xz/tests/files/README

.lzma Test Files
----------------

0. Introduction

    This directory contains bunch of files to test handling of .lzma files
    in .lzma decoder implementations. Many of the files have been created
    by hand with a hex editor, thus there is no better "source code" than
    the files themselves. All the test files (*.lzma) and this README have
    been put into the public domain.


1. File Types

    Good files (good-*.lzma) must decode successfully without requiring
    a lot of CPU time or RAM. If the decoder supports only Single-Block
    Streams, then good-multi-*.lzma won't decode, of course.

    Bad files (bad-*.lzma) must cause the decoder to give an error. Like
    with the good files, these files must not require a lot of CPU time
    or RAM before they get detected to be broken.

    Malicious files (malicious-*.lzma) are good in terms of the file format
    specification, but try to trigger excessive CPU, RAM or disk usage in
    the decoder. To prevent malicious files from putting the decoder in
    inifinite loop (*), eating all available RAM or disk space, decoders
    should have internal limitters that catch these situations.

    (*) Strictly speaking not infinite, but if decoding of a small file
        would take a few weeks or even years, it's an infinite loop in
        practice.


2. Descriptions of Individual Files

2.1. Good Files

    good-single-none.lzma uses implicit Copy filter with known Uncompressed
    Size.

    good-single-none-pad.lzma is good-single-none.lzma with Footer Padding.

    good-cat-single-none-pad.lzma is two good-single-none-pad.lzma files
    concatenated as is. Fully decoding this file requires that the decoder
    supports decoding concatenated files.

    good-single-lzma.lzma is LZMA compressed file with EOPM.

    good-single-subblock-lzma.lzma has basic combination of Subblock and
    LZMA filters.

    good-single-subblock_rle.lzma takes advantage of Subblock filter's
    run-length encoding.

    good-single-delta-lzma.tiff.lzma is an image file that compresses
    better with Delta+LZMA than with plain LZMA.


2.2. Bad Files

    bad-single-data_after_eopm.lzma has LZMA+Subblock, where the Subblock
    filter gives one byte of data to LZMA after LZMA has detected EOPM.

    bad-single-data_after_eopm_2.lzma is like
    bad-single-data_after_eopm.lzma but Subblock gives 256 MiB of data to
    LZMA after LZMA has detected EOPM.

    bad-single-subblock_subblock.lzma has Subblock+Subblock, where the
    Subblock decoder is given End of Input in the middle of a Subblock.

    bad-single-subblock-padding_loop.lzma contains huge amount of
    consecutive Padding bytes, which isn't allowed by the Subblock filter
    format. If it were allowed, this file would hang the decoder for very
    long time (weeks to years).

    bad-single-subblock1023-slow.lzma is similar to
    malicious-single-subblock31-slow.lzma except that this uses 1023 bytes
    of Padding in every place instead of 31 bytes. The Subblock filter
    format specification allows only 31-byte Padings, thus this file must
    get detected as bad without producing any output. Allowing larger
    Padding than 31 bytes was considered (so this test file was created),
    but it seemed to be a bad idea since it would increase worst-case CPU
    usage.


2.3. Malicious Files

    malicious-single-subblock31-slow.lzma requires quite a bit of CPU time
    per decoded byte. It contains LZMA compressed Subblock filter data that
    has as much Padding as the specification allows. LZMA is also used as
    a Subfilter, to further slowdown the decoder. Every Subfilter instance
    produces only one byte of output. If you can create a file that wastes
    notably more CPU cycles than this file, please contact Lasse Collin.

    malicious-single-subblock-256MiB.lzma is a tiny file that produces
    256 MiB of output. It uses Subblock filter's run-length encoding
    to achieve this.

    malicious-single-subblock-64PiB.lzma is a tiny file that produces
    64 PiB of output (if you have patience to wait). This is done by
    chaining two Subblock filters and using their run-length encoders.

    malicious-multi-metadata-64PiB.lzma is like
    malicious-single-subblock-64PiB.lzma but the huge amount of output
    is in a Metadata Block. Trying to decode this file may take years
    unless the decoder catches that the Metadata has unreasonable size.