ZIP file encryption

Hi
    Zip file is a compress file. It consists of a sequence of
   [local file header]
    [file data]
    [data descriptor]

Where second field(file data ) contains actual file data which is in encrypted form. I found its format from www.worsit.org
I'm interested to know about this encryption. How we convert it back to original data. As there are some Mathematics algorithms appied on Zip file. Where can i find the details of such algo?  In header of zip file, there is 4 bytes value, CRC. What is this and how we calculate this value? I want complete descryption. If anybody help then thanks in advance.



ManojKrChauhanAsked:
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ManojKrChauhanAuthor Commented:
Hi
     Sorry for late reply. Actually my account has been suspended due to some reason. Actually my motive for  zip file is to knowing about its internal format. Since there are many types of zip file as i found. Some are encrypted.Some contains password. How much types of zip files are available?
Also For a encrypted file, there are 12 bytes we used after "local file header". These bytes contains encryption information. How we calculate these bytes?
The sites which you told don't have enough information for me. So please tell me other links.
                          Thanks
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Jaime OlivaresSoftware ArchitectCommented:
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Jaime OlivaresSoftware ArchitectCommented:
Also notice that in some links posted earlier, they mention a ZLib library, which is the fundamental library that access .zip files in different compresssion formats.
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ManojKrChauhanAuthor Commented:
There is a 12 bytes encryption header after local file header. These 12 bytes are initialized using password. I want to know
1. Password only stores in encryption header?
2. What is difference between encryption headers for simple zip file, PKZip and TD.
As you gave a link http://www.codeproject.com/cpp/zip.asp, I found how encryption header is made(by generating random numberand initialized with password). But when i apply same technique on other zip files then it failed. So i want to know how many types of techniques used for different zip files?
 Thanks
This is last query regarding zip file.
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ManojKrChauhanAuthor Commented:
Thanks
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