  We study the security of interaction protocols when incentives of
participants are taken into account. We begin by formally defining correctness
of a protocol, given a notion of rationality and utilities of participating
agents. Based on that, we propose how to assess security when the precise
incentives are unknown. Then, the security level can be defined in terms of
defender sets, i.e., sets of participants who can effectively "defend" the
security property as long as they are in favor of the property.
  We present some theoretical characterizations of defendable protocols under
Nash equilibrium, first for bijective games (a standard assumption in game
theory), and then for games with non-injective outcomes that better correspond
to interaction protocols. Finally, we apply our concepts to analyze fairness in
the ASW contract-signing protocol.
