This morning I attended Martin Hellman’s stimulating keynote address at the 2013 Georgia Tech Cyber Security Summit. Martin Hellman is the co-inventor, with Whitfield Diffie, of the Diffie-Hellman Key Exchange Protocol, which began the (public) public-key cryptography revolution. Among the interesting things I learned during Martin Hellman’s talk are:

1. Hellman feels that Ralph Merkle deserves equal credit for inventing public-key cryptography and refers to his own invention as the Diffie-Hellman-Merkle key exchange protocol. (Merkle was the director of the Georgia Tech Information Security Center from 2003-2006.)

2. Hellman came up with the famous “double padlock” thought experiment after the invention of the Diffie-Hellman-Merkle key exchange protocol, as a way to explain it to others. The mathematics came first. (I had always wondered about this.)

3. Most interestingly, Hellman said that he got the idea to use modular exponentiation/discrete logarithms as a “one-way function” from the engineer and mathematician John Gill (who I never heard of before this morning). John Gill’s other suggestion was to use multiplication/factoring, which forms the basis of RSA! It’s all the more amazing that I’ve never heard of John Gill because he earned his bachelor’s degree in Applied Mathematics from Georgia Tech (where I now teach) and his Ph.D. in Mathematics from U.C. Berkeley (where I got my Ph.D.)! Hellman also recounted a conversation in which Gill (who is African-American) mentioned having encountered very little racial intolerance during his undergraduate studies in the 1960’s — apparently Georgia Tech was (relatively speaking) an oasis of tolerance among southern universities during that time.

Now on to the mathematical part of this post, which is an unusual proof of the existence of primitive roots modulo primes which I came up with recently while preparing a lecture for my course on Number Theory and Cryptography. The proof is much less elementary than every other proof I’ve seen, but I would argue that it nevertheless has some merit. Continue reading →

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