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I recently download a torrent and am curious about a part of the file name, which is about eight characters long with random numbers and letters.

Example filename of a video which I downloaded:

[SF]_Naruto_Shippuuden_205_(720p_AAC)[eddcc0fe].mp4

The part I'm curious about: eddcc0fe.

Is that some kind of video ID? What is it called and how do they get it?

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3 Answers 3

It could be the CRC32 checksum of the file.

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These eight characters are in fact a number – in hexadecimal. More precisely, it's the CRC-32 checksum (also called a hash) of the entire file. It's likely to have been put in there to let you check whether your file is not corrupted or truncated.

Regarding the choice of CRC: Algorithms such as MD5 and SHA are much more common these days, but their output is much longer (starting with 128 bits, or 32 hex digits), and CRC, although weak, serves its purpose well of detecting environmental damage.


Personally, I see little point in that. BitTorrent already includes a hash (hashes for every piece actually) and every piece is verified during the download. But I suppose putting the hash in the name could be useful when using FTP for distribution.

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checksums are very different than hashes –  Joel Coehoorn Apr 10 '11 at 0:23
    
Thank you grawity, so it is a hash sum. Why I never thought of that beforehand? Well thank you again. –  Matt Apr 10 '11 at 16:26
1  
I expect that the same file is distributed via multiple means (BT, HTTP/FTP, Usenet) such that having a CRC in the filename is useful for some types of distribution. –  afrazier Nov 19 '11 at 17:37

It is the result of a hash function, used to identify the contents of the data being transferred.

See: BitTorrent vocabulary.

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