Active attack

In cryptography an active attack on a communications system is one in which the attacker changes the communication. He may forge, alter, replace, block or reroute messages. This contrasts with a passive attack in which the attacker only eavesdrops; he may read messages he is not supposed to see, but he does not alter messages.

Active attack include: Successful active attacks are devastating; if the attacker can replace messages and have them taken as genuine, it is all over. The security system is then at best worthless; at worst it is of great value to the enemy. Fortunately, active attacks are often hard to execute.
 * man-in-the-middle attack; the attacker tricks both communicating parties into communicating with him; they think they are talking to each other
 * rewrite attacks; the attacker can replace a message with anything he chooses

Moreover, cryptographic authentication provides a complete defense against active attackers. Public key techniques are often used to authenticate people or devices, to ensure communication is with the right party and prevent man-in-the-middle attacks. Packets or messages are often authenticated with a hashed message authentication code; this prevents rewrite attacks, among others.