NSF/ENG/ECCS-BSF: Collaborative Research: Random Channel Cryptography

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Physical-Layer Key Distribution Using Multimode FibersInformation security is of paramount importance in today's connected world. Currently, information is secured through public-key cryptography, which is based on the inability of the state-of-the-art computers to solve certain mathematical problems such as prime number factorization in an efficient manner. Therefore, these cryptographic methods are not secure against advances in computing paradigms and computing power. As a result, quantum key distribution (QKD) has received significant academic and commercial attention in recent years. QKD is fundamentally secure by virtue of the quantum properties of light including the no-cloning theorem and the uncertainty principle. However, QKD cannot satisfy the increasing capacity (key rate and distance) demand of commercial applications. In the meantime, even though classical key distribution (CKD) can provide higher capacity, none of the optical CKD methods proposed so far can guarantee security. Instead, existing optical CKD methods can only provide deterrence to hacking by imposing hacking asymmetry: making equipment for eavesdropping prohibitively more complicated than that for key distribution between legitimate users. Given its importance in today's information-based economy, physical-layer secure key generation and distribution represents a technology gap that can only be addressed by transformative research. We propose a physical-layer key distribution method using multimode fibers, which we call Random Channel Cryptography (RCC), that offers the best of both worlds: capacity and hacking asymmetry of CKD, and security of QKD.Random Channel Cryptography is based on a central result in information-theoretic security that the security of key distribution at the physical-layer is guaranteed as long as the legitimate users have access to a common source of randomness, through channels that are less noisy than the channel of the hacker. We exploit communication channels such as a multimode optical fiber with distributed mode coupling that is inherently random, but deterministically symmetric as a result of reciprocity, for simultaneous key generation and distribution. In RCC, both Alice and Bob send a continuous-wave single-mode laser through an arbitrary degree of freedom in space into a random, spatially-, spectrally- and temporally-varying multidimensional channel, such as a multimode fiber, and both receive the time-varying intensities in the same degree of freedom in space. A common key can be established between Alice and Bob from the measured intensities, which are correlated with each other because the CW lights traverse the reciprocal paths. Neither Alice nor Bob needs to generate a key. Instead, the secure key is generated in a distributed fashion along the multidimensional channel and becomes simultaneously available to Alice and Bob. Security of RCC is enhanced as a consequence of hacking asymmetry. In RCC, Alice and Bob only need to make a small number of measurements whereas, in order to break the key, the eavesdropper must make M simultaneous measurements, where M is the number of fiber modes, which could be on the order of several hundred. Thus, if measurements performed by Alice and Bob represent the state-of-the-art, measurements required of the eavesdropper will be several orders of magnitude beyond the state-of-the-art. We believe that the key rate and distance of RCC can be 10Gb/s and 300 km, respectively, using off-the-shelf components. We propose to conduct research to:- determine the performance limits of RCC, - design methods to reach those limits, and further - prove the security of RCC against general passive and active attacks. In conclusion, RCC has the potential to become a secure, high-capacity key-distribution method for commercial applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.