In November 1976, Whitfield “Whit” Diffie and Martin Hellman published a paper entitled “New Directions in Cryptography” in IEEE Transactions in Information Theory. The paper demonstrated that it was possible to securely exchange information over non-secure channels, which they called public key cryptography.

Since Ellis, Cocks, and Williamson’s work on “non-secret” encryption was kept secret by the GCHQ, prior to Diffie and Hellman’s publication it was believed that encrypted communication was only possible by exchanging a cryptographic key in advance. Because of this limitation, cryptography was limited to important communications—diplomatic, military—and outside of the reach of civilians.

Prior to 1976, secure communication required setting cryptographic technologies with identical cryptographic keys (such as with the famous Enigma, Purple, and SIGABA machines). This was an onerous and risky process that needed to be repeated often (it is critical to change cryptographic keys frequently to maintain security). This process was complicated and labour intensive, requiring trusted personnel to transport codes on sheets of paper or electro-mechanical “fill” devices. Even the state-of-the-art Arpanet, which later became the Internet, encrypted communication required the careful coordination of cryptographic keys across distant “Private Line Interfaces.” As the number of nodes in the secure network increased, so too did the labour of exchanging keys.

Diffie and Hellman had invented, for the second time (unbeknownst to them), a way to encrypt communication over insecure channels without the prior exchange of keys. This process required generating a special “split” key that contained mathematically-linked parts. The “public” part could be freely exchanged on insecure channels, and when combined with the “private” part in a complicated back and forth exchange (later known as the Diffie-Hellman key exchange), ad hoc encrypted communication over insecure channels was possible. In short, their invention provided the basis for secure transactions on the Internet, and set in motion a fundamentally new way to communicate, to organize, and to socialize.

While Ellis, Cocks, and Williamson were inventing “non-secret” encryption at GCHQ, Diffie had become inspired by John McCarthy’s investigation of cryptography for large computer networks (at the behest of Larry Roberts at IPTO). This spark of inspiration led Diffie to spend the next few years traveling across the US in search of a solution. Diffie moved between archives, universities, and colleagues to discover everything he could about cryptography. He read David Khan’s famous survey of cryptography, The Codebreakers, met the famous computer scientist Donald Knuth, and finally, in 1974 travelled to Stanford to meet with Martin “Marty” Hellman, a professor of electronic engineering (and former employee of IBM) on the referral of friend and colleague Alan Konheim.

Diffie and Hellman started working together immediately, and drafted an early version of “New Directions in Cryptography.” Some of this early work was presented to an information theory workshop in 1975, and was then revised to substantively address similar work on cryptography also being developed by Ralph Merkle. It was submitted to IEEE Transactions in Information Theory in June, 1976.

On the eve of the Arpanet that would soon become the Internet, this idea was a revolution in cryptography and soon became the backbone of digital communication. They called their invention “public key” cryptography, and it would soon enable ecommerce, global banking and finance, private personal communication, and—now that it had escaped the confines of the intelligence community—all of the ills associated with the dark corners of today’s digital world.

Diffie and Hellman had developed public key cryptography, for the second time, in the complex context of military projects, academic associates, and government funding. This time, however, the idea was in the wild, and would soon be pursued by young computer scientists, Ronald Rivest, Adi Shamir, and Leonard Adleman, who were quick to see the commercial possibilities for public key cryptography on the emerging Internet.

At this point, Alice and Bob did not yet exist. In their paper, as was the tradition in cryptology research, Diffie and Hellman referred to the communicating parties as “A” and “B.” A and B were largely featureless—presumptively male, symbolic, and abstract.

Diffie and Hellman later won the 2015 Turing Award (the “Nobel prize” of computer science) for their work in the field. Their pioneering work has stood the test of time, and has been capable of adapting to and resisting challenges.