Brute-force attack

A brute force attack against a cipher refers to breaking a cipher by trying all possible keys. Statistically, if the keys were originally chosen randomly, the plaintext will become available after about 1/2 of the possible keys are tried. The underlying assumption is, of course, that the cypher is known. Since A Kerckoffs first published it, a fundamental maxim of cryptography has been that security must reside ONLY in the key. As Claude E. Shannon said a few decades later, 'the enemy knows the system'. In practice, it has been excellent advice.

As of the year 2002, ciphers with 64 or fewer keys are vulnerable to brute force attacks.

see also Unicity distance