#97 Archive libraries needs cleaner compression/crypto support

[Anonymous]

Althogh 7-Zip supports ZIP files and supports deflate,
implode, and BZIP2 decompressions, it does not
recognize BZIP2 and DCL Implode compressions used
by PKZIP 6.0 (files inside ZIP archive compressed by
DCL Implode is recognized as 'PKImploding' in 7-Zip File
Manager, but not extractable). Similarly, 7Zip does not
recognize zip files using AES-128 encryption, the
method used by both PKZIP 6.0 and WinRAR. I do not
recommended the quick fix solution involving statically
inserting codes into the archive format library. Instead,
it should be fixed more cleanly by separating
compression and encryption methods from file formats,
with the archive format library linking to external
compression and encryption libraries. This way enables
cleaner and more complete support of different archive
types.

I realize this may involve more work and may have to
wait until 7-Zip 3, but IIRC such solution is partially
available to 7z archive, and it is important to extend the
efforts into other archive formats, especially now that
other compression titles are incorporating existing
techniques to build custom archive formats.

[Igor Pavlov]

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I'm not sure that these methods are documented.

[Nobody/Anonymous]

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Implode was in info-zip 1.1 IIRC. You can find all PKZIP
packing methods and C source (up to deflate) in G64
homepage[1] or PKWARE's homepage[2]. It is hinted
Deflate64 is just deflate with 64K dictionary, and 7-Zip already
does the decompression part so the compression end should
be no problem. AES-256 is already used by 7-ZIP, and AES-
128 is just a lowered bit version of AES-256. You can find
reference code and specs at NIST[3]. Updated PKZip 4.5 file
format is on PKWARE[2]. Bzip2 documentation can be
found on Redhat[4].

I suspect DCL implode is just another variant of the old
implode method, but there is no guarantee.

[1] http://www.cs.tut.fi/~albert/Dev/gunzip/
[2]
http://www.pkzip.com/products/enterprise/white_papers/appno
te.html
[3] http://csrc.nist.gov/CryptoToolkit/aes/rijndael/
[4] http://sources.redhat.com/bzip2/

[Igor Pavlov]

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AES and BZip are not problems.
But there is no documentation how pk uses them.

deflate64:
-mm=9
or m=9 in GUI.

[Nobody/Anonymous]

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I suspect Bzip2 is probably done the same way as original
Bzip2. But crypto is another matter. However, the detailed
document view used in PKZIP 6 Professional shows that 16
bytes are added to the end of each central directory file
header when AES-128 encryption is used. I suppose this is
the password, probably encrypted with AES.

[Igor Pavlov]

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Can you send me small zip examples?
1) using bzip
2) using AES

[Nobody/Anonymous]

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I have play around the BZip2 feature with PKZIP 6.0 and
discovered PKZIP 6.0 compresses BZip2 identically as 7-Zip.
First, Zip files are created with PKZIP 6.0 using BZip2 at level
9, and create BZip2 files from 7-Zip with maximum
compression ratio. After ripping the compressed data from
the ZIP file and comparing it to the BZip2 archive, it turns out
both files are identical. It seems there are too many software
patents even for PKWARE to 'optimize' BZip2 algorithm.
Either way, it means 7-Zip can soon export ZIP files with
BZip2 compression with ease.

No such luck for AES-256, however. It seems the crypto keys
are generated in different methods. Probably with the old
PKZIP 2.04 methods, but there is no way to test it.

BTW how do I sent attachments via sourceforge forums?

[Nobody/Anonymous]

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I have play around the BZip2 feature with PKZIP 6.0 and
discovered PKZIP 6.0 compresses BZip2 identically as 7-Zip.
First, Zip files are created with PKZIP 6.0 using BZip2 at level
9, and create BZip2 files from 7-Zip with maximum
compression ratio. After ripping the compressed data from
the ZIP file and comparing it to the BZip2 archive, it turns out
both files are identical. It seems there are too many software
patents even for PKWARE to 'optimize' BZip2 algorithm.
Either way, it means 7-Zip can soon export ZIP files with
BZip2 compression with ease.

No such luck for AES-256, however. It seems the crypto keys
are generated in different methods. Probably with the old
PKZIP 2.04 methods, but there is no way to test it.

BTW how do I sent attachments via sourceforge forums?

[Nobody/Anonymous]

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Apparantly attachment feature only works when starting new
message, but not when replying threads.

[Igor Pavlov]

Test Attached file

[Igor Pavlov]

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I'll try to install pkzip myself.

[Igor Pavlov]

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I've added Bzip2 supporting.
About AES - seems my pkzip's version doesn't support
it.
If you can create small archive with AES, you can send it
to me via e-mail.
I don't think that I'll be able to add AES suport without
documentation about implementing aes in zip.
But anywhere I must have some example archive.

[Nobody/Anonymous]

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WinZip posted its version of encryption in it's site[1]. Note
that it generates different ZIP from PKWARE's. Also, PKWare
updated its file format document, but still no certificate check.

[1] http://www.winzip.com/aes_info.htm