JZlib is a re-implementation of
zlib
in pure Java.
The first and final aim for hacking this stuff is
to add the packet compression support to pure Java SSH systems.
Zlib
The zlib is designed to be a free, general-purpose,
legally unencumbered -- that is, not covered by any patents -- lossless data-compression library for use on virtually any computer hardware and operating system.
The zlib was written by
Jean-loup Gailly (compression) and
Mark Adler (decompression).
Features
- Needless to say, JZlib can inflate data, which is deflated by zlib
and JZlib can generate deflated data,
which is acceptable and is inflated by zlib.
- JZlib supports all compression level and
all flushing mode in zlib.
- JZlib does not support gzip file handling supports.
- The performance has not been estimated yet,
but it will not be so bad in deflating/inflating data stream
on the low bandwidth network.
- JZlib is licensed under BSD style license
- Any invention has not been done in developing JZlib.
So, if zlib is patent free,
JZlib is also not covered by any patents.
Why JZlib?
Java Platform API provides packages 'java.util.zip.*' for
accessing to zlib, but that support is very limited
if you need to use the essence of zlib. For example,
we needed to full access to zlib to add the packet compression
support to pure Java SSH system, but they are useless for our requirements.
The Internet draft SSH Transport Layer Protocol says in the section '4.2 Compression' as follows,
The following compression methods are currently defined:
none REQUIRED no compression
zlib OPTIONAL GNU ZLIB (LZ77) compression
The "zlib" compression is described in [RFC-1950] and in [RFC-1951]. The
compression context is initialized after each key exchange, and is
passed from one packet to the next with only a partial flush being
performed at the end of each packet. A partial flush means that all data
will be output, but the next packet will continue using compression
tables from the end of the previous packet.
To implement this functionality,
the Z_PARTIAL_FLUSH mode of zlib must be used,
however JDK does not permit us to do so.
It seems that this problem has been well known and
some people have already
reported to JavaSoft's BugParade(for example,
BugId:4255743),
but any positive response has not been returned from JavaSoft,
so this problem will not be solved forever.
This is our motivation to hack JZlib.
Of course, this stuff must be useful for any other pure java programs,
which need communications with compressed data stream.
Feel free to use this stuff in your hack and
we are very happy if we get some feedbacks.
Applications Using JZlib
-
According to Martin Smith and Karin Herm,
JZlib has been used in JMakeztxt, which converts text files into the format needed
by GutenPalm.
GutenPalm is a document reader for the Palm Pilot organizers.
- JSch uses JZlib for the packet compression.
Bibles for hacking JZlib
We have referred to following works in hacking JZlib.
Download
JZlib 1.0.7 is provided in tar+gz and zip formats.
Credits
JZlib has been developed by ymnk,
but he has just re-implemented zlib in pure Java(this stuff was just required to improve the service on WiredX.net).
So, all credit should go to authors
Jean-loup Gailly and
Mark Adler
and contributors of zlib.
Comments/suggestions are accepted at
jzlib@jcraft.com.
``SSH is a registered trademark and Secure
Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''.
|
