To decrypt, we check the MAC first by generating it again and making sure they are the same, and then we decrypt the output. Internally there are differences between how HMAC, GCM and Poly1305 generate these signatures, but you don't need to worry about that.
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I said the other problem is timing. Obviously the timing of each message is public, but is that a big deal? It can be! For example, if you send a message for every use keystroke, it's trivial to figure out what they're typing through timing analysis. WOW.
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Another example is VOIP. If your call app only sends data when people are speaking, but not during the silences, that's still enough to guess about 70% of English-language speech. Just from the silences! Scary cool.
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These examples underling: even when you use encryption algorithms and schemes we've been perfecting for about 80 years, there's still some gaps you can walk into and break the security. Which is why this stuff is worth knowing!
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Anyway, that's the level I'm going to stick at for now, but we've covered a lot of ground. If you've finished this thread, thank you! But also you should now have some kind of better understanding of what's going on, and what to be wary of. Feel free to AMA.
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Oh the truth table for XOR is wrong. I guess it's more of a lies table. Should be: a | b | c 0 | 0 | 0 1 | 0 | 1 0 | 1 | 1 1 | 1 | 0
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End of conversation
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This thread is amazing, if even for just this tweet. Colm, quick question while I search for the answer online: is the CRIME vulnerability viable because of the use of compression on messages that include cipher texts, or because TLS encrypts compressed data?
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CRIME is viable when you compress attacker-controlled "guess" data AND victim-valuable secret data. Both in the plaintext that's compressed, then encrypted. If the attackers guess overlaps with the secret, the overall size will be smaller, and the size isn't hidden by encryption.
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