[RC5] 999 to 1000 blocks and P5: MMX vs Classic

Mike S wickedchicken at plasticpenguins.com
Fri Jan 16 17:44:21 EST 2004

Tom Laermans wrote:
> I thought there was some problem with the fact that now 72 bits were
> needed and those aren't as easily stored in MMX registers as 64 bits or
> so..

"RC5 is word-oriented: all of the basic computational operations have 
w-bit words as inputs and outputs. RC5 is a block-cipher with a two-word 
input (plaintext) block size and a two-word (ciphertext) output block 
size. The nominal choice for w is 32 bits, for which RC5 has 64-bit 
plaintext and ciphertext block sizes."

D.net uses a 32-bit wordsize, so all of the computations are done within 
a standard 32-bit register. MMX can help because it is possible to do A) 
two keys in tandem (like mine) or B) do 64 keys in tandem and simulate 
gates (a DES core did this, I think it was called bitslice). Note that 
the word size has nothing to do with the key size.


wickedchicken at plasticpenguins.com

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