  Cyber-physical attacks impose a significant threat to the smart grid, as the
cyber attack makes it difficult to identify the actual damage caused by the
physical attack. To defend against such attacks, various inference-based
solutions have been proposed to estimate the states of grid elements (e.g.,
transmission lines) from measurements outside the attacked area, out of which a
few have provided theoretical conditions for guaranteed accuracy. However,
these conditions are usually based on the ground truth states and thus not
verifiable in practice. To solve this problem, we develop (i) verifiable
conditions that can be tested based on only observable information, and (ii)
efficient algorithms for verifying the states of links (i.e., transmission
lines) within the attacked area based on these conditions. Our numerical
evaluations based on the Polish power grid and IEEE 300-bus system demonstrate
that the proposed algorithms are highly successful in verifying the states of
truly failed links, and can thus greatly help in prioritizing repairs during
the recovery process.
