Finding the prime factors of large numbers is hard. Cryptography is almost exclusively about picking two Large Primes, Multiplying them, and then using that very large, very hard to factor number as a key.
This is overly simple, but it is the very core of every modern crypto. So understanding it is the best place to start.
This is overly simple, but it is the very core of every modern crypto. So understanding it is the best place to start.