A "brute-force attack" can be made on any cryptographic algorithm that uses keys. It's only occasionally the best attack possible on an algorithm (or protocol), but it always sets an upper boundary on how good an algorithm can be. A brute-force attack is nothing more than an attempt to guess every possible key that might be in use. For example, Mallory might intercept an encrypted message and wish to determine its plain text. To do this, Mallory tries decrypting using key index one, then tries with key index two, and so on. Of course, Mallory needs to determine when he has hit upon the correct decryption key. There are things the encryptor Alice can do to make Mallory's job in this determination more difficult, but in most systems, Mallory will not have too much trouble knowing when he has guessed the right key.