This paper considers secure network coding over networks with unequal link
capacities in the presence of a wiretapper that can wiretap any subset of k
links. Existing results show that for the case of equal (unit) link capacities,
the secrecy capacity is given by the cut-set bound, whether or not the location
of the wiretapped links is known, and can be achieved by injecting k random
keys at the source which are decoded at the sink along with the message. In
contrast, we show that for unequal link capacities, the secrecy capacity is not
the same in general when the location of the wiretapped links is known or
unknown. An example is given to show that when the location of the wiretapped
links is unknown the cut-set bound is not achievable. We give achievable
strategies where random keys are canceled at intermediate non-sink nodes, or
injected at intermediate non-source nodes. Finally, we show that determining
the secrecy capacity is a NP-hard problem.