In cryptology, there are different models of attacks or attackers.
So far we have assumed that an adversary can only read messages during transmission over a channel. Such an adversary is called a passive adversary. An active adversary, on the other hand, can mutilate or delete messages during transmission and/or generate false messages. An attack mounted by an active (resp. a passive) adversary is called an active (resp. a passive) attack. In this book, we will mostly concentrate on passive attacks.
 Throughout the book, resp. stands for respectively.
A two-party communication involves transmission of ciphertext messages over a communication channel. ...