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Ed2k-hash: Difference between revisions
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===Why does anidb require ed2k-hashes? | __NOTOC__ | ||
The main reason for this is that it avoids adding of double db entries. AniDB will not allow you to add a file with the same ed2k-hash as an existing one. | == What's an ed2k hash? == | ||
The file size and ed2k-hash of a file is used to identify it globally. | |||
A hash is a <insert non-laziness here> | |||
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 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? == | |||
The main reason for this is that it avoids adding of double db entries. AniDB will not allow you to add a file with the same ed2k-hash as an existing one. | |||
The file size and ed2k-hash of a file is used to identify it globally. | |||
You are allowed to add files without ed2k-hashes to AniDB, however you should edit those files later and add the missing ed2k-hashes. Once you added a certain number of files without ed2k-hashes you may no longer add new files without ed2k-hashes until you edit your old files first. | You are allowed to add files without ed2k-hashes to AniDB, however you should edit those files later and add the missing ed2k-hashes. Once you added a certain number of files without ed2k-hashes you may no longer add new files without ed2k-hashes until you edit your old files first. | ||
== Why use ed2k instead of another type of hash if you do not support downloads? == | |||
* The combination of ed2k-hash and file size makes ed2k effective for uniquely identifying files. | |||
* Since ed2k-hashes can be passed back and forth within a URL with both the hash and file size in a widely recognized format it is a convenient method for adding or checking files. | |||
* Other hashes are good for validating if a file is corrupt if you already know what file you are comparing it against, but cannot necessarily globally identify a file in the system like the ed2k. | |||
* AniDB was designed around ed2k, although other hashes have been added to the file records for validation, the internal structure is based on ed2k. If the site goes through a complete redesign then maybe another hash will be made the primary hash, but at this point, this is not likely to change. | |||
== 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: | 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: | ||
* [[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://sourceforge.net/projects/ed2k-tools/ ed2k_hash] another tool which is commandline based and also available for Linux. | |||
* [http://malich.content.no-ip.org/filehash/filehash-0.5.2.zip Filehash] a little java program written by Malich. for further info on it read [http://www.anidb.net/forum/viewtopic.php?t=77 here] | |||
* [http://www.slavasoft.com/hashcalc/ Hashcalc] can create md5, sha-1, crc32, ed2k-hashes and various other hashes in 1 go. though at least 1 case is known in which it created a wrong ed2k! | |||
== How is an ed2k hash calculated exactly? == | |||
A file is hashed in 9728000 byte ''chunks'', using the md4 algorithm, and produces a 128 bit hash for each chunk. For files with only one chunk, the ed2k hash ''is'' the md4 of the file, however for hashes with 2 or more chunks the the hash of each chunk is appended to those before it, and an further md4 of the hashes themselves provides the ed2k hash of the file. Pseudo code is given below: | |||
:if filesize is less than <font color="blue">or equal to</font> 9728000: | |||
:: return md4 of file | |||
:for chunk of size upto 9728000 in file: | |||
:: append md4 of chunk to hashlist | |||
:<font color="red">if filesize is a multiple of 9728000: | |||
::append md4 of null to hashlist</font> | |||
:return md4 of hashlist | |||
Note that there are two different ways in practice that implimentations treat the 9728000 byte boundary, given as either the red code or the blue code above, black is common to both. In practice this difference only affects a tiny number of files, however is the one case where two 'valid' ed2k hashes might be produced from one file. | |||
<list of which clients use which method> | |||
<list of affected files, filesize> | |||
*http://anidb.info/f7047 145920000 | |||
*http://anidb.info/f24359 136192000 | |||
*http://anidb.info/f31383 136192000 | |||
*http://anidb.info/f48530 175104000 | |||
*http://anidb.info/f51131 107008000 | |||
*http://anidb.info/f51330 175104000 | |||
*http://anidb.info/f55744 97280000 | |||
*http://anidb.info/f56411 165376000 | |||
*http://anidb.info/f57766 145920000 | |||
*http://anidb.info/f73921 184832000 | |||
*http://anidb.info/f78552 194560000 | |||
*http://anidb.info/f80216 165376000 | |||
*http://anidb.info/f92884 68096000 | |||
*http://anidb.info/f123554 243200000 | |||
*http://anidb.info/f126410 184832000 | |||
*http://anidb.info/f130233 184832000 | |||
*http://anidb.info/f142402 155648000 | |||
*http://anidb.info/f165143 165376000 | |||
*http://anidb.info/f166304 184832000 | |||
*http://anidb.info/f174421 223744000 | |||
[[Category: Definitions]][[Category: Guidelines]] | [[Category: Definitions]][[Category: Guidelines]] | ||