  This letter studies the problem of maintaining information freshness under
passive eavesdropping attacks. The classical three-node wiretap channel model
is considered, in which a source aims to send its latest status wirelessly to
its intended destination, while protecting the message from being overheard by
an eavesdropper. Considering that conventional channel capacity-based secrecy
metrics are no longer adequate to measure the information timeliness in status
update systems, we define two new age of information-based metrics to
characterize the secrecy performance of the considered system. We further
propose, analyze, and optimize a randomized stationary transmission policy
implemented at the source for further enhancing the secrecy performance.
Simulation results are provided to validate our analysis and optimization.
