(S)DES input/key complement property
- P3.10 (a)
-
Using input X' and key K', both inputs to the first XOR
are complemented, therefore the XOR output, and the result
of F, will be the same as using uncomplemented X and K
The second XOR has inputs L' and F, therefore its output
is complemented.
- P3.10 (b)
-
Due to the (S)DES complement property, we can get two encryptions
for the price of one, since after computing Y = K{X} we know
that Y' = K'{X'}. But this does not seem to reduce the work
needed for a brute-force attack on a particular key K, since
K' is a different key.
If (S)DES had the property Y'=K'{X} then the search space would
be reduced by 1/2; you'd compute Z=K{X} and if Z==Y then K is
the key, and if Z==Y' then K' is the key.
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