Quantum key to unbreakable cryptography

ANU researchers develop completely secret, unhackable quantum key transmission using laser beams and common optics

Researchers at the Australian National University have developed, and are now commercializing, an unhackable means of transmitting information that is much more secure than existing methods.

The researchers are working on quantum key distribution (QKD), generally known as quantum cryptography, which uses laser beams that are encoded in a way that makes interception physically impossible.

"It's somewhat cheekily titled the ultimate cryptographic system," said ANU physicist and Stanford Sloan Fellow, Vikram Sharma, referring to a presentation on the technology.

Sharma is part of a team that won the 2006 Eureka Prize for Scientific Research for demonstrating an end-to-end working prototype of a QKD system.

The team, which has worked on the project for three years, is looking to commercialise the QKD system through QuintessenceLabs -- a Canberra-based company setup by the ANU researchers -- which has attracted funding from various government agencies and investors.

There are currently two main commercial companies developing Quantum encryption devices; MagiQ in the US, and idQuantique in Switzerland.

"These devices in their early forms are quite expensive, at more than US$100,000. What we would like to do is scale those costs down progressively. We feel with our setup we have a good chance of achieving this as we employ a fair amount of off-the-shelf componentry, as opposed to the other two [designs] where specialised single photon sources and detectors are required" Sharma said.

"So an eavesdropper could intercept these quantum sets that you are sending but they will never be able to reproduce them perfectly. This is in stark contrast to electronic data, 1s and 0s, which you can copy perfectly as many times as you like"

ANU physicist, Vikram Sharma

Sharma said that tried and tested optics and electronics from the telecoms industry are used in his team's QKD, resulting in a considerably cheaper and much more robust system.

While explaining the potential of quantum cryptography, Sharma quoted the novelist Edgar Allen Poe who once said 'it can be roundly asserted that human ingenuity cannot concoct a cipher which human ingenuity cannot resolve'.

This may be true for traditional cryptography which is based on complex mathematics. But quantum cryptography uses the laws of physics to guarantee that no human can resolve its cipher.

"With quantum cryptography, we try and harness some of the fundamental properties that nature can offer us," Sharma said.

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He explained that QKD essentially leverages two concepts: The Heisenberg Uncertainty Principle, which means that information is encoded onto a laser beam by manipulation at the quantum level, and any attempt to monitor it necessarily disturbs the beam in some detectable way; and the No Cloning Theorem, a law which basically states that it is impossible at the quantum level to make a perfect copy of something. A series of clean up steps also ensures any intercepted information is rendered useless.

"So an eavesdropper could intercept these quantum sets that you are sending but they will never be able to reproduce them perfectly. This is in stark contrast to electronic data, 1s and 0s, which you can copy perfectly as many times as you like," Sharma said.

"Although several groups around the world have quantum cryptographic technology, our group was one of the first in the world to demonstrate the transmission of a completely secret key via bright laser beams and common optics," said Dr Thomas Symul, also part of the research team, in an ANU Newsletter.

Sharma and his group are hoping to commence commercial operations of QuintessenceLabs early next year, and will be pushing a commercial version of their QKD system that will be able to securely transmit unbreakable secret keys over optic fibres.

"Of course interested organisations would include defence, intelligence agencies and various other government departments handling sensitive data. Also tier-one financial institutions, and companies with deep intellectual property like pharmaceutical companies," Sharma said.

In the longer term the technology could be used to relay secret keys all over the world via satellite communications.

Sharma also sees its future potential in personal banking.

"There is billions of dollars in credit card fraud every year. So how do you secure either an Internet or telephone banking transaction?"

He cites recent proposals to couple a mobile phone with a special device that enables quantum transmissions between itself and an Automatic Teller Machine (ATM).

"You have a quantum cryptographic transmission between your mobile and the ATM, and a one-time key which would be downloaded onto your telephone," he said.

"Subsequently you can make an Internet transaction, put in your card number and that one time key can be downloaded via Bluetooth onto your PC. Through the ATM the bank would have a copy of that one time key already, so when you go to use it in an Internet transaction the bank can validate that you are in fact really you."

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How it works

(Courtesy of the Australian National University)

Vikram Sharma explains how the group's quantum key distribution system works. In classical cryptography, the sender is referred to as Alice, the recipient as Bob, and the eavesdropper as Eve.

Step [1]

A laser beam has billions of photons travelling per second. We call this the carrier frequency. We make sure that beam is as precise and noise-free as possible. From the carrier frequency, we shift a small number of photons out of the billions, and drop them at frequencies that are displaced, plus or minus, from the carrier frequency. In this way, we encode the random number ones and zeroes on the side bands of the laser. Using this method we've setup an experimental prototype to simulate transmissions over 50 kilometres of fibre.

Step [2]

Bob's receiver station makes very sensitive measurements of the laser beam to strip out the carrier and look at what was coded on the sidebands. This gives him the raw key, but the encoded data has been jiggled around by quantum noise. The key that Bob has received differs from that which Alice has sent. Also, in quantum cryptography we are totally paranoid, so we always assume that Eve is omnipresent and omniscient. We assume that all the losses in transmission are somehow captured by Eve and she is able to make optimal use of this information.

Step [3]

The raw data that Bob has received is manipulated in various ways to adjust for characteristics of the electronics and the transmission channel. We then carry out a procedure called post selection. This involves capitalising on the random nature of quantum noise by making the most of those events where Bob received more information than Eve. The idea is for Bob and Alice to capture those transmissions where Bob did really well in his measurements, and discard the rest.

Step [4]

Via a process called advantage distillation, we amplify the advantage that Alice and Bob have eked through the post selection procedure. This comes at the expense of discarding more bits of the raw key.

Step [5]

We then engage in a process of secret key reconciliation, which is essentially a procedure for error correction. Recall that the data is noisy. Bob makes his key match Alice's through a series of indirect disclosures that are somewhat analogous to the game Master Mind. But if Bob discloses information about his bits, even if it is indirect, then an eavesdropper could also get to know this information. Again we assume that Eve is eavesdropping and is trying to maximise her knowledge of the secret key using a composite of all the information she has gained from all her eavesdropping activities.

Step [6]

After the key reconciliation step we've got a perfectly clean key. The final stage is to negate the usefulness of all the information that Eve has. We draw on abstract algebra to carry out an operation that is somewhat analogous to kneading dough. Imagine that the information that Alice and Bob share is represented by a piece of pizza dough. We assume that Eve has gained considerable knowledge of the key as well from her eavesdropping, however she still has some uncertainty about some of the bits. If we represent those bits that she doesn't know anything about by a dot of blue ink on the dough, by kneading it we spread the ink throughout her dough. Alice and Bob don't have a problem because their bits have been perfectly aligned prior to this operation - even if you jumble them, it doesn't matter. And just to be sure that she can't reverse engineer the process, we chop off a bit of the dough and use only this small portion for the final key. Even if Eve had some incredibly powerful computer, she'd never be able to reconstruct Alice and Bob's final secret key.

Vikram Sharma can be contacted at