Difference between revisions of "Tutorial B2 Viewing Instruction Power Differences"

Prerequisites

You should have already completed Tutorial_B1_Building_a_SimpleSerial_Project. This tutorial assumes you are capable of building a new AVR/XMEGA code, programming the code, and connecting to the ChipWhisperer.

Setting Up the Example

1. Copy the directory simpleserial-base which is found at chipwhisperer\hardware\victims\firmware\ of the chipwhisperer release to a new directory called simpleserial-base-lab2. You must keep it in the same directory, as it will reference other files within that directory for the build process.

   If you just completed Tutorial_B1_Building_a_SimpleSerial_Project, you can simply reuse that code (this builds upon it).

   At this point we want to modify the system to perform a number of operations. We won't actually use the input data. To do so, open the file simpleserial-base.c with a text editor such as Programmer's Notepad (which ships with WinAVR).

2. Find the following code block towards the end of the file, which may look different if you just completed Tutorial_B1_Building_a_SimpleSerial_Project.

   /**********************************
    * Start user-specific code here. */
   trigger_high();
   
   //16 hex bytes held in 'pt' were sent
   //from the computer. Store your response
   //back into 'pt', which will send 16 bytes
   //back to computer. Can ignore of course if
   //not needed
   
   trigger_low();
   /* End user-specific code here. *
    ********************************/
   

3. Modify it to do some work with no-ops and multiplication instructions:

   /**********************************
    * Start user-specific code here. */
   trigger_high();
   
   //16 hex bytes held in 'pt' were sent
   //from the computer. Store your response
   //back into 'pt', which will send 16 bytes
   //back to computer. Can ignore of course if
   //not needed
   
   asm volatile(
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   "nop"       "\n\t"
   ::
   );
   
   asm volatile(
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"          
   "mul r0,r1" "\n\t"
   "mul r0,r1" "\n\t"
   ::
   );
   
   trigger_low();
   /* End user-specific code here. *
    ********************************/
   

4. Change the terminal to the directory with your source, and run make PLATFORM=CW303 to build the system. Remember you can press the up arrow on the keyboard to get recently typed commands in most OSes:

   make PLATFORM=CW303

   Which should have the following output, you will want to check the build platform is what you expect:

   ...Bunch of lines removed...
   Creating Extended Listing: simpleserial-base.lss
   avr-objdump -h -S -z simpleserial-base.elf > simpleserial-base.lss
   
   Creating Symbol Table: simpleserial-base.sym
   avr-nm -n simpleserial-base.elf > simpleserial-base.sym
   
   Size after:
   AVR Memory Usage
   ----------------
   Device: atxmega128d3
   
   Program:    1568 bytes (1.1% Full)
   (.text + .data + .bootloader)
   
   Data:        224 bytes (2.7% Full)
   (.data + .bss + .noinit)
   
   
   Built for platform CW-Lite XMEGA
   
   -------- end --------

5. Following the instructions given in Tutorial_B1_Building_a_SimpleSerial_Project, program the AVR or XMEGA with your new code. Note you __do not__ need to close the programming window. If you will be doing frequent modifications to the source code, this can simplify your life since you only need to hit the Program button in AVRStudio to download new code.
6. Ensure the hardware is setup as in Tutorial_B1_Building_a_SimpleSerial_Project. If using the CW1002, ensure the SMA cable is also connected as described in the previous tutorial.

Capturing Power Traces

The basic steps to connect to the ChipWhisperer device are described in Tutorial_B1_Building_a_SimpleSerial_Project. They are repeated here as well, however see Tutorial_B1_Building_a_SimpleSerial_Project for pictures & mode details.

1. Start ChipWhisperer-Capture
2. Under the Python Console tab, find the connect_cwlite_simpleserial.py script and double-click.
3. Check there are no errors on the connection.
4. Under the Python Console tab, find the setup_cwlite_xmega.py script and double-click.
5. Both the Target & Scope should switch to CON and be green circles.
6. Open the status monitor (Tools > Encryption Status Monitor).
7. Hit the Run 1 button. You may have to hit it a few times, as the very first serial data is often lost. You should see data populate in the Text Out field of the monitor window. The Text In and Text Out aren't actually used in this example, so you can close the Monitor dialog.

At this point you've completed the same amount of information as the previous tutorial. The following section describes how to setup the analog capture hardware, which is new (to you). The following is entirely done in the Scope Settings tab:

8. The ADC Freq should show 29.5 MHz (which is 4x 7.37 MHz), and the DCM Locked checkbox __MUST__ be checked. If the DCM Locked checkbox is NOT checked, try hitting the Reset ADC DCM button again.

9. At this point you can hit the Capture 1 button, and see if the system works! You should end up with a window looking like this:

   

   Whilst there is a waveform, you need to adjust the capture settings. There are two main settings of importance, the analog gain and number of samples to capture.

16. Under Gain Setting set the Mode to high. Increase the Gain Setting to about 25. You'll be able to adjust this further during experimentations, you may need to increase this depending on your hardware and target device.
17. Under Trigger Setup set the Total Samples to 500.
18. Try a few more Capture 1 traces, and you should see a 'zoomed-in' waveform.

Modifying the Target

Clock Phase Adjustment

A final area of interest is the clock phase adjustment. The clock phase adjustment is used to shift the ADC sample clock from the actual device clock by small amounts. This will affect the appearance of the captured waveform, and in more advanced methods is used to improve the measurement.

The phase adjustment is found under the Phase Adjust option of the ADC Clock setting:

To see the effect this has, first consider an image of the power measured by a regular oscilloscope (at 1.25GS/s):

And the resulting waveforms for a variety of different phase shift settings:

The specifics of the capture are highly dependent on each ChipWhisperer board & target platform. The phase shift allows customization of the capture waveform for optimum performance, however what constitutes 'optimum performance' is highly dependent on the specifics of your algorithm.

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.

Links