Active attack

In cryptography an active attack on a communications system is one in which the attacker changes the communication. He may create, 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 attacks include:

• 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

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. The attacker must not only intercept messages, break whatever cryptography is in use, and send off his bogus message; he also has to block delivery of the genuine message. Moreover, he has to do it all in real time, fast enough to avoid alerting his victims and to beat whatever synchronisation mechanisms the network may be using. A cryptosystem that an enemy can break in hours or days would generally be considered insecure, even worthless, but it will prevent active attacks as long as the enemy cannot break it quickly enough to replace messages.

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. Systems that combine these techniques are called hybrid cryptosystems.