A “golden key” is just another, more pleasant, word for a backdoor—something that allows people access to your data without going through you directly. This backdoor would, by design, allow Apple and Google to view your password-protected files if they received a subpoena or some other government directive. You’d pick your own password for when you needed your data, but the companies would also get one, of their choosing. With it, they could open any of your docs: your photos, your messages, your diary, whatever.