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→Breaking AES-CBC Encryption with unknown I.V.
If you've performed a standard CPA attack, you'll realize the problem with attacking AES-CBC is we don't directly control the input, which we call <math>PT</math>. Instead it's XORd with some unknown bytes (the AES-CBC ciphertext output).
But if we are always attacking the same block (that is, we reset the AES state to initial by say resetting the device, and rerun the algorithm up to the first block), the unknown bytes are constant. As it turns out this is a pretty easy problem to solve. The first step is to perform a standard CPA attack. The only issue is we won't recover the actual encryption key used <math>k</math>, instead we recover <math>k \oplus CBC_{m-1} \oplus PT</math>, since we basically roll all the constant inputs into what we call a `modified key'. Note <math>CBC_{m-1}</math> is the output of the previous-block AES-CBC ciphertext.
In what might seem like magic, we can use this modified key to directly determine the second-round key (the true key). This was originally presented by J. Jaffe in [https://www.iacr.org/archive/ches2007/47270001/47270001.pdf A First-Order DPA Attack Against AES in Counter Mode with Unknown Initial Counter]. The reason this works is if you remember we recovered <math>k' = k \oplus CBC_{m-1}</math>. In the AES algorithm the first thing we do is the AddRoundKey, which is: