Introducing Quantum Cryptography: This Is What It Will Take To Keep Secrets Secret

The Enigma coding machine used by the Germans in WWII. Photo by Ian Waldie / Getty Images

History is full of instances of the most brilliant code-makers being matched by the ingenuity of code-breakers.

Even today’s encrypted data is vulnerable to technological progress and recent revelations about government surveillance have thrown a spotlight on the lack of security of digital communications.

In the latest issue of the science journal Nature, researchers Artur Ekert and Renato Renner review what physics says about keeping secrets secret.

We already know that one of today’s most widely used encryption systems, RSA, will become insecure once a quantum computer, using subatomic particles to store information, is built.

“Recent developments in quantum cryptography show that privacy is possible under stunningly weak assumptions about the freedom of action we have and the trustworthiness of the devices we use,” says Ekert, Professor of Quantum Physics at the University of Oxford, UK, and Director of the Centre for Quantum Technologies at the National University of Singapore.

More than 20 years ago, Ekert and others independently proposed a way to use the quantum properties of particles of light to share a secret key for secure communication.

The key is a random sequence of 1s and 0s, derived by making random choices about how to measure the particles (and other steps) which is used to encrypt the message.

In the Nature Perspective, he and Renner describe how quantum cryptography has since progressed to a commercial prospect and into new theoretical territory.

“As long as some of our choices are not completely predictable we can keep our secrets secret,” says Renner, Professor of Theoretical Physics at ETH Zurich, Switzerland.

This arises from a mathematical discovery by Renner and his collaborator about ‘randomness amplification’. They found that a quantum trick can turn some types of slightly random numbers into completely random numbers. Applied in cryptography, such methods can reinstate our ability to make perfectly random choices and guarantee security even if we are partially manipulated.

“As well as there being exciting scientific developments in the past few years, the topic of cryptography has very much come out of the shadows. It’s not just spooks talking about this stuff now,” says Ekert, who has worked with and advised several companies and government agencies.

The authors say: “The day we stop worrying about untrustworthy or incompetent providers of cryptographic services may not be that far away”.

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