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<!DOCTYPE html>
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<title>Case Study: The GIF Format — Fuzzing with Fandango</title>
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<section class="tex2jax_ignore mathjax_ignore" id="case-study-the-gif-format">
<span id="sec-gif"></span><h1>Case Study: The GIF Format<a class="headerlink" href="#case-study-the-gif-format" title="Link to this heading">#</a></h1>
<div class="attention admonition">
<p class="admonition-title">Under Construction</p>
<p>To be added later.</p>
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<p>The <a class="reference external" href="https://www.fileformat.info/format/gif/egff.htm">GIF format</a> is widely used to encode image sequences.</p>
<p>We start with a very short GIF to keep things simple (<a class="reference external" href="http://probablyprogramming.com/2009/03/15/the-tiniest-gif-ever">source</a>): <a class="reference download internal" download="" href="_downloads/76eaac8615c4db457e2d37ab8fe4afcd/tinytrans.gif"><span class="xref download myst">tinytrans.gif</span></a>.</p>
<p>We can parse this file using Fandango:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="o">!</span>fandango<span class="w"> </span>parse<span class="w"> </span>-f<span class="w"> </span>gif89a.fan<span class="w"> </span>tinytrans.gif<span class="w"> </span>-o<span class="w"> </span>-<span class="w"> </span>--format<span class="o">=</span>grammar<span class="w"> </span>--validate
<span class="k">assert</span> <span class="n">_exit_code</span> <span class="o">==</span> <span class="mi">0</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <GifHeader> <LogicalScreenDescriptor> <GlobalColorTable> <Data_1> <Trailer> # b'GIF89a\x01\x00\x01\x00\x80\x01\x00\xff\xff\xff\x00\x00\x00!\xf9\x04\x01\n\x00\x01\x00,\x00\x00\x00\x00\x01\x00\x01\x00\x00\x02\x02L\x01\x00;'
<GifHeader> ::= <GIFHEADER>
<GIFHEADER> ::= <Signature> <Version> # b'GIF89a'
<Signature> ::= <char> <char> <char> # b'GIF'
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'G' # Position 0x0000 (0)
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'I' # Position 0x0001 (1)
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'F' # Position 0x0002 (2)
<Version> ::= <char> <char> <char> # b'89a'
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'8' # Position 0x0003 (3)
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'9' # Position 0x0004 (4)
<char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'a' # Position 0x0005 (5)
<LogicalScreenDescriptor> ::= <LOGICALSCREENDESCRIPTOR>
<LOGICALSCREENDESCRIPTOR> ::= <Width> <Height> <PackedFields> <BackgroundColorIndex> <PixelAspectRatio> # b'\x01\x00\x01\x00\x80\x01\x00'
<Width> ::= b'\x01' b'\x00' # Position 0x0006 (6); b'\x01\x00'
<Height> ::= b'\x01' b'\x00' # Position 0x0008 (8); b'\x01\x00'
<PackedFields> ::= <LOGICALSCREENDESCRIPTOR_PACKEDFIELDS>
<LOGICALSCREENDESCRIPTOR_PACKEDFIELDS> ::= <GlobalColorTableFlag> <ColorResolution> <SortFlag> <SizeOfGlobalColorTable> # 0b10000000
<GlobalColorTableFlag> ::= 1 # Position 0x000a (10), bit 7
