This is documentation on the .chm format used by Microsoft HTML Help. This format has been reverse engineered in the past, but as far as I know this is the first freely available documentation on it. One Usenet message indicates that these .chm files are actually IStorage files documented in the Microsoft Platform SDK. However, I have not been able to locate such documentation.
The word "section" is badly overloaded in this document. Sorry about that.
All numbers are in hexadecimal unless otherwise indicated in the text. Except in tabular listings, this will be indicated by $ or 0x as appropriate. All values within the file are Intel byte order (little endian) unless indicated otherwise.
The .chm file begins with a short ($38 byte) initial header. This is followed by the header section table, the offset to the content, and a number of bytes of information of unknown use. Collectively, this is the "header".
The header is followed by the header sections. There are two header sections. One header section is the file directory, the other contains the file length and some unknown data. Immediately following the header sections is the content.
The header starts with the initial header, which has the following format
0000: char[4] 'ITSF'
0004: DWORD 3 (Version number)
0008: DWORD Total header length, including header section table and
following data.
000C: DWORD 1 (unknown)
0010: DWORD a timestamp.
0014: DWORD Windows Language ID. The two I've seen
$0409 = LANG_ENGLISH/SUBLANG_ENGLISH_US
$0407 = LANG_GERMAN/SUBLANG_GERMAN
0018: GUID {7C01FD10-7BAA-11D0-9E0C-00A0-C922-E6EC}
0028: GUID {7C01FD11-7BAA-11D0-9E0C-00A0-C922-E6EC}
Note: a GUID is $10 bytes, arranged as 1 DWORD, 2 WORDs, and 8 BYTEs.
It is followed by the header section table, which is 2 entries, where each entry is $10 bytes long and has this format:
0000: QWORD Offset of section from beginning of file 0008: QWORD Length of section
Following the header section table is 8 bytes of additional header data. In Version 2 files, this data is not there and the content section starts immediately after the directory.
0000: QWORD Offset within file of content section 0
This section contains the total size of the file, and not much else
0000: DWORD $01FE (unknown) 0004: DWORD 0 (unknown) 0008: QWORD File Size 0010: DWORD 0 (unknown) 0014: DWORD 0 (unknown)
The central part of the .chm file: A directory of the files and information it contains.
The directory starts with a header; its format is as follows:
0000: char[4] 'ITSP'
0004: DWORD Version number 1
0008: DWORD Length of the directory header
000C: DWORD $0a (unknown)
0010: DWORD $1000 Directory chunk size
0014: DWORD "Density" of quickref section, usually 2.
0018: DWORD Depth of the index tree
1 there is no index, 2 if there is one level of PMGI
chunks.
001C: DWORD Chunk number of root index chunk, -1 if there is none
(though at least one file has 0 despite there being no
index chunk, probably a bug.)
0020: DWORD Chunk number of first PMGL (listing) chunk
0024: DWORD Chunk number of last PMGL (listing) chunk
0028: DWORD -1 (unknown)
002C: DWORD Number of directory chunks (total)
0030: DWORD Windows language ID
0034: GUID {5D02926A-212E-11D0-9DF9-00A0C922E6EC}
0044: DWORD $54 (This is the length again)
0048: DWORD -1 (unknown)
004C: DWORD -1 (unknown)
0050: DWORD -1 (unknown)
The header is directly followed by the directory chunks. There are two types of directory chunks -- index chunks, and listing chunks. The index chunk will be omitted if there is only one listing chunk. A listing chunk has the following format:
0000: char[4] 'PMGL'
0004: DWORD Length of free space and/or quickref area at end of
directory chunk
0008: DWORD Always 0.
000C: DWORD Chunk number of previous listing chunk when reading
directory in sequence (-1 if this is the first listing chunk)
0010: DWORD Chunk number of next listing chunk when reading
directory in sequence (-1 if this is the last listing chunk)
0014: Directory listing entries (to quickref area) Sorted by
filename; the sort is case-insensitive.
The quickref area is written backwards from the end of the chunk. One quickref entry exists for every n entries in the file, where n is calculated as 1 + (1 << quickref density). So for density = 2, n = 5.
Chunklen-0002: WORD Number of entries in the chunk Chunklen-0004: WORD Offset of entry n from entry 0 Chunklen-0008: WORD Offset of entry 2n from entry 0 Chunklen-000C: WORD Offset of entry 3n from entry 0 ...
The format of a directory listing entry is as follows
BYTE: length of name
BYTEs: name (UTF-8 encoded)
ENCINT: content section
ENCINT: offset
ENCINT: length
The offset is from the beginning of the content section the file is in, after the section has been decompressed (if appropriate). The length also refers to length of the file in the section after decompression.
There are two kinds of file represented in the directory: user data and format related files. The files which are format-related have names which begin with '::', the user data files have names which begin with "/".
