[Mldonkey-users] Re: wrong md4 digest

[Robin Hill]

On Sun Dec 08, 2002 at 11:31:57AM -0200, unixach <address@hidden> wrote:

> I was playing with openssl dgst -md4 to create a script which echoes a
> ed2k:// link to the screen. It seemed to work (I compared the MD4 generated
> by openssl with the one generated by ed2k_hash). But only for some files. For 
> "big" files, the MD4 differs.
> 
Yes, edonkey hashes aren't straight MD4 hashes.  What it does is split
the file into 9M chunks and calculate an MD4 hash for each chunk.  These
hashes are put, consecutively, into a buffer and an MD4 hash is created
for this buffer.  This final hash is the one used by edonkey.  This
means that for small files (under 9M) a straight MD4 will get you the
same result but for > 9M files it won't.

I assume the reason for doing it this way is that edonkey works using
these 9M chunks for verification, etc (if you look in the
shared_files.ini for mldonkey you'll see the MD4 hashes for each chunk)
so it has to calculate the hashes for each block as it downloads.  It's
a lot quicker then to generate an overall hash from these rather than
having to go through the entire file again.

HTH,
        Robin

-- 
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    ( ' }     |       Robin Hill        <address@hidden> |
   / / )      | Little Jim says ....                            |
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