Trapdoor is an older term that means the same thing as "backdoor". I think this author chose to use it in order to distinguish between the modern colloquial mea

Trapdoor is an older term that means the same thing as "backdoor". I think this author chose to use it in order to distinguish between the modern colloquial meaning of "backdoor" as "a method for an attack to eavesdrop on communication", and the more abstract meaning of a correctness vulnerability in an allegedly secure counting algorithm.