Project C-43: A Final Piece of Public-Key Cryptography History

"The event which changed this view was the discovery of a wartime Bell Telephone report by an unknown author describing an ingenious idea for secure telephone speech… The relevant point is that the receiver needs no special position or knowledge to get secure speech. No key is provided; the interceptor can know all about the system; he can even be given the choice of two independent identical terminals. If the interceptor pretends to be the recipient, he does not receive; he only destroys the message for the recipient by his added noise. This is all obvious. The only point is that it provides a counter example to the obvious principle of paragraph 4. The reason was not far to seek. The difference between this and conventional encryption is that in this case the recipient takes part in the encryption process. Without this the original concept is still true. So the idea was born. Secure communication was, at least, theoretically possible if the recipient took part in the encipherment.1

Ellis refers to a document titled “Final Report on Project C-43″ without any additional identifying information. For years, this passing reference has intrigued the cryptographic community with the possibility that Bell Labs researchers might have made important progress on private key encryption as early as the 1940s. It turns out that the mysterious Final Report exists and is available (if obscurely) online.1,2"