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Researchers say they have devised a foolproof way to encrypt messages that can be unlocked only by a recipient physically located in a specific place, solving a problem that has vexed cryptographers for years.

The technique for position-based quantum cryptography is scheduled to be presented at the 2010 IEEE Symposium on Foundations of Computer Science in October. It makes it theoretically possible for people to securely encrypt and decrypt messages without the use of pre-shared keys. Instead, the messages would be encrypted using keys based on a recipient's physical presence at a secure facility.

The technique builds off of previously reported research that suggested position-based crypto was impossible to pull off against multiple colluding adversaries scattered in different places. The researchers solved this problem by devising a way to use quantum mechanics to determine a party's location that can't be spoofed.

“Our results open a fascinating new direction for position-based security in cryptography where security of protocols is solely based on the laws of physics and proofs of security do not require any pre-existing infrastructure,” their paper states.

The task of verifying a recipient's location involves sending the quantum equivalent of bits using a protocol that requires the receiver to respond to random challenges. The so-called no-cloning principle of quantum mechanics makes impossible for people elsewhere to provide the correct answer.

The technique guarantees that the person sending the message shares a secret key with the recipient only if the latter is located at a specific location. Anyone located elsewhere will be unable to convert the message into plain text.

While the research solves an important problem, it's unlikely to see practical applications anytime soon, crypto and security expert Bruce Schneier said.

“Don't expect this in a product anytime soon,” he blogged. “Quantum cryptography is mostly theoretical and almost entirely laboratory-only. But as research, it's great stuff.”

The researchers are Nishanth Chandran, Ran Gelles, and Rafail Ostrovsky of the University of California, Los Angeles; Serge Fehr of the Cryptology and Information Security Group in Amsterdam; and Vipul Goyal of Microsoft Research in India. A PDF of the paper is here. ®

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