Passive attacks attempt to learn or make use of information from the system but do not affect system resources. A passive attack is one where the attacker only monitors the communication channel. A passive attacker only threatens the confidentiality of data.
Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. The goal of the opponent is to obtain information that is being transmitted.
Two types of passive attacks are related to message contents and traffic analysis: