Cryptographers are exploring new rules for quantum encryption

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Original version Of this story Printed in Quanta Magazine.

Let’s say you want to send a private message, cast a secret vote, or securely sign a document. If you do any of these things on a computer, you rely on encryption to keep your data safe. That encryption needs to withstand attacks from code breakers on your computer, so modern encryption methods rely on assumptions about which mathematical problems are hard for computers to solve.

But when cryptographers laid the mathematical foundations for this approach to information security in the 1980s, some researchers discovered that computational hardness was not the only way to protect secrets. Quantum theory, originally developed to understand the physics of atoms, turned out to have deep connections to information and cryptography. Researchers found ways to base the security of certain cryptographic tasks directly on the laws of physics. But these tasks were peculiar – for all others, there was no alternative to the classical computational approach.

By the turn of the millennium, quantum cryptography researchers thought that was the end of the story. But in the past few years, the field has undergone another major change.

“There has been a rearrangement in what we thought was possible with quantum cryptography,” said Henry Yuen, a quantum information theorist at Columbia University.

In a number of recently published papers, researchers have shown that most cryptographic tasks can still be performed securely, even in a hypothetical world where practically all computations are easy. It is only the difficulty of a particular computational problem that matters in terms of quantum theory.

“The assumptions you need can be very, very, very weak,” said Fermi Ma, a quantum cryptographer at the Simons Institute for the Theory of Computing in Berkeley, California. “This is giving us new insights into computational hardness.”

this message will self destruct

The story begins in the late 1960s, when a physics graduate student named Stephen Wiesner began thinking about the destructive nature of measurement in quantum theory. Measure any system governed by the laws of quantum physics, and you will alter the quantum state that mathematically describes its configuration. This quantum measurement mess was a stumbling block for most physicists. Wiesner, who took an unconventional information-centric view of quantum theory, wondered if it could be made useful. Perhaps it could serve as built-in tamper protection for sensitive data.

But Wiesner’s ideas were far ahead of their time, and he left academia after graduate school. Fortunately, he discussed his ideas with his friend and fellow physicist Charles Bennett, who spent a decade trying unsuccessfully to interest others in the topic. Finally, in 1979, Bennett met computer scientist Gilles Brassard while swimming off the coast of Puerto Rico during a conference. Together, they wrote an important paper describing a new approach to a crucial cryptographic task. Their protocol was based on quantum measurement perturbations, and did not require any assumptions about the difficulty of any computational problem.

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