###
Question
67. Should One Test for Weak Keys in DES?

DES has four weak keys *k* for which *E*_{k}(E_{k}(m)) = m (see
Question 60). There are also twelve semi-weak keys which come
in pairs* k1* and* k2* and are such that *E*_{k1}(E_{k2}(m))
= m.

Since there are 2^{56} possible DES keys the chance of picking a
weak or semi-weak key at random is 2^{52}. As long as the user-provided
key is chosen entirely at random, they can be safely ignored when
DES is used for encryption. Despite this, some users prefer to
test whether a key to be used for DES encryption is in fact a
weak key. Such a test will have no significant impact on the time
required for encryption.