  This letter considers a network comprising a transmitter, which employs
random linear network coding to encode a message, a legitimate receiver, which
can recover the message if it gathers a sufficient number of linearly
independent coded packets, and an eavesdropper. Closed-form expressions for the
probability of the eavesdropper intercepting enough coded packets to recover
the message are derived. Transmission with and without feedback is studied.
Furthermore, an optimization model that minimizes the intercept probability
under delay and reliability constraints is presented. Results validate the
proposed analysis and quantify the secrecy gain offered by a feedback link from
the legitimate receiver.
