Capstone: Modern Cryptography

In 1976, Whitfield Diffie and Martin Hellman asked an impossible-sounding question: can two people who've never met derive a shared secret over a public channel full of eavesdroppers? They showed it can be done — using the discrete logarithm problem in the very modular arithmetic you've learned. That single idea birthed public-key cryptography: RSA, elliptic curves, and now the post-quantum schemes preparing for a world where quantum computers break the old guard. Every concept in this branch converges here.