Actually, it's better than that. It's the hash of the whole file. The file size is determined by the key, but the minimum is 1kB IIRC. It's a rolling hash of the whole file...
On Wed, Sep 10, 2003 at 06:45:37PM -0400, Greg Wooledge wrote: > [EMAIL PROTECTED] ([EMAIL PROTECTED]) wrote: > > On Wed, 10 Sep 2003 13:53:49 -0700 =?iso-8859-1?Q?J=E9r=F4me_ou_Sally_Bonnet?= > > <[EMAIL PROTECTED]> wrote: > > >Is it possible to design a program that creates a file that matches > > >a specific CHK, so that it would be possible to prevent users from > > >accessing a specific file by injecting the colliding file in the > > >network before the target file becomes too popular? > > > > Yes, it is possible. But it is computationally very hard. For any given > > CHK there are an infinite number of files that yield that CHK after insertion. > > The problem is finding one of them. > > We're using SHA-1 (160 bit) hashes for this, right? Let's assume > so (the math is similar for other hashes). Let's assume that the > distribution of hashes over the space of possible files is perfectly > random. > > 2^160 is 1461501637330902918203684832716283019655932542976, which is > how many different possible hashes there are. On the average, you'd > have to create and hash half this many files (2^159) to find one > that generates the target CHK. > > Let's assume you can do 1000 of these files every second. Oh hell, > let's say you can do over a million (2^10) per second. That means > you'd have to hash files for somewhere around 2^149 seconds to find > one that matches. That's about 22613994246837820632943248145966286506 > *years* worth of work, give or take a little. > > -- > Greg Wooledge | "Truth belongs to everybody." > [EMAIL PROTECTED] | - The Red Hot Chili Peppers > http://wooledge.org/~greg/ | > _______________________________________________ > Devl mailing list > [EMAIL PROTECTED] > http://dodo.freenetproject.org/cgi-bin/mailman/listinfo/devl -- Matthew J Toseland - [EMAIL PROTECTED] Freenet Project Official Codemonkey - http://freenetproject.org/ ICTHUS - Nothing is impossible. Our Boss says so.
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