“A failed mathematician and an amateur physicist” – that’s how Gilles Brassard, one of the world’s top computer scientists, describes himself. Approaching his 70th birthday, the Université de Montréal professor is also a bit of Cassandra, warning of dangers both present and future.
The past? Well, his own is ancient history to him now – except for his abiding love of science.
Brassard was born in north-end Montreal, in Ahuntsic, the youngest of four brothers and one sister. Their dad was an accountant and mom a yoga instructor; the family lived across from what was then the Bois Saint-Hubert, a forest where Collège Ahuntsic, the local CEGEP, is today.
All four boys were keen on science, Gilles included, and all would go on to careers in the discipline: one as a physicist, one as a mathematician, and two as computer scientists. “I wanted to become a scientist for as long as I can remember,” Brassard recalls. “Actually, what I really wanted was to become an astronaut. Maybe I should have!”
Joking aside, Brassard today is a man on a mission, self-imposed.
He’s busy spreading the word on the growing risks that quantum computing – one of the theoretical fields he’s been involved in for decades – will lead to huge breaches in the security of communications over the Internet and elsewhere. That threat, Brassard says, can be mitigated by quantum cryptography, the field that he pioneered starting in the late 1970s, shortly after obtaining his Ph.D. in theoretical computer science in the United States, at Cornell University.
An ‘oracle’ is born
At the time, academic cryptography was in its infancy, and in his thesis Brassard made a splash: he proved the theoretical existence of an “oracle,” a mathematical construct that would make “public-key” cryptography, the newest form, provably secure – but again, only in theory. Brassard presented his findings at an international conference in Puerto Rico, where he met Charles H. Bennett, a physicist with the IBM Research Center north of New York City.
Together, they went on to develop a protocol, now known as BB84, that in principle can encode information and transmit it safely using the principles of quantum theory. Once more, that was something totally new at the time. “Quantum cryptography would not have been possible without this alliance between physics and computer science,” Brassard says. “By combining our two fields of knowledge, Charles and I created a new field of investigation.”
Until then, cryptography had been based on “unproven assumptions about the computational difficulty of solving some number-theory problems,” he explains, “and sadly, cryptography today is still based on those same unproven assumptions.”
Since the mid-20th century, it’s been understood that in order to encrypt a message with perfect secrecy – that is, nothing about the cleartext is revealed to the eavesdropper upon intercepting the ciphertext, except its length – the two parties involved must first share a secret “key,” as long as the message itself, made up of string of numbers or letters.
But this theorem concerning classical communication breaks down when quantum theory is taken into account, as Bennett and Brassard showed in their protocol.
Named after their initials and the year they published their theory, BB84 guarantees absolute confidentiality; there’s no need to have the key beforehand. “What’s quantum is the method we use to obtain the key,” Brassard explains. “The result is that the transmitted message is indecipherable to outsiders. In theory, it can’t be hacked – not now and not ever, not even if the eavesdropper is equipped with a full-scale quantum computer.”
Those don’t yet exist, he points out. But the very idea of them presents a clear danger.
Back in 1994, American mathematician Peter Shor first posited that quantum computers would be able to harm the entire cryptographic infrastructure on which the security of the Internet is based. “At the time, this seemed to be a remote threat, but today nobody can deny that quantum computers will soon be powerful enough to run Shor’s algorithm,” Brassard notes, adding a warning.
“In the not-so-distant future, hackers will be able to outrun the encryption, opening up the world’s secrets – in national defence, in banking, in medicine.”
Much worse, he says, is that everything that’s been online since the dawn of the Internet is vulnerable.
‘Harvest now, decrypt later’
“Any communication that has ever been sent over the Internet under the claimed cover of confidentiality could have been intercepted and stored by untrustworthy people who were simply unable to decrypt the communication at the time,” he points out. “But as soon as they have access to a quantum computer, they’ll be able to take the encrypted information out of storage and decrypt it retroactively. This is known as the ‘harvest now, decrypt later’ paradigm.”
