Explore the practical applications of number theory in computer security through this 81-minute lecture from MIT's Mathematics for Computer Science course. Discover how fundamental mathematical concepts like greatest common divisors (GCDs) and modular arithmetic form the backbone of modern cryptographic systems. Learn why these seemingly abstract mathematical tools are essential for protecting digital information and securing computer communications. Understand the connection between theoretical mathematics and real-world cybersecurity applications, building upon previous number theory foundations to see their critical importance in computer science. Gain insights into how mathematical principles translate into practical security measures that protect data in our digital world.