On Thu, 30 Oct 2008 08:14:48 -0700, Craig Lalley wrote:

> How difficult would it be to modify the MD5 value to a benign
> value...
> camouflage so to speak.

The purpose of MD5 was to provide strong integrity assurance that a
message (file contents) had not been tampered with. It should, in theory,
be enough to change one bit in a file to generate a completely different
MD5 digest from its unmodified ancestor. Given the single pass
construction of the MD5 compression algorithm it would probably be best to
change this bit as close to the beginning of a file as possible.

Of more concern for individuals who prize their liberty, given the
implications of national authorities collecting MD5 hash sums attributed
to "illicit" files, is the weakness of the MD5 algorithm to collisions,
producing duplicate MD5 hash sums from completely different files. In
this case is is not beyond reason to contemplate the scenario wherein an
innocent victim is deliberately sent a file purposely constructed to
duplicate the MD5 sum (but not the contents) of a known pornographic or
other illegal file whose MD5 signature is contained within the national
authority surveillance database.

For such a victim, the resultant turmoil following their being brought
under the suspicion, and likely thereafter the surveillance, of the
authorities can only be guessed at. How many of us relish the thought of
becoming suspected of downloading or receiving (even as spam) files whose
MD5 sums match those of files containing child-pornographic images?

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James B. Byrne mailto:ByrneJB@Harte-Lyne.ca
Harte & Lyne Limited http://www.harte-lyne.ca
9 Brockley Drive vox: +1 905 561 1241
Hamilton, Ontario fax: +1 905 561 0757
Canada L8E 3C3

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