And that, he adds, is the supreme danger: if the data is successfully unlocked, privacy will be destroyed. “Absolutely nothing can be done to prevent this. All the encrypted information that for the moment, we hope, is undecipherable is already in the hands of the adversary. The past is lost. We can only hope to save the future by relying on various cryptographic techniques.”
Chief among them: quantum cryptography.
“It and it alone is secure against unlimited computing power and technology limited only by the known laws of physics,” Brassard says. And though still not yet fully developed, encrypted communication based on quantum principles is now a reality that goes far beyond simple laboratory experiments. “It isn’t science fiction; it’s a part of the quantum revolution that is already a reality,” says Brassard.
Trouble is, will it be developed and adopted quickly enough to outpace quantum computing?
Worldwide, industry is investing billions of dollars in quantum technologies – particularly in China, where a quantum cryptographic ‘backbone’ today links the vast nation over a distance of more than 10,000 kilometres. And Google recently unveiled a quantum processor capable of performing a standard benchmark computation in under five minutes – something they estimate would take one of today’s fastest supercomputers 10 septillion years to do.
“Eventually, even a small quantum computer – say, 1,000 qubits in size – could perform a calculation that a conventional computer could not complete before the very end of time,” Brassard warns. “We’re not there yet, but that day is coming.”
Caution in 2025
These days, it’s a warning he’s bringing to all who will listen, at home and abroad, especially now, in 2025, which Unesco has proclaimed to be the International Year of Quantum Science and Technology.
Among his current efforts at outreach:
Cryptology isn’t Brassard’s only area of expertise. He also helped invent quantum teleportation, laying its foundations in 1992 with fellow Quebecer Claude Crépeau, now a computer-science professor at the École de technologie supérieure. With three other researchers, they showed it’s possible to transfer quantum information contained in subatomic particles such as photons or electrons from one part of the galaxy to another, without physically moving the particles.
The process is grounded in quantum theory, which says that a particle can exist in more than one state simultaneously and two particles separated by arbitrary distances seem to react instantaneously when only one of them is stimulated, “though that’s only an illusion,” as Brassard points out. For now, despite compelling demonstrations, quantum teleportation is still in the experimental stage, “but one day, it will fuel the quantum Internet,” says Brassard.
Harbours few secrets
In his day-to-day life, for someone so dedicated to issues of privacy and security, the scientist says he personally harbours remarkably few secrets. “I consider that fighting Big Brother is the most important battle I can wage, but as for myself and the privacy precautions I take, well, have you ever heard of the French expression ‘le cordonnier mal chaussé’? In English, you’d say ‘The baker’s children have no bread.’ I need to practise what I preach!”
To his students, too, Brassard prides himself on being an open book, delivering his lectures in a folksy, approachable style. In December, for instance, he sent a video invitation in advance of his Belgian lecture, addressing his audience thus: “I’ll be coming to tell you some nice stories about art and science […] We live in a quantum world: is it to the advantage of the codemakers or the codebreakers? Good question. See you soon with an answer!”
In his personal appearance, as well, Brassard cuts a no-nonsense figure. He wears unstylish glasses in steel half-rim frames, rarely tucks in his shirts, eschews jacket and tie, keeps his grey hair long and sometimes swept back in a ponytail, like an old hippie or a cool monk. The unspoken message: what you see is what you get. And underlying that message is a fundamental democratic impulse.
“Science is for everyone,” says Brassard, who first “haunted the corridors of Université de Montréal” in 1968, when at the tender age of 13 he began his university studies, spending the next 57 years at the university, 46 of those as a professor. (“I’ve held all the ranks, from assistant to full professor,” he says. “The only thing awaiting me is to retire and hopefully become professor emeritus!”)
“We should all do research we’re interested in, and not be afraid of failure,” he says. “At first your ideas might seem far-fetched, and maybe they are, but they could also be the start of a big idea. Remember what Einstein said: ‘Imagination is more important than knowledge.’
“With that, I totally agree.”
