[RC5] Which sub-keyspace comes next

gindrup at okway.okstate.edu gindrup at okway.okstate.edu
Wed May 20 15:45:30 EDT 1998

     Not unless much has changed sine RC5-64 started.  RC5-64 was chosen 
     for a couple of reasons, one of them being that nothing (or at least 
     no more than epsilon) would have to be changed in the key 
     distribution network.  RC5-72 wouldn't have that property so the 
     key-distribution network would have to be overhauled...
     Also, even *I* think RC5-72 is too long to contemplate with the 
     current d.net machine.  There would be 65kilo-subspaces to crack.  
     Unless the DES attack this June rally increases participation, this 
     is *way* out of our league.  I'd quit D.Net if RC5-72 was the next 
     And, for contrast, I was one of the larger proponents for doing 
     RC5-64 when -56 ended.  I saw -64 as still within reach (and current 
     keyrate growth is confirming that).  -72 is too far away unless we 
     use smarter algorithms.
     Speaking of which, I've seen a preprint that claims to be able to 
     reverse a few steps of the encryption phase so that one has a 
     (slightly parametric) partially encrypted message.  In principle, if 
     three rounds could be removed, keyrates would immediately increase 
     by ~33% (because only 3/4 of the work would have to be done) minus a 
     little overhead to see if any set of parameters would match.  It's 
     still essentially brute-force because one still has to check every 
     key, but the amount of work is reduced noticably.
     Anyway...  If RC5-72 were to be started in the next month or two, 
     you can bet that I would be working on OGRs and N-OGRs.
            -- Eric Gindrup ! gindrup at okway.okstate.edu

______________________________ Reply Separator _________________________________
Subject: [RC5] Which sub-keyspace comes next 
Author:  <rc5 at llamas.net> at SMTP
Date:    5/19/98 5:10 PM

Looks like we are going over the 1%-margin today. Might be still a long 
way to go, but if the right key would be found these days, are the current 
clients prepared to do RC5-72 ?
Thomas Grewe
     email: Thomas.Grewe at strz.uni-giessen.de 

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