We have beautiful technology called MD5 Hash for banning files, then why file hosting sites never implement this thing on DMCA files?

closed as off-topic by Neil Smithline, Matthew, Ohnana, tim, Rory Alsop May 12 '16 at 18:17

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  • 4
    Why should they? It's not their stuff, and they make money on ads. MD5 is also easy to bypass, just change 1 byte. Side note: YouTube actually does have video recognition software which is used to fight piracy, and sometimes abused for other purposes. – Alexander O'Mara May 11 '16 at 3:27
  • Some services do, but most don't. Managing the list of hashes is usually a more costly overhead than just taking content down whenever you get a notice. – wireghoul May 11 '16 at 4:46

Because every version of a protected file would have to have their MD5 hash submitted, and a pirate can alter the file (add or remove something) and thereby modify the hash.

Hashes are the wrong technology to do what you are thinking. Hashes are good for claiming that 2 files are the same, not that all files must be the same file.

  • 2
    Not to mention pirated media come in many different forms, with different file sizes, codecs, container formats, quality settings, etc. to begin with. – Alexander O'Mara May 11 '16 at 3:56
  • Indeed, cryptographic hashes are entirely the wrong tool for this job. They are designed such that even tiny changes in the input data result in drastically different hashes. What you want is the exact opposite: a hash function where small changes to the input result in small changes of the hash (ideally, cognitively irrelevant changes should result in no change to the hash at all). Such hash functions do exist, this technique is called fingerprinting, and doesn't really have much to do with cryptography. – Jörg W Mittag May 11 '16 at 15:19

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