[HARDWARE] Read blocks of subkeys from storage instead of calculate

stoney at sequent.com stoney at sequent.com
Mon Oct 4 12:45:17 EDT 1999


Since most of the time in RC5 is spent calculating the 
2r + 2 subkeys for each key where r is the number of rounds and
the subkeys are dependent on the key.

What if you read subsets of the set of subkeys instead of generating
the subkeys.  This would reduce the complexity of the algorithm
since you would only calculate the subkeys for the keyspace once.
You would be able to concentrate on accelerating the actual decryption.

You could use a storage/memory hiearchy like
  1. cdrom/dvd (all of the subkeys)
  2. ide/scsi
  3. sdram
  4. sram
               sram  sram sram
  dvd   scsi   xcel  xcel xcel 
   |    |      |     |    |
   \____|______|_____|____|

It seems as if Rivest had the right idea by creating an algorithm
that is ideal for certian microprocessors you eliminate the 
advantage that LOW COST special purpose hardware gives you over 
certian general purpose microprocessors.  You also reduce your 
risk to brute force attacks by increasing the complexity going 
from key to key.  Also the assignments are dependent upon the 
result of the previous assignment.  So the operations in a 
single decryption engine are serialized.

Stoney



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