# [RC5] Is this calculation wrong? If so why?

Kwoody kwoody at citytel.net
Wed Jan 14 23:14:43 EST 1998

``` Can someone explain this simple calc as it makes me curious:

my block size is 2^30 which = 1073741824 keys. This machine process's
des keys at about 126kkeys/sec so:

2^30/126kkeys/sec = 8521 seconds/60 = 142 minutes which is 2 hours 22
mins.

But doing a des block is taking 4 hours 43 mins which is double what
this
calculation shows.

Doing rc5 stuff of 2^28 on this machine gets about 63kkeys/sec so a
simple calc works to this:

2^28/63kkeys/sec = 4260 seconds/60 = 71 minutes which is 1 hour 11 mins
which is how long it took to process an rc5 block exaclty (give or take
a
minute depending on process's running and system load of course)

This is on a dx2/66.
This is sort of apple and oranges type comparison as the block sizes are
different but is the above calculation right?

But on my cyrix 166 an rc5 block took 16 mins at about 275kkeys/sec. For
des it does about 483kkeys/sec so a simple calculation:

2^28/483kkeys/sec = 555 seconds/60 = 9 mins per block, but in acutal
fact this machine does it in double the time: 18 mins.

Have I expressed these values correctly? Or am I way off on what Ive
done here? Are des keys double the size of an rc5 key? I like math but
Ive never been especially good at it.

thanks,
keith

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