1 of 7 HIGH DATA RATE QUANTUM NOISE PROTECTED ENCRYPTION OVER LONG DISTANCES T. Banwell, P. Toliver, J. C. Young, J. Hodge, M. Rauch, M. S. Goodman Telcordia Technologies Red Bank, NJ and G. Kanter, E. Corndorf, V. S. Grigoryan, C. Liang, P. Kumar Northwestern University Evanston, IL ABSTRACT We report on a new physical layer optical encryption ap- proach based on quantum noise limited optical signals and M-ary optical phase shift keying that operates at high data rates. In contrast to established encryption methods that rely solely on deterministic algorithms, this system utilizes quantum noise to realize a randomized cipher. Keyed M- ary optical phase modulation is used to encrypt quantum- noise limited mesoscopic signals (50k photons/bit) that are compatible with current directions in optical networking: the physically encrypted signals may be optically ampli- fied, routed through optical switches, and can propagate over long distances approaching 1000 km. We describe the approach in detail and report on results of experiments in which these encrypted signals were transmitted over an 850 km network at 622 Mb/s. Bit-error rate measurements were performed under varying network conditions. Our paper concludes with the engineering challenges for ex- tending this approach to 2.5 Gb/s and beyond and how these are being addressed. INTRODUCTION Information dominance is a pre-requisite to achieving rapid success in future military conflicts. Fiber and free- space optical links are found at the core of the emerging communications systems. High speed encryption plays an important role in assuring that complex communications systems meet essential information assurance objectives [1]. Classical encryption relies on the presumed difficulty of inverting certain mathematical operations in the absence of “key” information [2]. Figure 1(A) illustrates a classical method for encrypting high speed data (Gb/s) using a keyed pseudorandom number generator, such as AES in counter mode, to approximate a one time pad. A binary data stream d k is exclusive OR’ed (XOR) with a pseudorandom cipher stream a k to create a ciphertext stream a k ⊕d k . The XOR approach readily supports data rates exceeding 10Gb/s. Figure 1(A) also illustrates the transmission of this ciphertext over an optical network using DPSK modulation. An eavesdropper viewing the optical signal can determine a k ⊕d k with nearly absolute certainty using an appropriate standard receiver. Classical cryptanalysis attempts to deduce a k and d k using presumed properties of data d k or known weakness in the creation of cipherstream a k . Chained algorithms that operate across blocks of data bits may offer greater security than the simple XOR approach; however, their ciphertext output is generally observable by an eavesdropper and therefore remains susceptible to many methods of cryptanalysis [2]. PRNG + d k ⊕ a k d k a k PRNG d k a k OPTICAL ENCRYPTION OPTICAL TX w k OPTICAL RCVR PRNG IMPERFECT OPTICAL RCVR VERY POOR ESTIMATE OF w k & d k ⊕ a k ALICE EVE (GOAL) ANALYSIS NOT FEASIBLE CLASSICAL OBSERVED SIGNALS QUANTUM NOISE LIMITED CLASSICAL ANALYSIS OF d k ⊕ a k (A) (B) + Figure 1(A) Basic elements of classical encryption: eavesdropper has complete access to ciphertext. Figure 1(B) Proposed optical ( quantum ) encryption limits the eavesdropper to imperfect observation of ciphertext and greatly complicates cryptanalysis. Security against cryptanalysis can be greatly improved if the eavesdropper’s observation of ciphertext a k ⊕d k is se- verely impaired. The AlphaEta approach includes optical encryption methods to create an optical signal that cannot be readily demodulated by an eavesdropper [3-5]. This is accomplished using an optical signal space with a large number of states. These states become nearly indistin- guishable when the signal intensities are chosen such that neighboring states are obscured by inherent quantum shot noise. In contrast to single photon methods, these mesoscopic optical states can be sent long distances over optical networks that include optical amplifiers, filters and multiplexers. In Figure 1(B), a specific “quantum con- strained” optical state is created for each data bit d k by a pseudorandom word w k . The intended recipient possessing