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    <p>This is an add-on to MizardX's answer, giving some explanation and background.</p> <p>See <a href="http://www.chiramattel.com/george/blog/2007/09/09/deflatestream-block-length-does-not-match.html" rel="noreferrer">http://www.chiramattel.com/george/blog/2007/09/09/deflatestream-block-length-does-not-match.html</a></p> <p>According to <a href="http://www.ietf.org/rfc/rfc1950.txt" rel="noreferrer">RFC 1950</a>, a zlib stream constructed in the default manner is composed of:</p> <ul> <li>a 2-byte header (e.g. 0x78 0x9C) </li> <li>a deflate stream -- see <a href="http://www.ietf.org/rfc/rfc1951.txt" rel="noreferrer">RFC 1951</a> </li> <li>an Adler-32 checksum of the uncompressed data (4 bytes) </li> </ul> <p>The C# <code>DeflateStream</code> works on (you guessed it) a deflate stream. MizardX's code is telling the zlib module that the data is a raw deflate stream.</p> <p>Observations: (1) One hopes the C# "deflation" method producing a longer string happens only with short input (2) Using the raw deflate stream without the Adler-32 checksum? Bit risky, unless replaced with something better.</p> <p><strong>Updates</strong></p> <p><strong>error message <code>Block length does not match with its complement</code></strong></p> <p>If you are trying to inflate some compressed data with the C# <code>DeflateStream</code> and you get that message, then it is quite possible that you are giving it a a zlib stream, not a deflate stream. </p> <p>See <a href="https://stackoverflow.com/q/762614/12892">How do you use a DeflateStream on part of a file?</a></p> <p>Also copy/paste the error message into a Google search and you will get numerous hits (including the one up the front of this answer) saying much the same thing.</p> <p><strong>The Java <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/zip/Deflater.html" rel="noreferrer"><code>Deflater</code></a></strong> ... used by "the website" ... C# DeflateStream "is pretty straightforward and has been tested against the Java implementation". Which of the following possible Java Deflater constructors is the website using?</p> <blockquote> <p><code>public Deflater(int level, boolean nowrap)</code></p> <p>Creates a new compressor using the specified compression level. If 'nowrap' is true then the ZLIB header and checksum fields will not be used in order to support the compression format used in both GZIP and PKZIP.</p> <p><code>public Deflater(int level)</code></p> <p>Creates a new compressor using the specified compression level. Compressed data will be generated in ZLIB format.</p> <p><code>public Deflater()</code></p> <p>Creates a new compressor with the default compression level. Compressed data will be generated in ZLIB format. </p> </blockquote> <p><strong>A one-line deflater</strong> after throwing away the 2-byte zlib header and the 4-byte checksum: </p> <pre><code>uncompressed_string.encode('zlib')[2:-4] # does not work in Python 3.x </code></pre> <p>or </p> <pre><code>zlib.compress(uncompressed_string)[2:-4] </code></pre>
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    1. POPython: Inflate and Deflate implementations
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    1. CO+1 Thanks for the additional info.
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    2. CO@John Machin: To reply to your first observation... the result is only longer in the case of shorter strings (header? padding?). When I feed in 161 bytes of data for deflation, prior to base64 encoding the result is 126 bytes.
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    3. CO@John Machin: Great insights and information. The Java signature of deflater used is the one with two parameters, with nowrap==true. I used your one-line deflater example and it inflates well in .NET and Java, despite looking different than the value produced by deflating with the libraries in those languages. This is great. Now I am working on inflate - taking the deflated data produced by Java or .NET and adding on an adler32 checksum and the zlib header to see if I can get Python to consume it well. I'll let you know how it goes.
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