  The problem of securing a network coding communication system against an
eavesdropper adversary is considered. The network implements linear network
coding to deliver n packets from source to each receiver, and the adversary can
eavesdrop on \mu arbitrarily chosen links. The objective is to provide reliable
communication to all receivers, while guaranteeing that the source information
remains information-theoretically secure from the adversary. A coding scheme is
proposed that can achieve the maximum possible rate of n-\mu packets. The
scheme, which is based on rank-metric codes, has the distinctive property of
being universal: it can be applied on top of any communication network without
requiring knowledge of or any modifications on the underlying network code. The
only requirement of the scheme is that the packet length be at least n, which
is shown to be strictly necessary for universal communication at the maximum
rate. A further scenario is considered where the adversary is allowed not only
to eavesdrop but also to inject up to t erroneous packets into the network, and
the network may suffer from a rank deficiency of at most \rho. In this case,
the proposed scheme can be extended to achieve the rate of n-\rho-2t-\mu
packets. This rate is shown to be optimal under the assumption of zero-error
communication.
