Examining the infrastructure in order to gain information.
Examples include watching air ticketing information in order to see when
particular people go to particular places and using this as an intelligence
indicator, tapping a PBX system in order to record key telephone
conversations, and watching for passwords on the Internet in order to gain
identification and authentication information to multiple computers.
Complexity: Except in cases where cryptography, spread spectrum, or other
similar technology is used to defend against such an attack, it appears that
infrastructure observation is simple to accomplish and expensive to detect.
No mathematical analysis has been published to date.