Here's why! If we go back to my XOR example:https://twitter.com/colmmacc/status/1101572361365516288 …
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So in my example 3 was the plaintext, and 7 was the key. 3 ^ 7 = 4, so that's the encrypted text. Let's say we encrypted 5 too. 5 ^ 7 = 2. Well it turns out that XORing the encrypted text is just like XORign the plaintext. 3 ^ 5 = 6, and 4 ^ 2 = 6.
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So re-using the same stream reveals the "difference" between the plain-texts. There's enough information in there to make guesses at what the plaintext is. O.k. so the bug is bad, and has to be fixed. Real world things could hit this: e.g. people might be encrypting a snapshot.
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But here's what interesting: the fix also breaks anything that's already encrypted! if you stored a > 256GiB encrypted image using Salsa20 ... it will now partially decrypt to garbage.
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The normal defense against this in cryptography is the MAC. A MAC is basically a keyed checksum of the data; if the data ever changes, even by one bit, the MAC should fail to validate. But in this case, an encrypt-then-MAC style MAC will be absolutely valid!
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To detect this kind of corruption: you'd need to have a MAC of the plaintext. MAC-then-encrypt is usually considered a bad practice, because it leaves the cryptography open to side-channel experimentation by attackers, but in this case you absolutely need it!
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So really you need MAC-then-encrypt-then-also-MAC! I call this scheme Combined Online Linear Message MAC And Corruption Check (it's ok to shorten that to COLMMACC).
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Replying to @colmmacc
EtM is fine. If you’re worried about implementation bugs, you can combine (not cascade!) two stream ciphers. Ditto for the MAC. These can even run simultaneously on different cores. For very large messages, the synchronization overhead will be negligible anyway.
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Tests should be run on individual platforms the software will be used on. But in practice this is rarely the case. Especially since people now just use pre-built packages, containers, etc.
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