  We propose the notion of {\it resistance of a graph} as an accompanying
notion of the structure entropy to measure the force of the graph to resist
cascading failure of strategic virus attacks. We show that for any connected
network $G$, the resistance of $G$ is
$\mathcal{R}(G)=\mathcal{H}^1(G)-\mathcal{H}^2(G)$, where $\mathcal{H}^1(G)$
and $\mathcal{H}^2(G)$ are the one- and two-dimensional structure entropy of
$G$, respectively. According to this, we define the notion of {\it security
index of a graph} to be the normalized resistance, that is, $\theta
(G)=\frac{\mathcal{R}(G)}{\mathcal{H}^1(H)}$. We say that a connected graph is
an $(n,\theta)$-{\it resistor graph}, if $G$ has $n$ vertices and has security
index $\theta (G)\geq\theta$. We show that trees and grid graphs are
$(n,\theta)$-resistor graphs for large constant $\theta$, that the graphs with
bounded degree $d$ and $n$ vertices, are $(n,\frac{2}{d}-o(1))$-resistor
graphs, and that for a graph $G$ generated by the security model
\cite{LLPZ2015, LP2016}, with high probability, $G$ is an $(n,\theta)$-resistor
graph, for a constant $\theta$ arbitrarily close to $1$, provided that $n$ is
sufficiently large. To the opposite side, we show that expander graphs are not
good resistor graphs, in the sense that, there is a global constant
$\theta_0<1$ such that expander graphs cannot be $(n,\theta)$-resistor graph
for any $\theta\geq\theta_0$. In particular, for the complete graph $G$, the
resistance of $G$ is a constant $O(1)$, and hence the security index of $G$ is
$\theta (G)=o(1)$. Finally, we show that for any simple and connected graph
$G$, if $G$ is an $(n, 1-o(1))$-resistor graph, then there is a large $k$ such
that the $k$-th largest eigenvalue of the Laplacian of $G$ is $o(1)$, giving
rise to an algebraic characterization for the graphs that are secure against
intentional virus attack.
