# DES algorithm exhaustive key search 2^55 instead of 2^56 [duplicate]

How is it possible to do an exhaustive key search (DES algorithm) and only search for 2^55 instead of 2^56 keys ?

This is a slide from my course if this helps you. I don't understand it.

## marked as duplicate by Maarten Bodewes♦, otus, fgrieu, e-sushiSep 21 '16 at 13:15

• In the question's quote, underline (rather than the usual overline as in $\overline K$) is used for bitwise complement. – fgrieu Sep 21 '16 at 5:54