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

Dan Oetting oetting at gldmutt.cr.usgs.gov
Mon Oct 4 15:40:20 EDT 1999

At 11:45 -0700 10/4/1999, stoney at sequent.com wrote:
>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.


For RC5-64 there are 2^64 (1.8e+19) keys which expand to 4.7e+20 subkeys or
1.9e+21 bytes. These subkeys will require over 3 Trillion cd's for storage.

One 600MB CD can hold 5.8 million sets of subkeys. A 200Mhz PPC can
generate about700,000 keys per second so could generate those 5.8 million
sets of subkeys in under 8.2 seconds. You would need a 430x CD to read the
subkeys faster than they are already generated.

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