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For the rest of this tutorial, we'll be focusing on the STM32F3, which is the microcontroller on the CW303 Arm target (though other targets should demonstrate the same principles). Since the STM32F3 is an Arm Cortex M4 device, we'll need to refer to the Cortex M4 Instruction Set and the Cortex M4 Instruction Set Summary.
The first thing we'll do is replace the <code>nop</code> instructions, since from it's documentation page we can see the processor may not execute them. Instead, let's add some <code>add.w</code> (which is the 32 bit wide version of the add instruction) instructions. We'll be doing this since the <code>mul</code> instruction is always 32 bits wide and the 16 bit thumb instruction has a different power profile than the 32 bit Arm instruction. From the earlier links, we can see that both add and mul take 1 cycle each to complete.
Now hit the ''Run 1'' [[File:Capture One Buttonwe should have 10 <code>add.PNG|image]] button w</code> instructions and capture a single trace. It's recommended that you only capture a single trace. You may also want to change the colour of the next trace to make differentiating them easier10 <code>mul</code> instructions:<syntaxhighlight lang="c">trigger_high();
asm volatile(
"mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t" "mul add.w r0,r1r0" "\n\t""mul add.w r0,r1r0" "\n\t"
::
);
asm volatile(
"nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t""nopmul r0, r0" "\n\t"
::
);
trigger_low();</syntaxhighlight>Now hit the ''Run 1'' [[File:B2 cap2Capture One Button.PNG|frameless|1149x1149pximage]]button and capture a single trace. You should now have something that looks like this:
== Clock Phase Adjustment ==
== Conclusion ==
In this tutorial you have learned how power analysis can tell you the operations being performed on a microcontroller. In future work we will move towards using this for breaking various forms of security on devices. In particular, [[Tutorial B3-1 Timing Analysis with Power for Password Bypass]] will examine how we can use this information to exploit a password check.
== Links ==
