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Tutorial A5 Breaking AES-256 Bootloader

507 bytes added, 12:57, 5 November 2017
14th Round Key
##: [[File:Tutorial-A5-Plot-Traces.PNG|image]]
##: Notice that the traces are synchronized for the first 7000 samples, but become unsynchronized later. This fact will be important later in the tutorial.
# Set up the attack in the ''Attack'' settings tabscript:## Leave Make a copy of the Crypto Algorithm set to AES-128''attack_cpa. py'' script, call it something new (Remember that wesuch as ''''re applying the AES-128 attack to half of the AES-256 key!)## Change Adjust the Leakage Model model from ''SBox_output'' to ''HWInvSBox_output''. This is done by finding the following line in the script: AES Inv SBox Output##: <pre>from chipwhisperer.analyzer.attacks.models.AES128_8bit import AES128_8bit, First Round SBox_output</pre>##: and change that line to:##: <pre>from chipwhisperer.analyzer.attacks.models.AES128_8bit import AES128_8bit, InvBox_output</pre>## and then also change this further down where we set the leakage model:##: <pre>leak_model = AES128_8bit(DecInvSBox_output)''. </pre>## If you're finding the attack very slow, narrow down the attack a bit. Normally, this requires a bit of investigation to determine which ranges of the trace are important. Here, you can use the range from 2900 for 4200. The default settings will also work fine!To do this adjust the following line to look as follows:##: [[File:Tutorial-A5-Hardware-Model<pre>attack.PNG|image]]setPointRange((2900, 4200))</pre>
# Note that we do ''not'' know the secret encryption key, so we cannot highlight the correct key automatically. If you want to fix this, the ''Results'' settings tab has a Highlighted Key setting. Change this to Override mode and enter the key <code>ea 79 79 20 c8 71 44 7d 46 62 5f 51 85 c1 3b cb</code>.
# Finally, run the attack by switching to the ''Results Table'' tab and then hitting the ''AttackRun'' buttonwhile your script is selected.#: [[File:A5_run_script_round14.png|400px]]
There are a few ways to check the results of the attack. First, the results table will show the best guesses for each subkey. With the highlight override enabled, the red bytes should be the best guesses for every single subkey:
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