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Note that network coding allows immediate nodes to mix information from different data flows. On the basis of this observation, we prove that under suitable conditions, communicators can communicate with perfect secrecy over wiretap networks. Our method of secure communication over wiretap networks without keys is fundamentally distinct from cryptographic means. Furthermore, a linear network code with perfect secrecy is available for a given wiretap multicast network provided that the cardinality of the alphabet is larger than a constant. Comparing with other approaches, no network capacity is lost by our method.