  In this paper, we show how to exploit real-time communication applications to
determine the IP address of a targeted user. We focus our study on Skype,
although other real-time communication applications may have similar privacy
issues. We first design a scheme that calls an identified targeted user
inconspicuously to find his IP address, which can be done even if he is behind
a NAT. By calling the user periodically, we can then observe the mobility of
the user. We show how to scale the scheme to observe the mobility patterns of
tens of thousands of users. We also consider the linkability threat, in which
the identified user is linked to his Internet usage. We illustrate this threat
by combining Skype and BitTorrent to show that it is possible to determine the
file-sharing usage of identified users. We devise a scheme based on the
identification field of the IP datagrams to verify with high accuracy whether
the identified user is participating in specific torrents. We conclude that any
Internet user can leverage Skype, and potentially other real-time communication
systems, to observe the mobility and file-sharing usage of tens of millions of
identified users.
