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

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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.

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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@pacifier.com