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I think the question is that how is it useful if you happen to have two hashes that match - it's very unlikely that they will be

Where the problem comes is when you can generate a matching 128 bit hash with your own content without having to do 2^128 different hashes (or strike ridiculously (lottery winner being struck by lightning) lucky and only need to do 2^100 hashes)

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Lets say you have an anti-malware service that sees file1, analyzes it, and decides the file is benign. The service then updates a database saying 'hash xyz has been analyzed and is benign'.

Then you send file2 which is actually malicious, and the service decides it has already analyzed this file and lets it go.

This would work on a lot of big AV appliances and software. A surprising amount of these are still using md5 for de-duplication.

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