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Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz> Reviewed-by: Andrew Bartlett <abartlet@samba.org>
48 lines
2.1 KiB
Plaintext
48 lines
2.1 KiB
Plaintext
# Test vectors for MS-XCA [de-]compression
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There are currently two supported variants of the Xpress Compression
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Algorithm, "Plain LZ77" and "LZ77 + Huffman". For each we have two
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directories of files compressed on Windows, corresponding to the two
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compression levels that Windows offers.
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The subdirectories are
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./decompressed - test files to compress with .decomp extension.
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./compressed-huffman - LZ77+Huffman compressed, with .lzhuff extension.
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./compressed-more-huffman - LZ77+Huffman compressed, with .lzhuff extension.
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./compressed-plain - Plain LZ77 compressed, with .lzplain extension.
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./compressed-more-plain - Plain LZ77 compressed, with .lzplain extension.
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where the more-compressed-* versions have the files that Windows put
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more effort into compressing (largely in vain -- they are similar in
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size). Windows probably does not use this more effortful compression
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in network protocols, but these files must be decompressible.
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The compressed files were made using the Windows Compression API,
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which uses the same underlying code as MS-XCA, but which puts some
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annoying hurdles in the way. In particular, it won't perform
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LZ77+Huffman compression on any file smaller than 300 bytes. The
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relationship between the two is covered in various messages in
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https://lists.samba.org/archive/cifs-protocol/2022-October/
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https://lists.samba.org/archive/cifs-protocol/2022-November/
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To recreate these files or add more, use
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lib/compression/tests/scripts/generate-windows-test-vectors.c under
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Cygwin or MSYS2. This file is also in the decompressed directory.
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Some of the decompressed files were found via fuzzing, some are designed
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to test one aspect or another of the format, while others are public
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domain texts.
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These are used in compression and decompression tests.
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- For decompression tests, we need the decompressed versions to
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compare against.
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- For compression tests, we do not assert that the compressed file is
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identical to the Windows compressed file. Exact equality is not
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expected by MS-XCA, which leaves room for implementation tricks, but
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the size of the compressed file allows us to make ballpark
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assertions about expected compression ratios.
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