Cryptography Reference
In-Depth Information
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Figure 10.3
The DES key schedule calculation.
Because of their strange properties, weak and semiweak DES keys should not
be used in practice. Because there are only 16 = 2
4
such keys, the probability of
randomly generating one is only
2
4
2
56
=2
−
52
10
−
16
.
≈
2
.
22
·
This probability is not particularly worrisome. It's certainly equally insecure
to use a very small (large) key, as an adversary is likely to start searching for keys
from the bottom (top). Consequently, there is no need to worry much about weak
and semiweak keys in a specific application setting.
