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Comment by jagged-chisel | Hacker News Reader

Mallory sends a message, forged as if from Alice, to Bob. How can Bob determine that it came from Alice and wasn’t forged by Mallory?

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No, no, in these systems Alice and Bob both know a secret. Mallory doesn't know the secret, so, Mallory can't forge such a message.

However, Bob can't prove to the world "Alice sent me this message saying she hates cats!" because everybody knows Bob knows the same secret as Alice, so, that message could just as easily be made by Bob. Bob knows he didn't make it, and he knows the only other person who could was Alice, so he knows he's right - but Alice's cat hatred cannot be proved to others who don't just believe what Bob tells them about Alice.

Now it makes sense why Alice was sending me that kitten in a mixer video.

But seriously, in a case before a court or jury, wouldn't there be much more evidence? Down to your own lawyer sending a complete dump of your phone with all those Sandy-Hooks-conspiracies and hate messages to the opposing side?

Sometimes instead of a complete dump, you might mutually agree on a third party forensic lab to analyze your phone and only provide relevant details to the opposing side. Usually there's a few rounds of review where you can remove some details/records that are not relevant before the distilled records are sent to the opposing counsel.

Deniability is just that, the opportunity to deny. Will denying change what people believe? Well, maybe or maybe not. I'm sure you can think of your own examples without me annoying the moderators.

The idea is to use a MAC instead of a signature. As long as Alice isn't compromised and sharing her key with Mallory (which she could do even in the signature case), when Bob receives a message with a valid MAC on it, he knows that Alice authorized the message.

This is always a possibility but I'm guessing the happy path is the more likely believable scenario and could be verified OOB.

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