Statistical tests can be used for analyzing the statistical behavior of block
ciphers. A strong block cipher should behave like a random permutation of
the plaintext for a random key so that it is impossible to get information
about the key or plaintexts from ciphertexts except by exhaustive search.
Commonly used statistical tests include randomness tests on ciphertext,
correlation tests between plaintext, key, and ciphertext, etc. We want to
remark that good statistical behaviors are only a necessary condition for the
security of block ciphers, and that block ciphers that pass such statistical
tests may well still remain catastrophically weak