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takyon writes:

Quantum data locking could provide an efficient and secure alternative to one-time pad encryption (which requires a key at least as long as the message), according to two new papers:

Researchers at the University of Rochester have moved beyond the theoretical in demonstrating that an unbreakable encrypted message can be sent with a key that's far shorter than the message—the first time that has ever been done. Until now, unbreakable encrypted messages were transmitted via a system envisioned by American mathematician Claude Shannon, considered the "father of information theory." Shannon combined his knowledge of algebra and electrical circuitry to come up with a binary system of transmitting messages that are secure, under three conditions: the key is random, used only once, and is at least as long as the message itself. The findings by Daniel Lum, a graduate student in physics, and John Howell, a professor of physics, have been published in the journal Physical Review A.

[...] Let's assume that Alice wants to send an encrypted message to Bob. She uses the machine to generate photons that travel through free space and into a spatial light modulator (SLM) that alters the properties of the individual photons (e.g. amplitude, tilt) to properly encode the message into flat but tilted wavefronts that can be focused to unique points dictated by the tilt. But the SLM does one more thing: it distorts the shapes of the photons into random patterns, such that the wavefront is no longer flat which means it no longer has a well-defined focus. Alice and Bob both know the keys which identify the implemented scrambling operations, so Bob is able to use his own SLM to flatten the wavefront, re-focus the photons, and translate the altered properties into the distinct elements of the message.

Along with modifying the shape of the photons, Lum and the team made use of the uncertainty principle, which states that the more we know about one property of a particle, the less we know about another of its properties. Because of that, the researchers were able to securely lock in six bits of classical information using only one bit of an encryption key—an operation called data locking. "While our device is not 100 percent secure, due to photon loss," said Lum, "it does show that data locking in message encryption is far more than a theory."

2013 paper by Seth Lloyd: Quantum enigma machines

Quantum enigma machine: Experimentally demonstrating quantum data locking (DOI: 10.1103/PhysRevA.94.022315) (DX)

Experimental quantum data locking

Classical correlation can be locked via quantum means--quantum data locking. With a short secret key, one can lock an exponentially large amount of information, in order to make it inaccessible to unauthorized users without the key. Quantum data locking presents a resource-efficient alternative to one-time pad encryption which requires a key no shorter than the message.

Original Submission