[RC5] Thoughts on cracking DES

Sanford Olson sanford at msn.fullfeed.com
Mon Jan 12 22:44:27 EST 1998


At 08:45 PM 1/12/98 -0700, you wrote:

[snip]
>OK, from this, it would seem to me that there are not 2^56-1 possible key
>values, but in fact, there are only (2^56-1)/2 possibilities, due to the
>fact that one half of the values are going to generate keys with invalid
>parity.


The key is 8 bytes (64-bits including the parity bits).  Thus, in each
byte, the 7 "key" bits can have any value and then you set the parity bit
accordingly.  8 bytes of 7 "key" bits yields a 56-bit key.  So, we could
check all 2^64 possible 64-bit keys, testing each one for correct parity
(only 1 in 256 will pass the parity check) before doing the DES crunching
or we can test all 2^56 possible 56-bit keys, expanding them into the
required 64-bit parity format.  The latter way will be *much* faster.

- Sanford

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