[RC5] d.net progress

gindrup at okway.okstate.edu gindrup at okway.okstate.edu
Fri Nov 21 14:30:30 EST 1997


     The plaintext is not completely known.
     
     I was intending that to mean that someone has done the theoretical 
     work to show that some significant portion of the keyspace can be a 
     priori eliminated due to the concerns I listed.  We aren't applying 
     this information until *after* trying the decrypt.  And even then, we 
     don't apply all of it.  I.e., the clients *only* check bits 32-63 if 
     bits 0-31 match the text "The " (of "The unkn").  No attempt has been 
     made (well as far as anybody who reads this list and hasn't been able 
     to pore over the source knows) to require that the rest of the message 
     decrypt to printable ASCII, et c...
            -- Eric Gindrup ! gindrup at Okway.okstate.edu


______________________________ Reply Separator _________________________________
Subject: Re: [RC5] d.net progress 
Author:  <rc5 at llamas.net > at SMTP
Date:    1997/11/21 12:00


gindrup at okway.okstate.edu wrote:
     
>      But, remember, a *real* effort would use the information that the
>      first several bytes of the decrypted message are "The unknown message 
>      is: ", the next byte is capitalized ASCII and all the remaining bytes 
>      are printable ASCII characters followed by some (possibly empty)
>      sequence of NULLs (padding).
>      We aren't even remotely using all this information.  We're attacking 
>      by pure brute-force.
     
I was under the impression that we *were* using this information.  I certainly 
was
when I whipped together an RC5-40 cracker a couple of weeks ago (which was 
woefully
inadequate to the task, if you must know ... one PPro 200 just doesn't cut it 
for
breaking RC5-40 though my program was not as optimized as it could be).
     
My impression is that the RSA contest is a known plaintext, known ciphertext 
attack.
     
Keith W. Jones -- keith at streamdata.com
     
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