Brute Force Attack

This consists of simply going through every possible key, deciphering the ciphertext with each key, and trying to find some recognizable cleartext in the result. If you use a 128-bit key there are 2128 possible keys.

A would-be code-breaker would regard this type of attack as the last resort, to be used only if there is no feature - that is, weakness - of the cipher that makes possible some more intelligent attack. A cipher that forces the code-breaker to resort to this method is a strong cipher.

However, ciphers that use weak (small) keys are susceptible to brute force attack, in particular RC4-40, RC2-40 and DES-56. There have been many successful brute force attacks against RC4-40 since its release in February 1995.

A brute force attack against ciphers with 128 bits or more is probably impractical in the foreseeable future.