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Re^7: On showing the weakness in the MD5 digest function and getting bitten by scalar context

by Anonymous Monk
on Aug 28, 2004 at 22:16 UTC ( #386657=note: print w/ replies, xml ) Need Help??


in reply to Re^6: On showing the weakness in the MD5 digest function and getting bitten by scalar context
in thread On showing the weakness in the MD5 digest function and getting bitten by scalar context

Maybe I misunderstood what I read? Maybe, there is an application that allows the fact the the alternative plaintext is gibberish to be ignored and compromise something?
Don't know if I got that correctly, but sabotaging a p2p network could be such an application. If a p2p network uses hashes to identify files, you could use this weakness to supply gibberish data (parts) to prevent successful downloads.


Comment on Re^7: On showing the weakness in the MD5 digest function and getting bitten by scalar context
Re^8: On showing the weakness in the MD5 digest function and getting bitten by scalar context
by BrowserUk (Pope) on Aug 28, 2004 at 23:07 UTC

    Yes, I think that would work.

    A workaround might be quite easy though.

    Produce 2 md5s. One from the whole file and another from the file minus 1 byte (first, last or middle). Or make the second md5 just the first half of the file; or from just the 10th, 20th, 30th etc. bytes (or whichever bytes the attack modifies to compromise the md5).

    Now the attackers not only have the task of finding a duplicate file with the same md5, they have to produce one that matches two md5s.

    Again, my math lets me down, but doesn't that make their job much, *much* harder?


    Examine what is said, not who speaks.
    "Efficiency is intelligent laziness." -David Dunham
    "Think for yourself!" - Abigail
    "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon

      Yes, it's probably possible to come up with a workaround that would allow us to continue using MD5 securely, much like triple DES allows us to keep using DES. Now that there's a good alternative to DES, though (namely AES), there's no reason to keep using triple DES (which is slow). There's already a good alternative to MD5, so there's no reason to kludge up a workaround.

      Note that nobody ever uses double DES. That's because it is vulnerable to a meet-in-the-middle attack. Secure workarounds are hard to design.

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