SimpleSerial protocol

What will this section cover?

• What is the SimpleSerial protocol?
• How to we write source code?
• How do we trigger power traces to start and stop?

In order to run your own algorithms, they first have to be written as source code. The ChipWhisperer framework mostly uses C as the language for the source code of encryption algorithms. This means that any existing C implementation of an encryption algorithms can (most of the time) easily be used on the ChipWhisperer targets. There are a few things to take into account, however. The protocol to send data back and forth between the capture board and the target board, some recommendations when writing or choosing software, and how to start and stop power traces. These will all be covered here.

The SimpleSerial Protocol

In order to send data back and forth between the capture board and the target board and tell the target to start running its algorithm, a specific protocol is often used by ChipWhisperer. It is called the SimpleSerial protocol. It basically explains how to send data to the target (such as keys, plain texts, commands, etc.) and return data back from the target (such as cipher texts, hashes, errors, etc.). Some documentation on the C implementation of the algorithm can be found here. Although, if you use either the official simpleserial-base or the simpleserial-c-template, you almost don't need to worry about the specifics.

Writing / selecting your code

When writing your own algorithms for the ChipWhisperer, it is recommended to either not use the heap or use it sparingly. Most of the boards used as targets by ChipWhisperer don't have a lot of RAM and thus using it sparingly is preferred.

Also, when selecting existing implementations, it is recommended to use embedded hardware or heapless implementations. MBedTLS contains some of the more used encryption algorithms.

Starting / Stopping power traces

On the ChipWhisperer Lite ARM and on many other boards, we use the trigger_high function to start a power trace and the trigger_low to stop a power trace. Both of these functions are available from the hal.h, which is also used by the SimpleSerial protocol.

This means somewhere in your code it should look something like the following.

// ...

trigger_high();

encrypt(...);
// or
decrypt(...);

trigger_low();

// ...

This way your entire encryption or decryption will be captured by the power trace. You can also move the triggers to a specific point of the encryption or decryption, if you want to focus on a specific part of the algorithm.

This should give you most of the information needed to write and/or compile your own implementations of algorithms. The next section will contain some other resources which can help you in this process.