Laser can crack 'unbreakable' quantum communications

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London, Oct 4 : A new research has found that by shining a bright laser into sensitive equipment that uses the technology of quantum cryptography, makes it possible to hijack communications without a trace.

Quantum cryptography has been used by some banks to protect data, and even to hide election results in Switzerland last year.

But, according to a report in New Scientist, it has been discovered that shining bright light into the sensitive equipment needed makes it possible to possible to intercept messages without detection.

"It turns the equipment into a puppet-box that an eavesdropper can control," said Vadim Makarov from the Norwegian University of Science and Technology in Trondheim, who uncovered the vulnerability.

Quantum cryptography relies on both users sharing a secret key, each digit of which is encoded into the polarisation of an individual light photon.

"Alice", the sender, transmits a stream of photons signalling either 1s or 0s. But for each one she randomly chooses from one of two ways to encode the digit.

Because the receiver, "Bob", doesn't know which system Alice has used he must be able to decode both types and has two pairs of photon detectors - one for each system.

A beam splitter randomly directs each photon received to one of the pairs. If a photon reaches the correct pair it is decoded correctly, if not Bob receives a false result.

Once the transmission is over, Alice uses an unencrypted channel to tell Bob which system she used for each photon. Digits decoded wrongly are discarded to reveal the final secret key used to secure later communications.

In practise, these steps are carried out automatically by a computer system.

An eavesdropper, "Eve", who intercepts the transmission, must emulate Bob's detection method and then pass the data on to him unaltered to fool him everything is normal.

But, quantum mechanics makes that impossible. The message will have been changed by Eve's interception to contain errors that reveal her presence when Alice and Bob compare notes later.

Now, however, Makarov and colleagues from Sweden and Russia have shown that Eve could control Bob's equipment, so that they both decode exactly the same digits from Alice's transmission.

When Alice later tells Bob which photons he encoded wrong, Eve can learn the key by listening in on the unencrypted message, and there are no extra errors to give her away.

The method exploits the way a common type of photon counter can have its sensitivity reduced by a very bright flash of light.

ANI

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