  In this paper, security analysis of block ciphers with key length greater
than block length is proposed. When key length is significantly greater than
block length and the statistical distribution of cipher system is like a
uniform distribution, there are more than one key which map fixed input to
fixed output. If a block cipher designed sufficiently random, it is expected
that the key space can be classified into same classes. Using such classes of
keys, our proposed algorithm would be able to recover the key of block cipher
with complexity O(max(2^n, 2^{k-n}) where n is block length and k is key
length. We applied our algorithm to 2- round KASUMI block cipher as sample
block cipher by using weakness of functions that used in KASUMI.
