The unbreakable man: How Gilles Brassard used the laws of physics to reinvent secrecy

Gilles Brassard spent forty years warning the world that its secrets could be stolen.

Light cannot be secretly observed. Touch a photon and it changes; intercept it and it leaves a trace. For forty years, Gilles Brassard built his career on that fact, and this week the Association for Computing Machinery gave him theTuring Award, computing’s highest prize, for what he made of it.Working with the physicistCharles Bennett, a physicist at IBM Research and co-recipient of the award, Brassard showed that this property of light could be turned into a lock whose security is guaranteed not by mathematics, but by physics itself. Their 1984 protocol, known as BB84, was the first provably secure method of exchanging an encryption key. Bennett and Brassard inventedquantum cryptography, which, in the simplest possible terms, is a method of locking a secret so that the laws of physics themselves stand guard.“What Charlie and I did was latch on to this gift from nature and turn it into a scheme for communication,” Brassard toldIBM Thinkin an interview.

Brassard grew up in Montreal, obtained his doctorate in computer science from Cornell University in 1979, returned to the Canadian city, and has been at the Université de Montréal ever since, where he has held a full professorship since 1988 and a Canada Research Chair since 2001.He is warm, slightly formal, fond of Bach and Mahler. He cooks. He goes to concerts in Amsterdam. The word he uses most often about his work is “fun.”The idea for BB84 came from a paper. In 1976, while Brassard was still a doctoral student, his advisor, John Hopcroft, handed him a paper titledNew Directions in Cryptographyby Whitfield Diffie and Martin Hellman.The paper proposed something that had never before appeared in the published scientific literature: that two strangers who shared no prior secrets could nonetheless conduct a private conversation, by exchanging information publicly and using mathematics to derive a shared secret that no eavesdropper could reconstruct.For most of human history, secret communication required both parties to possess the same key in advance, a shared code that could be used to scramble a message and unscramble it again. The Spartan generals used physical ciphers. Julius Caesar used letter substitution.During the Cold War, the hotline between Washington and Moscow was secured by keys printed onto physical tape and carried across the Atlantic by a diplomat in a briefcase handcuffed to his wrist. What Diffie and Hellman proposed was simpler and stranger: the idea that you could simply conjure a shared secret from nothing, using mathematics alone, was, in 1976, new.“It was so very, very elegant that I dropped what I was doing essentially, and moved on to cryptography,” Brassard said. The paper’s approach, and theRSA cryptosystemthat Ron Rivest, Adi Shamir and Leonard Adleman introduced the following year, became the foundation of the internet’s security architecture.Every time someone shops online, logs into a bank or sends an email, one of these two systems is encrypting the connection in the background. Most users never think about this, which is roughly the state of affairs the systems were designed to produce.Both systems rested on the same principle: that certain mathematical problems are so hard to reverse that solving them would take a conventional computer longer than the age of the universe. No one had proven they had to stay that way. They had simply never found a shortcut.