Do you have a link to any reading about this? I'm curious.
I actually spoke with a person who works with a specific convict who wants to learn math and programming. They apparently ran into this problem. I don't have a reference, though.
Is this an argument of sneaking in a book on elliptic curves that tangentially talks about cryptography?
(I kid, of course.)
I'd imagine the job prospects for ex cons specializing in cryptography aren't that great anyway.