Let’s say you had an encrypted conversation with Bob yesterday. Today Bob accidentally posts his secret key on the internet. You still want your conversation to be private, right? So what can you do? You can use a cryptographic protocol with perfect forward secrecy (PFS). That will protect you against similar blunders by your conversation partners and even your own blunders of this type. Sounds pretty desirable, right?
Since the Snowden leaks, we know for sure that a variety of state run intelligence services record your communication – some of them with the explicit intent to decrypt it later when possible. And with many protocols you just need to break one key to be able to decipher many messages. PFS also protects against that: you usually need to break every single message individually.
So both blunder, as well as outright malice, pose a sizeable threat, which therefore we need to consider when designing our threat models.
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