Change update function to use decryption rather than encryption. #173
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Signed-off-by: Tom Kaitchuck <tom.kaitchuck@emc.com>
Signed-off-by: Tom Kaitchuck <tom.kaitchuck@emc.com>
Signed-off-by: Tom Kaitchuck <tom.kaitchuck@emc.com>
Signed-off-by: Tom Kaitchuck <tom.kaitchuck@emc.com>
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Because the AES decrypt and encrypt operations are symmetric, it seems like it should not matter which is used. However in the context of thinking about how one could theoretically extend attacks like #163 to be more general, there is a critical difference. If one attempts to cancel a differential after an AES round, it changes the canceling change from being all in one word vs one byte in each of 4 words. This matters in terms of how it interacts with the addition on the other half of the state. The spread out update is harder for an attacker to control and subsequently cancel, both because of carries and because it means operating on both 64byte halves of the added state at the same time.