Quantum version of Nazi Enigma machine is uncrackable

Alan Turing and an army of codebreakers took years to crack the Enigma code during the second world war. Just be thankful the Nazis weren't using a quantum Enigma machine.

Quantum key distribution (QKD) can already keep secret the key to a code. "I was trying to think of alternatives," says Seth Lloyd at the Massachusetts Institute of Technology. "It occurred to me that it would be interesting to quantise the Enigma machine."

Enigma machines are like souped-up typewriters, with an illuminated panel above the keyboard that also displays the alphabet. Type in a letter and a different letter on the panel lights up, indicating how to encrypt that character.

Wired rotors do the scrambling by guiding an electrical signal along a certain path. Crucially, with each new letter, the path changes, so if you type the same letter again it will be encrypted differently ? avoiding the repeated letter patterns that codebreakers rely on. The change isn't random, but depends on the initial rotor settings, which work as the key.

If you know the settings ? Enigma operators had books specifying them ? the encrypted words can be typed into another Enigma machine to reveal the message.

Turing and his colleagues relied on external information, such as knowledge that a message was likely to be about the weather, to make educated guesses at its content, which often led them to the key. In other words, they turned an increase in information about the message into a reduction in the key's strength.

Space Enigma

To devise a quantum enigma machine, Lloyd preserved the basic idea of two people with versions of the same machine and shared settings. But instead of electricity and rotors that change the settings, the hypothetical machine uses photons, with the quantum ability to follow two paths at once.

The switch to quantum mechanics changes the equation: increasing information on the part of the codebreaker no longer leads to a significant reduction in the key strength ? unless the information is the key itself. Lloyd is now trying to build the machine: it might even fit into a typewriter-like form, he says.

Vadim Makarov, who runs a quantum hacking lab at the University of Waterloo, Canada, says quantum Enigma is more difficult to implement than QKD but has other benefits. QKD uses quantum mechanics to create the key, requiring back-and-forth communication, but in quantum Enigma, the key is pre-arranged, so the only transmissions are the messages themselves.

"It might come in handy for a future space civilisation spread through the solar system," says Makarov, given that transmissions will take longer to arrive in space.

Reference: arxiv.org/abs/1307.0380

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