  The problem of securing a network coding communication system against a
wiretapper adversary is considered. The network implements linear network
coding to deliver $n$ packets from source to each receiver, and the wiretapper
can eavesdrop on $\mu$ arbitrarily chosen links. A coding scheme is proposed
that can achieve the maximum possible rate of $k=n-\mu$ packets that are
information-theoretically secure from the adversary. A distinctive feature of
our scheme is that it is universal: it can be applied on top of any
communication network without requiring knowledge of or any modifications on
the underlying network code. In fact, even a randomized network code can be
used. Our approach is based on Rouayheb-Soljanin's formulation of a wiretap
network as a generalization of the Ozarow-Wyner wiretap channel of type II.
Essentially, the linear MDS code in Ozarow-Wyner's coset coding scheme is
replaced by a maximum-rank-distance code over an extension of the field in
which linear network coding operations are performed.
