Mathematics
The Mathematics of Factorization and the RSA Cryptosystem
Quick fact
RSA encryption works because it's easy to multiply two large prime numbers, but factoring the resulting product is computationally infeasible for sufficiently large primes—the security of the system rests on this asymmetry.
Why this is interesting
You rely on a simple math trick every time you send a secure message online—but what makes that trick so hard to break? The same multiplication you learned in grade school is the key to modern cryptography.