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File Hash Collision First time that it happens to me

#1 User is offline   pier4r 

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Posted 16 March 2017 - 08:47 AM

Just a little interesting fact.

After years of using emule (and mods), I guess I hashed some 30 thousand + files, and I never saw a collision between two different files. All the hashing collisions were, indeed, file duplicates.

Today, now that I'm sharing just 640 files (there were moment with 7000+) I saw a collision between two pdf, that are different at least in one page (the first one).

It was interesting. I know that the hashing function could generate collisions but I did not expect those with a little pool of files, and with contents that are very similar but not equal (therefore, when the content is similar but not equal, I know that an md5 checksum would be way different compared to the reference file).
>>> My wiki (ITA) on emule >>>Feature Request (ICS) or SOTN, ClientAnalyzer(fixing fastXs and reask punishment),, EmuleCollectionV2 >>> Emule on old hardware (intel pentium 2 or 3 - via c3 - and so on) with good OS settings and enough ram (256+ mb): great >>>user of: eMule - Xtreme - ZZUL bastard - SharX - SharkX 1.8b5 pierQR - ZZUL-Tra - ZZUL-Tra-TL - kMule - Beba

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#2 User is offline   Some Support 

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Posted 17 March 2017 - 09:13 AM

Well MD4 is long broken in that regard, however is you use an extended link (Filesize + SHA + MD4) I'd say collision (intentional or not) are pretty much impossible at this point.

#3 User is offline   fox88 

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Posted 18 March 2017 - 08:51 AM

View Postpier4r, on 16 March 2017 - 11:47 AM, said:

and with contents that are very similar but not equal (therefore, when the content is similar but not equal, I know that an md5 checksum would be way different compared to the reference file).

Avalance effect is one of the design goals, but MD5 article in wiki has an example of two data blocks with the same hash and differing in 6 high bits only.
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#4 User is offline   pier4r 

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Posted 20 March 2017 - 06:53 PM

View PostSome Support, on 17 March 2017 - 10:13 AM, said:

Well MD4 is long broken in that regard, however is you use an extended link (Filesize + SHA + MD4) I'd say collision (intentional or not) are pretty much impossible at this point.


I know that the md4 is not "foolproof" but for practical purposes should be fine with file hashes unless those files are very particular. So was interesting that two documents, about the same topic but with different content, collided. Just this. No big problems actually.

Would be more interesting with a large pool of files.
>>> My wiki (ITA) on emule >>>Feature Request (ICS) or SOTN, ClientAnalyzer(fixing fastXs and reask punishment),, EmuleCollectionV2 >>> Emule on old hardware (intel pentium 2 or 3 - via c3 - and so on) with good OS settings and enough ram (256+ mb): great >>>user of: eMule - Xtreme - ZZUL bastard - SharX - SharkX 1.8b5 pierQR - ZZUL-Tra - ZZUL-Tra-TL - kMule - Beba

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