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In short, a hash is a checksum. It is used to detect errors in data by checking the integrity of the message/file. There are various different hash algorithms used for differing purposes, some that are focused more on security and some on speed. AniDB sports both kinds; CRC is not particularly secure at all, while MD5, SHA-1 and the MD4/ed2k hashes are much more secure from a cryptographic standpoint. They are not fail-safe though, but having more than one hash makes it exponentially harder to counterfeit a file. You might be able to fool one of the hashes, but generally not two, or as is the case with most of the files found in AniDB: four hashes!
The ed2k hash is based on the ''md4 algorithm'', but rather than providing a single hash of the entire file, it breaks the file up into 9500kb ''chunks'' and produces a final hash based on the md4 sums of the chunks. While md4 is no longer considered secure from a cryptographic perspective, for the purpose of uniquely identifying files it's more than adequate. It is often listed as part of an ''ed2k link'', which also includes a size in bytes, and a name.
== Why does AniDB require ed2k-hashes? ==
== Which software can be used to generate them? ==
If you have the file(s) on your hard disk or on CD you can use all kinds of tools to generate the ed2k-hash and other additional info:
* [[AvdumpAvdump2]] is the preferred tool as it can automatically creq files.
* [[AniDB O'Matic]] by BennieB/PetriW can generate ed2k-hashes. It generates ed2k-hashes, md5, sha-1, crc32 in one go and also lists stuff like codec, resolution, bitrates, ...
* [http://ed2k-tools.sourceforge.net/index.shtml ed2k_hash] another tool which is commandline based (current Windows and OSX versions have a GUI as well) and also available for Unix operating systems (Linux/BSD/etc).
=== List of which clients use which method ===
[[AvdumpAvdump2]] use uses both methods. It will creq files from blue to red and store blue in file description ({{AniDBShortLink|f7047|Alternative ed2k}}).
*{{colour|blue|edonkey2000 v0.5.0 to v1.4.3}}
*{{colour|blue|mldonkey (2.5.30.17 tested)}}
|-
|http://anidb.net/f174421 || 223744000 || cd87540a7b48e87e78e7c714fbf7581e || a7c9a857b6d584bae0568495b92ea609
|-style="background-color: #eee;"
|http://anidb.net/f200233 || 58368000 || cfff67163f6ca9bbed26211e140b10e1 || 0943c164b8a076c88551f0b2c1757436
|-
|http://anidb.net/f220069 || 389120000 || a182915f5cd114937f760246da92234b || 43324839e08afca1d83a890468f87f5e
|-style="background-color: #eee;"
|http://anidb.net/f240925 || 126464000 || bae493f413037066ce5a597be4d97e8f || 725a209cf70715b3fdfb03348bb040c6
|-
|}
==See also==*[[Export ED2K Links]][[Category: Definitions]][[Category: Guidelines]]

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