  Increasing knowledge of paedophile activity in P2P systems is a crucial
societal concern, with important consequences on child protection, policy
making, and internet regulation. Because of a lack of traces of P2P exchanges
and rigorous analysis methodology, however, current knowledge of this activity
remains very limited. We consider here a widely used P2P system, eDonkey, and
focus on two key statistics: the fraction of paedophile queries entered in the
system and the fraction of users who entered such queries. We collect hundreds
of millions of keyword-based queries; we design a paedophile query detection
tool for which we establish false positive and false negative rates using
assessment by experts; with this tool and these rates, we then estimate the
fraction of paedophile queries in our data; finally, we design and apply
methods for quantifying users who entered such queries. We conclude that
approximately 0.25% of queries are paedophile, and that more than 0.2% of users
enter such queries. These statistics are by far the most precise and reliable
ever obtained in this domain.
