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By Gerald Gilbert What’s all this about "unbreakable codes"? Are they possible? "It may well be doubted whether human ingenuity can construct an enigma of the kind which human ingenuity may not, by proper application, resolve." This passage, taken from Edgar Allen Poe’s story "The Gold-Bug," conveys the sense of wonder that attaches to the possibility of developing an unconditionally secret, or "unbreakable," code. Unconditional secrecy was defined mathematically in 1949 by the great information scientist Claude Shannon, who referred to it as "perfect secrecy." In non-mathematical terms, unconditional secrecy prevails if, upon intercepting and analyzing a transmission, and upon performing any analysis whatsoever, the enemy remains unable to determine the content of the message with any greater likelihood than that provided by pure chance alone. Quantum cryptography is a technique that exploits the properties of quantum mechanics to allow separated parties to carry out unconditionally secret communications. MITRE researchers are carrying out groundbreaking experimental and theoretical work in this exciting new field, which offers both promise and specter. On 27 July 2000, MITRE became only the tenth organization in the world to successfully carry out quantum cryptography. At the beginning of September 2000, a MITRE Technical Report on the subject of practical quantum cryptography was publicly released via the international physics preprint repository Web site hosted by the Los Alamos National Laboratory. In July 2001 MITRE implemented the first multiplexed quantum cryptography system. Quantum cryptography combines quantum key distribution (QKD), a technique that utilizes features of quantum mechanics to distribute cryptographic keys between remotely located parties in unconditional secrecy, with subsequent encryption using the method of the Vernam cipher or "one-time pad." Both the initial distribution of the cryptographic key via QKD methods and the encryption using the Vernam method are unconditionally secret. QKD relies upon the Heisenberg Indeterminacy Principle, which fundamentally and crucially distinguishes quantum mechanics from classical mechanics. The important consequence of the Heisenberg Principle here is that any attempt on the part of a would-be eavesdropper to tamper with the transmission in any way will result in an inevitable alteration of the signal states, thus providing a telltale "fingerprint" that cannot be disguised. This "fingerprint" manifests itself as an elevated error rate that can be detected by the authorized communicators. On the theoretical side, in addition to analyzing various fundamental physics questions associated with quantum cryptography, such as the high-speed production and detection of individual photons, MITRE researchers have concentrated on understanding the practical constraints that determine the performance characteristics of actual system implementations. This has led, for the first time, to the determination in precise mathematical detail of all the processing required in order to carry out quantum cryptography via ground-ground, ground-space, and space-space links over both fiber-optic cable and free-space communications channels in real environments. MITRE’s work in this area has been presented at international conferences and universities as well as to various government organizations. On the experimental side, MITRE researchers have conducted successful experimental demonstrations of quantum cryptography over a 10-meter fiber-optic cable at throughput rates that are comparable to those achieved by groups at Los Alamos National Laboratory, IBM, British Telecom, Norwegian Telecom, the University of Geneva, the University of Hokkaido, and elsewhere. The long-term goal of the MITRE effort is to demonstrate quantum cryptography at high throughput rates in the 1 gigabit per second range. MITRE’s theoretical analysis has demonstrated that in order to achieve such high throughput values it is essential to combine, or multiplex, a number of individual quantum cryptography links. To that end, current experimental activities at MITRE include implementing for the first time anywhere a multiplexed, multicast quantum cryptosystem, as well as experimental work intended specifically to explore certain features of actual systems implementations involving satellite communications. The impact of MITRE’s work can be expected to extend across a wide variety of spaceborne and terrestrial communications architectures. MITRE’s activities in quantum cryptography draw from resources across the corporation, which supports the effort directly through the MITRE Technology Program in the Quantum Information Science MITRE Sponsored Research project. MITRE personnel participating in quantum cryptography research are located not only in the Army Systems and Technology Division but also in the Security and Information Operations, National Intelligence, Communications and Networking, and Sensors and Enabling Technology Divisions. The communications architectures of the not-too-distant future will increasingly consist of ultra-transparent optical fibers and all-optical switching. Since practical quantum cryptography makes use of optical photons, it will play a crucially important role in secure communications. For more information, please contact Gerald Gilbert using the employee directory. |
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