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Changes
→Power Traces: Added capture information and sample trace
== Power Traces ==
After the bootloader is finished the decryption process, it executes a couple of distinct pieces of code:
* To apply the IV, it uses an XOR operation;
* To store the new IV, it copies the previous ciphertext into the IV array;
* It sends two bytes on the serial port;
* It checks the bytes of the signature one by one.
We should be able to recognize these four parts of the code in the power traces. Let's modify our capture routine to find them.
Re-run the capture script and change a few settings:
<ol>
<li> We'd like to skip over all of the decryption process. The source code around this point is:
<pre>
trigger_high();
aes256_decrypt_ecb(&ctx, tmp32); /* encrypting the data block */
trigger_low();
</pre>
so we can skip straight over the AES-256 function by triggering on a falling edge instead of a rising edge. Change this in the scope settings.
<li> We don't need as many samples now. Change the number of samples to 3000.
<li> If we decrypt multiple ciphertexts in a row, only the first one will use the secret IV - all of the others will use the previous ciphertext instead. To avoid this, we'll have to automatically reset the board.
<ol>
<li> In the ''General Settings'' tab, change the Auxiliary Module to ''Reset AVR/XMEGA via CW-Lite''.
<li> In the ''Aux Settings'' tab, change both delays to around 100 ms.
</ol>
<li> Capture one trace and make sure that everything works.
</ol>
If everything worked out, you should be able to see all of the code's features:
[[File:Tutorial-A5-Bonus-Trace-Notes.PNG]]
== Matching Power to Code ==
