rsync and md4

[jim bell]

At 01:19 PM 6/29/96 -0400, Mark M. wrote:

>> A first guess might be 2^-128 but I know that this sort of thing is
>> rarely that simple. Is md4 that good?
>
>2^-64.
>> 
>> Note that I am not interested in "attacks" on md4 as such as the
>> source of the random data is just another file provided by the same
>> user, so it won't have been specially designed to defeat md4. 
>> 
>> If the probability is within a few orders of magnitude of 2^-128 then
>> can I also be sure that if I only use the first b bits of a md4
>> checksum it will be within a few orders of magnitude of 2^-b ? There
>> is an option in rsync to use a shorter checksum by truncating
>> md4. This saves some bytes on the link at the risk of lowering the
>> confidence. 
>
>The probability of failure is 2^-(b/2).

I don't think that's correct.  That would be the correct formula if I were 
looking for two strings that happened to have the same hash value, but 
that's not what he's asking for.

Jim Bell
jimbell at pacifier.com