Motorola has a patent—asserted in the Microsoft case—on a specific number used as an error correcting code.

No. To patent something you have to disclose it, so patenting a secret key would totally defeat the purpose.

Well, I remember that being part of the contention. Sony said, "we've patented it". Everyone else said, "how can you have patented it, when you won't tell anyone what it is!?". Or something along those lines.

If you can find the patent number that would make it easier to formulate a cogent response.

Some googling suggests it was not a patent they claimed, but that it was an "illegal number". Very possible I confused the two.

https://en.wikipedia.org/wiki/Illegal_number

I think at the time what I found ridiculous was that you were prohibited from "having" the number, but of course nobody could say what number it was you weren't allowed to have... seemed like something that belonged in Dr. Strangelove.