There are some algorithms where there's no obvious way to back door it, but it's always conceivable -- the person designing it may know some clever mathematics

There are some algorithms where there's no obvious way to back door it, but it's always conceivable -- the person designing it may know some clever mathematics that you don't. With Dual_EC_DRBG, everyone knew that it could be back doored. It's not some guess, or "maybe it could have". It was obviously designed to be back doored. It should have been called "NSA_BACKDOOR_RNG", because that's literally what it is. And yes, all organisations that are not under the thumb of the US Government laughed at the transparent attempt to introduce a back door and rejected Dual_EC_DRBG. Only US-based companies use it, which ought to give you a hint.