Information leakage

Information leakage happens whenever a system that is designed to be closed to an eavesdropper reveals some information to unauthorized parties nonetheless. For example, when designing an encrypted instant messaging network, a network engineer without the capacity to crack encryption codes could see when messages are transmitted, even if he could not read them. During the Second World War, the Japanese for a while were using secret codes such as PURPLE; even before such codes were cracked, some basic information could be extracted about the content of the messages by looking at which relay stations sent a message onward.