How does CW Lite Measure port actually work?

#1

I am collecting power traces using the ChipWhisperer Lite and the capture software GUI. What exactly am I looking at? Is it showing me the current measurement or the power (I*V) measurement?

The second thing I am confused about, is how does the software know how to give me the current measurement or power measurement when all I give it is the voltage measurement through the “Measure” SMA connector? If I am doing this on a external target and I put a shunt resistor on the external target, does the ChipWhisperer software need to know the resistor value and the supply voltage of the target device?

Clarification on these questions would be greatly appreciated. I looked over the documentation on the Wiki, but I was not able to find the answers to my questions.

Thanks.

#2

Hi,

CW measures a voltage; depending on the target and where the measurement is taken, it may be the voltage drop across a shunt resistor, or between either side of the shunt resistor and ground (for example the CW305 wiki explains the options for that target).

The key point is that side-channel attacks don’t care about absolute power measurements; relative power differences is all that matters. This is why the value of the shunt resistor doesn’t matter to the Capture software. Correlations and statistics are the magic that makes the attacks work.

Hope this helps,
Jean-Pierre

#3

Thanks for the response Jean-Pierre. That is helpful to know.

Could you (or anyone) explain the benefits of measuring power at different locations? For example in the options below, what are the pros and cons for each measurement location and why might I choose one over the other?

  1. VCC --> [MEASURE PORT] --> Shunt Resistor --> Device --> GND
  2. VCC --> Shunt Resistor --> [MEASURE PORT] --> Device --> GND
  3. VCC --> Device --> [MEASURE PORT] --> Shunt Resistor --> GND

It seems like the XMega target board that comes with the ChipWhisperer Lite uses option 2 if I am not mistaken.

Any insight would be appreciated. Thanks.