Date Added: Oct 2010
In the standard model of authentication theory, a transmitter wants to send some information to a receiver across an insecure channel while an opponent with access to the channel wants to deceive the receiver. The opponent can either insert new messages into the channel, or intercept messages from the transmitter and modify them into his own. In each case, the opponent's goal is to deceive the receiver into believing that the new messages are authentic (coming from the transmitter). The first attack based on insertion of new messages is known as impersonation and the second attack based on modification of messages from the transmitter is known as substitution.