Public-key cryptography: Diffie–Hellman and RSA

Every cipher before 1976 had the same unsolved problem: before two people could talk securely, they had to share a key securely. Governments used couriers. In November 1976 Whitfield Diffie and Martin Hellman, at Stanford, published “New Directions in Cryptography”, showing that two parties could agree a secret over a channel everyone could hear.

A year later at MIT, Ron Rivest, Adi Shamir and Leonard Adleman found a way to encrypt with one key and decrypt with another, published in 1978. Only in 1997 did GCHQ reveal that its own researchers had reached equivalent ideas years earlier, Clifford Cocks's version of RSA dating from 1973, and kept them classified.