Читать книгу Cryptography, Information Theory, and Error-Correction - Aiden A. Bruen - Страница 70

There are many attacks on cryptosystems, i.e. attempts by an intruder to break the system by recovering the key and the message or the message directly. By far, the attack most difficult to defend against is an impersonation or man‐in‐the‐middle attack. Another type of attack is a brute‐force attack (see Section 7.3) in which the attacker systematically tries all possible keys on the key‐space. A variety of attacks are discussed in Chapter 7.

A basic issue in cryptography is this: If we are trying to guess one of possible passwords in order to log on (or possible keys say), then how many guesses will we have to make on average until we are successful? The answer is easy to find.