<ColorResolution> ::= 0 0 0 # Position 0x000a (10), bits 6-4; 0b000
<SortFlag> ::= <bit>
<bit> ::= <_bit>
<_bit> ::= 0 # Position 0x000a (10), bit 3
<SizeOfGlobalColorTable> ::= 0 0 0 # Position 0x000a (10), bits 2-0; 0b000
<BackgroundColorIndex> ::= b'\x01' # Position 0x000b (11)
<PixelAspectRatio> ::= b'\x00' # Position 0x000c (12)
<GlobalColorTable> ::= <RGB> b'\x00' b'\x00' b'\x00' # Position 0x000d (13); b'\xff\xff\xff\x00\x00\x00'
<RGB> ::= <R> <G> <B> # b'\xff\xff\xff'
<R> ::= <UBYTE>
<UBYTE> ::= <ubyte>
<ubyte> ::= <uchar>
<uchar> ::= <unsigned_char>
<unsigned_char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'\xff' # Position 0x0010 (16)
<G> ::= <UBYTE>
<UBYTE> ::= <ubyte>
<ubyte> ::= <uchar>
<uchar> ::= <unsigned_char>
<unsigned_char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'\xff' # Position 0x0011 (17)
<B> ::= <UBYTE>
<UBYTE> ::= <ubyte>
<ubyte> ::= <uchar>
<uchar> ::= <unsigned_char>
<unsigned_char> ::= <byte>
<byte> ::= <_byte>
<_byte> ::= b'\xff' # Position 0x0012 (18)
<Data_1> ::= <DATA>
<DATA> ::= <GraphicControlExtension> <ImageDescriptor> <LocalColorTable> <ImageData> # b'!\xf9\x04\x01\n\x00\x01\x00,\x00\x00\x00\x00\x01\x00\x01\x00\x00\x02\x02L\x01\x00'
<GraphicControlExtension> ::= <GRAPHICCONTROLEXTENSION>
<GRAPHICCONTROLEXTENSION> ::= <ExtensionIntroducer_1> <GraphicControlLabel_1> <GraphicControlSubBlock> <BlockTerminator_2> # b'!\xf9\x04\x01\n\x00\x01\x00'
<ExtensionIntroducer_1> ::= b'!' # Position 0x0013 (19)
<GraphicControlLabel_1> ::= b'\xf9' # Position 0x0014 (20)
<GraphicControlSubBlock> ::= <GRAPHICCONTROLSUBBLOCK>
<GRAPHICCONTROLSUBBLOCK> ::= <BlockSize> <PackedFields_2> <DelayTime> <TransparentColorIndex> # b'\x04\x01\n\x00\x01'
<BlockSize> ::= b'\x04' # Position 0x0015 (21)
<PackedFields_2> ::= <GRAPHICCONTROLEXTENSION_DATASUBBLOCK_PACKEDFIELDS>
<GRAPHICCONTROLEXTENSION_DATASUBBLOCK_PACKEDFIELDS> ::= 0 0 0 0 0 0 0 1 # Position 0x0016 (22), bits 7-0; 0b00000001
<DelayTime> ::= b'\n' b'\x00' # Position 0x0017 (23); b'\n\x00'
<TransparentColorIndex> ::= b'\x01' # Position 0x0019 (25)
<BlockTerminator_2> ::= b'\x00' # Position 0x001a (26)
<ImageDescriptor> ::= <IMAGEDESCRIPTOR>
<IMAGEDESCRIPTOR> ::= <ImageSeparator_1> <ImageLeftPosition> <ImageTopPosition> <ImageWidth> <ImageHeight> <PackedFields_1> # b',\x00\x00\x00\x00\x01\x00\x01\x00\x00'
<ImageSeparator_1> ::= b',' # Position 0x001b (27)
<ImageLeftPosition> ::= b'\x00' b'\x00' # Position 0x001c (28); b'\x00\x00'
<ImageTopPosition> ::= b'\x00' b'\x00' # Position 0x001e (30); b'\x00\x00'
<ImageWidth> ::= b'\x01' b'\x00' # Position 0x0020 (32); b'\x01\x00'
<ImageHeight> ::= b'\x01' b'\x00' # Position 0x0022 (34); b'\x01\x00'
<PackedFields_1> ::= 0 0 0 0 0 0 0 0 # Position 0x0024 (36), bits 7-0; 0b00000000
<LocalColorTable> ::= b'\x02' b'\x02' b'L' # Position 0x0025 (37); b'\x02\x02L'
<ImageData> ::= <IMAGEDATA>
<IMAGEDATA> ::= <LZWMinimumCodeSize> <DataSubBlocks> # b'\x01\x00'
<LZWMinimumCodeSize> ::= b'\x01' # Position 0x0028 (40)
<DataSubBlocks> ::= b'\x00' # Position 0x0029 (41)
<Trailer> ::= <TRAILER>
<TRAILER> ::= <GIFTrailer_1>
<GIFTrailer_1> ::= b';' # Position 0x002a (42)
</pre></div>
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