  We introduce and investigate *targeting adversaries* who selectively attack
users of Tor or other secure-communication networks. We argue that attacks by
such adversaries are more realistic and more significant threats to those most
relying on Tor's protection than are attacks in prior analyses of Tor security.
Previous research and Tor design decisions have focused on protecting against
adversaries who are equally interested in any user of the network. Our
adversaries selectively target users---e.g., those who visit a particular
website or chat on a particular private channel---and essentially disregard Tor
users other than these. We present a model of such adversaries and investigate
three example cases where particular users might be targeted: a cabal
conducting meetings using MTor, a published Tor multicast protocol; a cabal
meeting on a private IRC channel; and users visiting a particular .onion
website. In general for our adversaries, compromise is much faster and provides
more feedback and possibilities for adaptation than do attacks examined in
prior work. We also discuss selection of websites for targeting of their users
based on the distribution across users of site activity. We describe
adversaries both attempting to learn the size of a cabal meeting online or of a
set of sufficiently active visitors to a targeted site and attempting to
identify guards of each targeted user. We compare the threat of targeting
adversaries versus previously considered adversaries, and we briefly sketch
possible countermeasures for resisting targeting adversaries.
