# Re: Complex Theoretical One Way Hash Question

• From: none <""mike\"@(none)">
• Date: Tue, 18 Apr 2006 16:50:34 +0100

xmath wrote:
none wrote:

Another question : take the MD5 hash of the phrase " the MD5 hash of
this phrase is n", where n is a string of hex digits. For what number n
is this true? Birthday paradox, should find one in about 2^64 tries...

There's no birthday paradox here, since there's no collision between
two elements of a set. Finding a match to that question has only
chance 2^-k on every try (k = size of hash in bits) regardless of how
many attempts you made. On average you'll therefore only find a
solution after 2^(k-1) tries, which is too much even when using MD5.

- xmath

I must admit that I jumped to conclusions here, assuming that this was similar to a collision, rather than the 2^128 case of finding n for which md5(phrase,n)=(some_fixed_number). Unfortunately, I am not adept enough to be sure either way, but I will take your word for it.
(/me goes back to the textbooks)
P.S. if you read my other post describing the competition, can I assume that my beer money is safe? And no, I wont raise the prize to 2^64 beers :)
.

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