An Old Mystery Solved: Project C-43 and Public Key Encryption

For most of history, it was believed that the only way a message could be encrypted was if the sender and the receiver shared the secret of srambling and unscrambling the text. That view changed sharply in 1976, when Stanford computer scientists Martin E. Hellman and Whitfield Diffie published a paper called ““New Directions in Cryptography” that described what is now known as public key encryption (PKE). Two years later, Ron Rivest, Adi Shamir, and Len Adelman of MIT described a simpler method. When the web came along, the Diffie-Hellman and RSA algorithms became the bedrock of secure communications.

But PKE had an unknown pre-history. As early as the 1960s, John H. Ellis of GCHQ/CESG, the British equivalent of the National Security Agency’s Central Security Service, was experimenting with ideas about “non-secret encryption.” He described his work in a 1970 paper entitled “The Possibility of Non-Secret Digital Encryption,” but it remained classified until 1997. In the 1970s, CESG researchers Clifford Cocks and Malcolm Williamson found ways to implement PKE, but this work, too, stayed secret for more than two decades. A 2004 Wired story by Steven Levy gives a detailed account of the British efforts. In his account, The History of Non-Secret Encryption,” Ellis drops a fascinating hint of earlier work. Reflecting on the “obvious” impossibility of secret communications without a shared secret, he wrote...

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