I think you're missing a key point. Yes, you can test AES at one moment in time. But when you're talking backdoors, they aren't necessarily active all the time.

I think you're missing a key point. Yes, you can test AES at one moment in time. But when you're talking backdoors, they aren't necessarily active all the time. Backdoors could wait for a specific trigger from outside input. I've been referring to these as "sleeper cell" attacks. For example, you could have a malicious memory module that alters its own contents when it sees a specific value and exploits the system in place.