Considering that most people couldn't even read regular text back then it was definitely pretty sound, but that's not really my point. We do? Got any reference

Considering that most people couldn't even read regular text back then it was definitely pretty sound, but that's not really my point. We do? Got any references for that? Take RSA -- it's not even proven that breaking it is equivalent with integer factorization. Even if it was, there are no proof that integer factorization needs to be time consuming at all, there might very well exist an algorithm with polynomial complexity.