A passive attack on a cryptosystem is one in which the cryptanalyst cannot interact with any of the parties involved, attempting to break the system solely based upon observed data (i.e. the ciphertext). This can also include known plaintext attacks where both the plaintext and its corresponding ciphertext are known.
While most classical ciphers are vulnerable to this form of attack, most modern ciphers are designed to prevent this type of attack above all others.
Attributes
Types of Passive Attacks are 1) Traffic Analysis 2)Release of message contents. Traffic Analysis: In this attack the eavesdropper analyzes the traffic, determine the location, identify communicating hosts, observes the frequency and length of message being exchanged. Using all these information they predict the nature of communication . All incoming and out going traffic of network is analysed but not altered.
Release of message content: A telephonic conversation, an E-mail message or a transferred file may contain confidential data. A passive attack may monitor the contents of these transmission.
Passive attacks are very difficult to detect because they do not involve any alteration of the data. When the messages are exchanged neither the sender nor the receiver is aware that a third party has read the messages. This can be prevented by encryption of data.