# Secure Network Coding for Multiple Unicast: On the Case of Single Source

## Abstract

This paper considers multiple unicast wireline noiseless networks where a single source wishes to transmit independent messages to a set of legitimate destinations. The primal goal is to characterize the secure capacity region, where the exchanged messages have to be secured from a passive external eavesdropper that has unbounded computational capabilities, but limited network presence. The secure capacity region for the case of two destinations is characterized and it is shown to be a function of only the min-cut capacities and the number of edges the eavesdropper wiretaps. A polynomial-time two-phase scheme is then designed for a general number of destinations and its achievable secure rate region is derived. It is shown that the secure capacity result for the two destinations case is not reversible, that is, by switching the role of the source and destinations and by reversing the directions of the edges, the secure capacity region changes.

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