[rc5] RC64 keyspace

Rob Winters rob at annapurna.com
Tue Oct 28 08:51:42 EST 1997


On Tue, 28 Oct 1997, Ray Vanlandingham wrote:
> However if you get a 'partial' hit, say a key that decodes the 'T' at the
> start of the message, could you use that information to start eliminating
> keys? I not really that familiar with the encryption mechanism, but if part
> of the key is used for each character, thenit such be possible to start
> only sending out keys that match in that particular 'part' of the key. If
> this would work, it could cut processing time immensely.

No, it's not like in the movies. You have to decode the thing completely
as if each key is the correct key. Then you either have it, or you don't. 

I do wonder whether RSA might intentionally spread the answers out to
different areas of the keyspace. They would want to ensure, for example,
that a brute force search from the bottom would not find their 128-bit key
in minutes :-). For that reason, I think that I would fish around a bit,
and not just go from beginning to end. Might not help, but shouldn't hurt.

 /// Rob

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