gets access to your secret key material.

* They can then decrypt their archive.

The session keys are exchanged by the asymmetrical systems that the imagined quantum computer would be able to break. So the attacker gets the session keys directly. So for, say, signal, they only have to break a new key exchange which doesn't happen all that often. They can just run the hash ratchet after that. Even for TLS that does a new session key per connection, that connection might last a fair time. The 10 min can be spread over multiple connections for this proposal. We are hardly talking about a massive increase of difficulty.

[bawolff]

I mean, it depends a little bit on what your threat model is. If it takes a week to break a key, and you have hundreds of thousands of tls sessions without knowing which is the relavent one, it is definitely something. But yeah it seems like it would quickly become a minor hurdle once real quantum computers become a thing and presumably have their own moore's law.