[rc5] Question for beginner

Jeff Woods jeff at delta.com
Fri Oct 24 10:22:57 EDT 1997

At 01:41 AM 10/24/97 +0300, you wrote:

>Ok, here is my question. What is changing from moving from the 56 bit
>key to the 64?  I know the keyspace expands a lot -how much, really?

The old keyspace had this many keys:      72,057,594,037,927,936
The new keyspace has this many keys:  18,446,744,073,709,551,616

The new keyspace is exactly 256 times as large as the old keyspace.   (Each
successive RSA contest is 256 times as hard.  In other words, there's VERY
LITTLE likelyhood of even RC5-32/12/9 ever being broken, much less the
128-bit keys.)

>But then the algorithm remains the same isn't it? Why we need new

The clients were hardcoded to the algorithm for RC5-32/12/7.  While the
UNDERLYING algorithm is identical, it varies based on the key length, and
thus had to be recoded.   Also, the client PROBABLY communicates on a
different TCP *port*, to differentiate 56-bit results coming in from the
new 64-bit results.  It is a safe bet that at least 10% of the RC5-56
clients will STILL BE OPERATING a month from now, even if the proxy network
is dismantled in the meantime.   The new proxies have to know that the
RESULTS they are receiving are for 64-bit trials.

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