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Many of the tutorials in this wiki discuss attacks on AES-128 encryption. It turns out that its big brother, AES-256, can be attacked by extending the same attacks. This page discusses AES-256 and how to reuse an AES-128 attack to obtain the key.
= The AES-256 Algorithm =
In AES-128, we used the following steps to encrypt 16 bytes of plaintext:
# Use a 16 byte key to generate a key schedule, which is 176 bytes long (11 words made up of 16 bytes).
Note that this implementation chooses to expand the key during the decryption process. This order of events isn't a big deal to us - the <code>subBytes()</code> operation will still be visible in a power trace.
= Attacking AES-256 Decryption =
From our experience with AES-128, we know that the AES substitution boxes are a good attack point. These boxes are non-linear, so we don't have any problems with nearly-correct key guesses. Since there are S-boxes operating on 1 byte each, we should be able to recover 16 bytes from the <code>SubBytes()</code> function. In the decryption code, this part of the algorithm corresponds to the first three lines: