Custom Firmware Documentation Example not working as expected

[xsimplex]

Hi,

as I would like to test my own crypto implementation on the CW, I read the documentation and example on "Custom Firmware on ChipWhisperer Targets" (this file) and tried following the steps and code outlined there as a first example.
However, when I copy the code as-is (for a simpleserial-xor.c and a Makefile) and use a basic Python script to flash the firmware and attempt to capture a trace, I keep getting a Device reported error invalid comand (0x1) error.
I build the firmware with make -j PLATFORM=CW308_STM32F3.

Interestingly, this problem disappears and everything works as expected if I change the letters used for the commands in the simpleserial-xor.c file. For example, if I change the letters for sending a key and plaintext from k and p to i and j, everything works.

I suspect this is due to the automatic translation of p and k commands done in the SimpleSerial2.py simpleserial_write() function.

Is my code failing because simpleserial_write() is not the intended way to communicate with the target when using SimpleSerial version 2.1 and I should rather use send_cmd(), or is there a different reason why I'm seeing this behaviour?

The Python code I am using is this:

import chipwhisperer as cw

def connect_cw(target_type=cw.targets.SimpleSerial):
    scope = cw.scope()
    target = cw.target(scope, target_type)

    return scope, target

def disconnect_cw():
    scope.dis()
    target.dis()

def reset_target(scope):
    scope.io.nrst = 'low'
    time.sleep(0.05)
    scope.io.nrst = 'high_z'
    time.sleep(0.05)


if __name__ == '__main__':
    scope, target = connect_cw(cw.targets.SimpleSerial2)

    scope.default_setup()
    prog = cw.programmers.STM32FProgrammer

    reset_target(scope)

    cw.program_target(
        scope, prog,
        "./firmware/simpleserial-xor/simpleserial-xor-CW308_STM32F3.hex"
    )

    ktp = cw.ktp.Basic()
    key, text = ktp.next()

    target.flush()
    target.simpleserial_write('k', key)
    ack = target.simpleserial_wait_ack()

    scope.arm()

    target.simpleserial_write('p', text)
    ret = scope.capture()

    if ret:
        raise IOError("Timeout during capture.")
    else:
        print("Capture successful.")

    trace = scope.get_last_trace();
    print(trace)