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This tutorial is an add-on to [[Tutorial A5 Breaking AES-256 Bootloader]]. It continues working on the same firmware, showing how to obtain the hidden IV and signature in the bootloader. '''It is not possible to do this bonus tutorial without first completing the regular tutorial''', so please finish Tutorial A5 first.

In this tutorial, we have the luxury of seeing the source code of the bootloader. This is generally not something we would have access to in the real world, so we'll try not to use it to cheat. (Peeking at <code>supersecret.h</code> counts as cheating.) Instead, we'll use the source to help us identify important parts of the power traces.

Inside the bootloader's main loop, it does three tasks that we're interested in:

* it decrypts the incoming ciphertext;

We'll use both of the source files throughout the tutorial.

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;

Let's grab a lot of traces so that we don't have to come back later. Save the project somewhere memorable, set up the capture routine to record 1000 traces, hit ''Capture Many'', and grab a coffee.

We need to find the IV before we can look at the signature, so the first half of the attack will look at the IV bytes.

The bootloader applies the IV to the AES decryption result by calculating

This is effectively a DPA attack on a single bit of the IV. We can repeat this attack 128 times to recover the entire IV.

Unfortunately, we can't use the ChipWhisperer Analyzer to attack this XOR function. Instead, we'll write our own Python code. One thing that we ''don't'' need to do is write our own AES-256 implementation: there's some perfectly fine code in the PyCrypto library. [https://pypi.python.org/pypi/pycrypto Install PyCrypto] and make sure you can use its functions:

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for i in range(numTraces):

ct = str(bytearray(textin[i]))

d = [bytearray(pt)[i] for i in range(16)]

dr.append(d)

[[File:Tutorial-A5-Bonus-Diff-0.PNG]]

[[File:A5-Bonus-Diff-0-Zoom.PNG]]

These peaks are about 60 samples later (or 15 cycles, since we're using an ADC clock that's 4 times faster than the microcontroller). Also, most of these peaks are upside down! This is a pretty clear indicator that we can find the IV bits from these differential traces. The only thing that we can't tell is the polarity of these signals; there's no way to tell if right-side-up peaks indicate a bit that's set or cleared. However, that means we can narrow down the IV to two possibilities, which is a lot better than <math>2^{128}</math>.

The best way to attack the IV would be to repeat the 1-bit conceptual attack for each of the bits. Try to do this yourself! (Really!) If you're stuck, here are a few hints to get you going:

* One easy way of looping through the bits is by using two nested loops, like this:

If you're ''really, really'' stuck, there's a working attack in [[#Appendix A: IV Attack Script]]. You should find that the secret IV is <code>C1 25 68 DF E7 D3 19 DA 10 E2 41 71 33 B0 EB 3C</code>.

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