Quantum enigma creates unbreakable messages using keys shorter than the message itself

Quantum 'Enigma' machine shortens encryption keys

A quantum encryption technology has been developed that for the first time enables the user to secure messages with keys shorter than the messages themselves. 

The system, developed by researchers at the University of Rochester, USA, relies on a method known as quantum data locking to send unbreakable messages between two entities.

Quantum data locking takes advantage of the fact that photons - elementary particles of light - can be altered in multiple ways when travelling in the form of waves. That means that a photon can encode more information in a single bit compared to the conventional binary system. As a result, the quantum key for encrypting and deciphering a message can be shorter that the message itself.

The two entities involved in the message exchange - in cryptography known as Alice and Bob - have both to be equipped with a system called the spatial light modulator (SLM).

Alice, who is sending the message, uses the modulator to alter the properties of photons she previously generated to encode the message. The SLM can flatten or tilt the waves and distort their shapes.

Bob, who is the recipient of the message and knows the key Alice used to encode the message, can then use his own SLM to bring the photon waves back to their original form and decode the message.

The technology takes advantage of the fact that a simple act of intercepting the message by an unauthorised party would alter the quantum system.

"While our device is not 100 per cent secure, due to photon loss, it does show that data locking in message encryption is far more than a theory," said Daniel Lum, a graduate student in physics who devised the encryption machine together with his supervisor professor John Howell.

The system is the first technology that can send unbreakable encrypted messages while not respecting the rules established by American mathematician Claude Shannon. Shannon, considered the father of information theory, stated in the 1940s that unbreakable messages can only be generated using a random key, which is at least as long as the message itself and used only once.

The possibility to use quantum data locking to encrypt messages was predicted previously by MIT professor Seth Lloyd but the Rochester machine represents the first practical application.

"It's highly unlikely that our free-space implementation will be useful through atmospheric conditions," said Lum. "Instead, we have identified the use of optic fiber as a more practical route for data locking.”

The Rochester team named their machine after the famous encryption technology used by Germany during the Second World War, which was eventually cracked by mathematician Alan Turing and the team at Bletchley Park, enabling British intelligence to decipher German messages. This turned the tide of the war in favour of the Allied Forces.

An article describing the new technology is published in the journal Physical Review A.

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