[RC5] Thoughts on cracking DES

Tim Charron tcharron at interlog.com
Mon Jan 12 23:21:51 EST 1998


>  1.1  Get a 64-bit key from the user. (Every 8th bit (the least
> significant bit of each byte) is considered a parity bit. For a key to
> have correct parity, each byte should contain an odd number of "1"
> bits.)
> 
> 
> 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.

Of all 2^64 possible 64 bit keys, only 2^56 of them will have proper 
parity.  This is why the DES keyspace is 2^56 in size.  

However, Distributed.net will be distributing 2^55 keys, and the core 
routines used will check pairs of complementary keys.  This is why 
(for example) the clients will show 'Benchmarking 2000000 keys' when 
a benchmark for DES is run.

-- Tim
Tim Charron
tcharron at interlog.com
tcharron at ctfinance.com
http://www.interlog.com/~tcharron/

An idle computer is a terrible thing!  http://www.distributed.net/

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