  We consider the problem of detecting and locating modifications in signed
data to ensure partial data integrity. We assume that the data is divided into
$n$ blocks (not necessarily of the same size) and that a threshold $d$ is given
for the maximum amount of modified blocks that the scheme can support. We
propose efficient algorithms for signature and verification steps which provide
a reasonably compact signature size, for controlled sizes of $d$ with respect
to $n$. For instance, for fixed $d$ the standard signature size gets multiplied
by a factor of $O(\log n)$, while allowing the identification of up to $d$
modified blocks. Our scheme is based on nonadaptive combinatorial group testing
and cover-free families.
