Ed2k-hash: Difference between revisions

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== What's an ed2k hash? ==

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 failsafe 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 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.

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.

== Why use ed2k instead of another type of hash? ==

* 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 URI with both the hash and file size in a widely recognized format it is a convenient method for adding or checking files.

== 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:

* [[Avdump]] 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).

* [http://www.slavasoft.com/hashcalc/ Hashcalc] can create md5, sha-1, crc32, ed2k-hashes and various other hashes in 1 go.

== 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:

=== List of affected files, by fileID ===

{| style="font-family: monospace; text-align: right; margin: auto;"

! style="font-family: sans-serif; text-align: center;" | File

|File of zeros || 19456000 || 194ee9e4fa79b2ee9f8829284c466051 || 114b21c63a74b6ca922291a11177dd5c

|- style="background-color: #eee;"

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[[Category: Definitions]][[Category: Guidelines]]