it does not modify anything when just reading a ecm out. edit. I reread what you wrote. I see what your saying, just not how I would word it.
The ecm sends a seed, and you must reply with the correct key.

There are 256 seed/key combinations plus a few extra orphans as I understand it. There will be a tool like antus's released soon that will do 411's, and if the dev can find the time, many more GM ecms