# 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.

• I've voted to close this question as this has already been explained in an answer. If you need more detail, please specify what you don't understand or want detailed from the given answer in the duplicate. – Maarten Bodewes Sep 20 '16 at 22:14
• 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
• @fgrieu Maybe they wanted to mirror the usual notation :P – Maarten Bodewes Sep 21 '16 at 7:04