How can I combine my interests for pure mathematics and computer science in college?
Math is heavily used in both computer security, machine learning and AI.


Computer Security, such as encryption and related fields is all about finding math that makes it impossible to retrieve information without being the intended recipient.  With quantum computing possibility being a reality within the next 10 - 20 years there is a lot of work needed to come up with new ways of securing data.  Number theory is very relevant here.
Machine learning is heavily based on statistics.  new algo's with better capability of prediction is always relevant, and a very hot topic.
AI, while strictly an area of machine learning, are typically resolving to other areas of of learning than typical stats.  Any advancements in this areas will be done by people with strong math background.