The most basic attack that can always be mounted on a block cipher is
that of exhaustive search. (If this is also the best attack available, then the
designer of the cipher has done a good job!) In such an attack, an adversary
obtains a plaintext and its corresponding ciphertext under the secret key
and simply tests each of the possible candidates for the key until a match
is found. If the key has n bits, then there are 2n possible keys to test,
and hence the amount of work for exhaustive search is closely related to
the key size. When key size is larger then the block size, multiple pairs of
plaintext/ciphertext may be needed in an exhaustive search attack.