Key exchange: Diffie–Hellman

Symmetric encryption needs both sides to share a key — but how do they agree on one over a wire an eavesdropper is watching? Diffie–Hellman key exchange is the answer, and it is the heart of every modern TLS handshake.

You build the exchange, watch both parties arrive at the same secret, and then mount the canonical attack: a man in the middle against unauthenticated DH — the reason TLS needs certificates (the next pillar, P53).