Before most of our time, but my understanding is that the past restrictions on cryptography research and export were reasonably effective?

Yes they were disliked and probably with good reason, but just mentioning it as a counterpoint that perhaps it is possible. You could have made a similar argument that anyone with pen and paper could do cryptography

Personally I think there's no way to be sure other countries aren't doing it, so perhaps it will continue in some government controlled way