Thank you Xavier,

But what I've asked is:

The MD5 generates a long string. Did got only the 
first 4 bytes?
Because I didn't find at MD5 a way to generate only 4 
bytes string.

> > I would like to know how is created the 4 byte MD5 
> > string. Or all MD5 strings that are used to create 
> > each file name.
> 
> ..using the MD5-128 hasing algorythm. You can take a 
> look at this algo in md5.* files and htsmd5.* files. 
> If you are interested by the principal, see the 
> excellent book of Bruce Scheiner (applied crypto)
> 
>