Terrorist attacks on transportation networks have traumatized modern
societies. With a single blast, it has become possible to paralyze airline
traffic, electric power supply, ground transportation or Internet
communication. How and at which cost can one restructure the network such that
it will become more robust against a malicious attack? We introduce a unique
measure for robustness and use it to devise a method to mitigate economically
and efficiently this risk. We demonstrate its efficiency on the European
electricity system and on the Internet as well as on complex networks models.
We show that with small changes in the network structure (low cost) the
robustness of diverse networks can be improved dramatically while their
functionality remains unchanged. Our results are useful not only for improving
significantly with low cost the robustness of existing infrastructures but also
for designing economically robust network systems.Comment: 9 pages, 9 figure