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PFS will ensure the same key will not be generated again, so forcing a new diffie-hellman key exchange. This would ensure if a hacker\\criminal was to compromise a private key, they would only be able to access data in transit protected by that key and not any future data, as future data would not be associated with that compromised key.
Let's look at how key exchange works in the common non-ephemeral case. Instead of giving a practical example using, say, Diffie-Hellman, I'll give a generalized example where the math is simple:
Alice (client) wants to talk to Bob (server).
Bob has a private key X and a public key Y. X is secret, Y is public.
Alice generates a large, random integer M.
Alice encrypts M using Y and sends Y(M) to Bob.
Bob decrypts Y(M) using X, yielding M.
Both Alice and Bob now have M and use it as the key to whatever cipher they agreed to use for the SSL session—for example, AES.
Pretty simple, right? The problem, of course, is that if anyone ever finds out X, every single communication is compromised: X lets an attacker decrypt Y(M), yielding M. Let's look at the PFS version of this scenario:
Alice (client) wants to talk to Bob (server).
Bob generates a new set of public and private keys, Y' and X'.
Bob sends Y' to Alice.
Alice generates a large, random integer M.
Alice encrypts M using Y' and sends Y'(M) to Bob.
Bob decrypts Y'(M) using X', yielding M.
Both Alice and Bob now have M and use it as the key to whatever cipher they agreed to use for the SSL session—for example, AES.
(X and Y are still used to validate identity; I'm leaving that out.)
In this second example, X isn't used to create the shared secret, so even if X becomes compromised, M is undiscoverable. But you've just pushed the problem to X', you might say. What if X' becomes known? But that's the genius, I say. Assuming X' is never reused and never stored, the only way to obtain X' is if the adversary has access to the host's memory at the time of the communication. If your adversary has such physical access, then encryption of any sort isn't going to do you any good. Moreover, even if X' were somehow compromised, it would only reveal this particular communication.
That's PFS.
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