For most of history, encryption had a chicken-and-egg problem: to send a secret message you first had to share a secret key — but sharing that key securely was itself the hard part. In 1976 Whitfield Diffie and Martin Hellman published New Directions in Cryptography and broke the deadlock.
Why it mattered
Their idea was public-key cryptography: each person has a pair of keys — a public one they hand out freely and a private one they keep. Anything scrambled with your public key can only be unscrambled with your private key, so two people who have never met can communicate securely over a completely open channel. The following year, RSA turned the concept into a practical algorithm.
This is the mathematics behind the padlock in your browser. Every time you see HTTPS, log into a bank, or send an encrypted message, public-key cryptography is quietly setting up the secure connection and proving the site is who it claims to be. Without it, there would be no safe online payments, no digital signatures — and, decades later, no cryptocurrencies.