Unlocking Quantum Cryptography

Quantum Physics holds the key to a new level of data security, where the laws of the universe, not just tough mathematics, are the greatest allies.

IN A NEW TWIST on the eternal triangle, this is the story of Alice, Bob and their unwanted guest, Eve.

Standard methods rely on setting Eve a near impossible mathematical puzzle. But physicists are working on a new model that no amount of computational power can crack, because it relies on the laws of the universe.

Since the mid-1990s, scientists have claimed to have achieved quantum cryptography at an experimental level. But a paper to be published in the journal Physical Review Letters, and in lectures delivered in universities in the US and Australia, Macquarie physicist Associate Professor Barry Sanders has demonstrated the goal is some way off yet.

"Previous attempts have assumed technical limitations for our eavesdropper, Eve," says Sanders. "Experimental physicists may have demonstrated that with today's technologies, or even future generations of today's technologies, their experimental quantum cryptography is secure. But that is only a provisional security."

Sanders proposes a model that is future-proof.

But first, let's understand the past.

Traditional cryptography-as used by World War II spies and the like-involved an encoded message and a key to unlock the code. The encoded information could be sent by insecure means, but the key had to be communicated in secret from the sender (Alice) to the receiver (Bob). All eavesdropper Eve had to do was tap the line or rob the courier, and her job was done.

The next generation was born in 1976, with public-key cryptography. It was a breakthrough that eliminated the need to send the key from one place to another.

In this model, Bob creates two keys, one that does the encoding, and another that does the decoding.

This system is most commonly described as like a safe with two keys, one that anyone can have and which allows you to put something into the safe, and one that only one person has and which allows that person to remove things from the safe.

Bob sends Alice the encoding key and keeps to himself the decoding key. Eve is out of the loop.

The flaw to this system is that there must be a mathematical relationship between the encoding key and the decoding key. If Eve could discover that relationship, she could derive the secret key from the public key.

The problem for Eve is that today's cryptographers set her a fabulously difficult mathematical problem. To attain the relationship between the two keys, Eve is asked to factorise a number that has very many digits. That is, she has to break down a complex number to all the possible numbers that, when multiplied with another number, will equal the original.

This takes advantage of the exponential rate the difficulty of factoring a number grows with every digit you add.

At the fastest rates of today's computation, the puzzles set by cryptography can take billions of years to solve.

It doesn't sound like there is a major security problem here, does it.

The potential trouble lies with the next generation of computers that scientists across the world are racing to develop-the quantum computer. If such a computer can be developed, suddenly this impossible mathematical task becomes a snack.

According to Sanders, the very fact that public-key encryption contains the seeds of its own destructionin however theoretical a formis a flaw that needs addressing.

He says quantum encryption holds the answer.

He and other quantum physicists propose a system that takes advantage of the uncertainty principle of Quantum Mechanics. Here, the very act of Eve looking at the transmitted information changes it. Alice and Bob will then always know if Eve is spying on them and be able to scramble the information accordingly.

Unlike other experimental models of quantum cryptography, which have assumed certain technical limitations to Eve's decrypting powerand therefore suffer a drawback similar to classical encryptionthe model Sanders proposes allows Eve to know and do all things. She is "omnipotent Eve".

But he's still got her beat.

The model Sanders outlines in his paper uses a technique to generate pairs of photons out of those produced randomly by a laser.

One stream contains the information sequence in the way each photon is polarised. The other contains the "partner" photons.

Being linked at the quantum level, Alice cannot identify the polarity of one photon in the pair without affecting the other. Sometimes the effect will amount to no change but, inevitably, changes will occur to disrupt the information and alert Alice and Bob that Eve is listening.

"The beauty of this," says Sanders, "is that the integrity of the information is protected by the laws of physics, not by today's technical inadequacies."

Having delivered his paper at the universities of Stanford and Oregon in the US, and University of Melbourne, Sanders is to present at the International Quantum Electronics Conference in Nice, France.

The papers on which the presentation is based were co-written with Gilles Brassard (University of Montreal), Norbert Ltkenhaus (University of Helsinki) and Tal Mor (College of Judea and Samaria).

Story by Bruce Williams
Images by Bruce Williams

For more information: Quantum Cryptography Tutorial. (Online)

and See our Computing CD for more about "Computing and Cryptography"

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