An index chunk has the following format
0000: char[4] 'PMGI' 0004: DWORD Length of quickref/free area at end of directory chunk 0008: Directory index entries (to quickref/free area)
The quickref area in an PMGI is the same as in an PMGL
The format of a directory index entry is as follows
BYTE: length of name
BYTEs: name (UTF-8 encoded)
ENCINT: directory listing chunk which starts with name
When higher-level indexes exist (when the depth of the index tree is 3 or higher), presumably the upper-level indexes will contain the numbers of lower-level index chunks rather than listing chunks
An ENCINT is a variable-length integer. The high bit of each byte indicates "continued to the next byte". Bytes are stored most significant to least significant. So, for example, $EA $15 is (((0xEA&0x7F)<<7)|0x15) = 0x3515.
The content typically immediately follows the header sections, and is at the location indicated by the DWORD following the header section table. All content section 0 locations in the directory are relative to that point. The other content sections are stored WITHIN content section 0.
There exists in content section 0 and in the directory a file called "::DataSpace/NameList". This file contains the names of all the content sections. The format is as follows:
0000: WORD Length of file, in words 0002: WORD Number of entries in file Each entry: 0000: WORD Length of name in words, excluding terminating null 0002: WORD Double-byte characters xxxx: WORD 0
Yes, the names have a length word AND are null terminated; sort of a belt-and-suspenders approach. The coding system is likely UTF-16 (little endian).
The section names seen so far are
"Uncompressed" is self-explanatory. The section "MSCompressed" is compressed with Microsoft's LZX algorithm.
For each section other than 0, there exists a file called '::DataSpace/Storage/<Section Name>/Content'. This file contains the compressed and/or encrypted data for the section. So, conceptually, getting a file from a nonzero section is a multi-step process. First you must get the content file from section 0. Then you decompress (if appropriate) the section. Then you get the desired file from your decompressed section.
There are several other files associated with the sections
This file contains $20 bytes of information on the compression. The information is partially known:
0000: DWORD 6 (unknown) 0004: ASCII 'LZXC' Compression type identifier 0008: DWORD 2 (Possibly numeric code for LZX) 000C: DWORD The Huffman reset interval in $8000-byte blocks 0010: DWORD The window size in $8000-byte blocks 0014: DWORD unknown (sometimes 2, sometimes 1, sometimes 0) 0018: DWORD 0 (unknown) 001C: DWORD 0 (unknown)
This file contains a quadword containing the uncompressed length of the section.
It appears this file was intended to contain a list of GUIDs belonging to methods of decompressing (or otherwise transforming) the section. However, it actually contains only half of the string representation of a GUID, apparently because it was sized for characters but contains wide characters.
The compressed sections are compressed using LZX, a compression method Microsoft also uses for its cabinet files. To ensure this, check the second DWORD of compression info in the ControlData file for the section — it should be 'LZXC'. To decompress, first read the file "::DataSpace/Storage/<SectionName>/Transform/{7FC28940-9D31-11D0-9B27-00A0C91E9C7C}/InstanceData/ResetTable". This reset table has the following format
0000: DWORD 2 unknown (possibly a version number)
0004: DWORD Number of entries in reset table
0008: DWORD 8 unknown
000C: DWORD $28 Length of table header (area before table entries)
0010: QWORD Uncompressed Length
0018: QWORD Compressed Length
0020: QWORD 0x8000 block size for locations below
0028: QWORD 0 (zeroth entry of table)
0030: QWORD location in compressed data of 1st block boundary in
uncompressed data
Repeat to end of file
Now you can finally obtain the section (from its Content file). The window size for the LZX compression is 16 (decimal) on all the files seen so far. This is specified by the DWORD at $10 in the ControlData file (but note that DWORD gives the window size in 0x8000-byte blocks, not the LZX code for the window size)
There is one change from LZX as defined by Microsoft: After each Huffman reset interval (defined in the ControlData file, but in practice equal to the window size) of compressed data is processed, the decoder state is partially reset: that is, the Huffman length tables are cleared and the one-bit preprocessing header is reread. The LZ window is not cleared.
The rule that the input bit-stream is to be re-aligned to a 16-bit boundary after $8000 output characters have been processed IS in effect, despite this LZX not being part of a CAB file. The reset table tells you when this was done, though there seems to be no need for that during decompression; you can just keep track of the number of output characters. Furthermore, while this does not appear to be documented in the LZX format, the compressed stream is padded to an $8000 (decimal) byte boundary.
Copyright 2001 Matthew T. Russotto
You may freely copy and distribute unmodified copies of this file, or copies where the only modification is a change in line endings, padding after the html end tag, coding system, or any combination thereof. The original is in ASCII with Unix line endings.