  This paper considers the problem of securing a linear network coding system
against an adversary that is both an eavesdropper and a jammer. The network is
assumed to transport n packets from source to each receiver, and the adversary
is allowed to eavesdrop on \mu arbitrarily chosen links and also to inject up
to t erroneous packets into the network. The goal of the system is to achieve
zero-error communication that is information-theoretically secure from the
adversary. Moreover, this goal must be attained in a universal fashion, i.e.,
regardless of the network topology or the underlying network code. An upper
bound on the achievable rate under these requirements is shown to be n-\mu-2t
packets per transmission. A scheme is proposed that can achieve this maximum
rate, for any n and any field size q, provided the packet length m is at least
n symbols. The scheme is based on rank-metric codes and admits low-complexity
encoding and decoding. In addition, the scheme is shown to be optimal in the
sense that the required packet length is the smallest possible among all
universal schemes that achieve the maximum rate.
