I'm not going to go too deep into how these work, but if you want to get a sense yourself: start by looking up linear congruential generators, which is a really simple function that can make a block of data "cycle" in a random non-repeating way.
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Now you can still screw up with AEAD. If you re-use the same IV, that's bad!! There are attempts to make this better, my colleague Shay has been working on a cool scheme called SIV, and it adds a measure of protection against that too.
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If you do use unique IVs, modern encryption is really robust. In general, you could publish some encrypted text in the New York Times, and no-one will be able to crack it. This Is true even if /some/ of the text is known. For example ...
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In internet protocols a lot of the text is known, a HTTP server always responds the same way and the first few bytes are known and totally guessable. This doesn't matter at all - doesn't help an attacker figure anything else out even one bit. We've come a long way from WWII.
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But there are attacks that do work! If you're sending this data over a network, and someone can see the timing and size of message. This opens us up to traffic analysis.pic.twitter.com/8qeI9A3Ozp
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Let's look at length first. O.k. so the length is obviously not hidden. That's fine if you're trying to protect your password or credit card number in the middle of a response. No big deal. But it does mean that someone might be able to fingerprint the content you're sending.
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Simple example: if you send a gif over a messaging app, if the size of that gif is unique, someone in the middle can probably guess what gif you just sent. There are more sophisticated versions of this for Google Maps, Netflix, WikiPedia, and so on.
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The way we protect against this is to "pad" messages, to make large numbers of messages appear to be the same size no matter what. Military grade network encryption actually pads all traffic all the time, so it's always the same!
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Another problem with length is that if you're using compression, and let attackers control any of the content on a page that a user sees, that can let the attackers figure out even small secrets. Look up the "CRIME" attacks. It's awesome, and scary.
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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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