What do you think they use the math for?

My point is that mathematicians are not necessarily or even probably a good fit for intelligence analysis ("spying"). I wouldn't describe the stuff NSA mathematicians ostensibly get up to as spying. See this for example: https://www.nsa.gov/careers/career_fields/mathematics.shtml

well, the scary part is that those maths purposes are being deployed for surveillance reasons. William Binney comes to mind