[RC5] DES-II question

Marc Sissom marcus at dfwmm.net
Wed Mar 25 19:07:01 EST 1998


Petr Novotny wrote:
> 
> Hi,
> 
> I was just curious - how many "false positives" were there during
> DES-II contest? (I hope this information is not classified...)

I bet it is classified since the number is directly related to the
number of plain/cypher-text that are tested on each key.

Test one byte, then 1 in 256 will yield a positive.
Test two, then 1 in 65535, three is 2^24, etc. 

We must assume that they test at least four bytes, else every
28 bit block would have at least one false positive. This assumes
that the keys that produce false positives for any one particular
"real" key are evenly distributed across the keyspace. I have no
idea whether or not it is true. I assume that it is because it
would otherwise be an exploitable weakness in the algorithm.
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