[rc5] Question for beginner

Wes Shull wshull at scicom.com
Mon Oct 27 22:10:03 EST 1997


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I sent this to the list earlier, but never saw it...  I'm thinking 
maybe I sent it from another email address and the list is too 
spamproof for its own good.  If you've already seen this, my 
apologies.

>Eric Gindrup <gindrup at okway.okstate.edu> wrote:
>
>>        I'm sort of curious if the new clients post about 88%
>performance 
>>     of the old clients (in keys/sec) due the the 7/8 utility
>multiplier...

At 07:49 PM 10/24/97 -0700, Barry Nathan wrote:

>I'm getting over 99% of the 56-bit performance with the 64-bit 
client
>on my Mac...

Then, Joseph Fisk asked:

>"The 7/8 utility multiplier" <- what exactly is that?

I'm guessing it's supposed to take account the fact that the old keys 


were 7 bytes and the new keys are 8 bytes, so the new clients would 
only be 7/8 the speed of the old.

Fortunately for us, though, it *doesn't* scale like that in this 
case.  As long as you 
stick with RC5-32/12/X, the only thing that changes for different 
values of X is how long it takes to copy the key initially at the 
beginning of the expansion.  The expansion algorithm's loop still 
does the same number of rounds (3*(2*(r+1))) = 78 in these cases.  
What it does within the loop is not dependent on the key size.  The 
encryption/decryption algorithms just use the expanded key table, the 

size of which is solely dependent on the number of rounds (r=12).

End result:  it should only be very, very slightly slower than the 56-

bit-key version.  <1%.

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-- 
Wes Shull * wes at kuoi.asui.uidaho.edu * Prince William County, Virginia, USA
Radio Amateur (KC7QCY) * Porsche 928 Fan * Ford Festiva Owner * cat lover
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