  Secure Message Transmission (SMT) is a two-party cryptographic protocol by
which the sender can securely and reliably transmit messages to the receiver
using multiple channels. An adversary can corrupt a subset of the channels and
commit eavesdropping and tampering attacks over the channels. In this work, we
introduce a game-theoretic security model for SMT in which adversaries have
some preferences for protocol execution. We define rational "timid" adversaries
who prefer to violate security requirements but do not prefer the tampering to
be detected.
  First, we consider the basic setting where a single adversary attacks the
protocol. We construct perfect SMT protocols against any rational adversary
corrupting all but one of the channels. Since minority corruption is required
in the traditional setting, our results demonstrate a way of circumventing the
cryptographic impossibility results by a game-theoretic approach.
  Next, we study the setting in which all the channels can be corrupted by
multiple adversaries who do not cooperate. Since we cannot hope for any
security if a single adversary corrupts all the channels or multiple
adversaries cooperate maliciously, the scenario can arise from a game-theoretic
model. We also study the scenario in which both malicious and rational
adversaries exist.
