may not be as bad as it appears

may not be as bad as it appears

Posted Aug 19, 2004 7:24 UTC (Thu) by ekj (subscriber, #1524)
In reply to: may not be as bad as it appears by ajax
Parent article: Crypto researchers abuzz over flaws (News.com)

That is not nessecarily so.

It depends on the details of the flaw. If the attack depends on custom-crafting the entire input, or worse yet, both inputs, to find a collision, then you are correct.

But it's possible to change only 20 bytes in a file and make the sha1sum equal. That little "garbage" could easily fit in say a comment in C code or an unused static variable in a binary program. The trick is, offcourse, how to select those 20 bytes.

With a good (as in cryptographically strong) hash there's no better way to do that than simply randomly try different garbage-strings until you find one that matches. That is impractical for a hash of sufficient size.

With a broken hash, all bets are off. It depends on the details.

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