1976

Public Key Cryptography

Ralph Merkle (b. 1952), Whitfield Diffie (b. 1944), Martin Edward Hellman (b. 1945)

For more than 2,000 years, cryptography’s Achilles’ heel was the need for two parties to privately, in person, agree upon a secret key (a string of letters and numbers) before they could send coded messages back and forth. This limitation didn’t pose a problem for diplomats and generals, who received keys in person before heading off on their assignments. But with the birth of email in 1971, computer scientists realized that the only way for email messages to be secure would be for them to be encrypted.

In 1974, Ralph Merkle, then a senior at University of California, Berkeley, came up with an inelegant, yet groundbreaking solution to this problem as part of a class project. Merkle’s approach let two people agree upon a cryptographic key by first exchanging millions of cryptographic puzzles over the internet. Merkle’s professor didn’t understand the significance of the solution, so Merkle dropped the course. Merkle wrote up his idea and submitted it to Communications of the ACM, then the premiere journal of computer science; the paper was rejected with the comment, “Experience shows that it is extremely dangerous to transmit key information in the clear.”

The following year, at Stanford in an embryonic cryptography research group headed by professor Martin Hellman, student Whitfield Diffie pursued an idea similar to Merkle’s but with a more efficient solution based on number theory. Now called the Diffie-Hellman key exchange, the system allows two parties to exchange specially crafted numbers from which each can derive the same encryption key. An outside observer—perhaps a wiretapper—cannot derive the key. The work was presented at the National Computer Conference in June 1976.

As for Merkle, his paper was finally published by Communications in 1978—complete with an apologetic note from the journal’s editor.

Diffie and Hellman were awarded the A.M. Turing Award in 2015 for their key-exchange algorithm. Merkle went on to invent cryptographic hashing and hash trees—the basis of cryptocurrencies like Bitcoin. He won the IEEE Hamming Medal in 2010.

Public key cryptography allows two parties to exchange specially crafted numbers from which each can derive the same encryption key.