LHA is an archiving program and file format created by Haruyasu Yoshizaki in 1988. It was originally called LHArc, then was briefly LH before settling on LHA. In the 1990s, it was the most popular archiving format on the Amiga platform. It also got some use on the PC platform including in the installers for id Software games such as Doom and Quake, because ZIP compression was inferior until the release of PKZIP 2.0, which brought the formats to parity. At present, it is mostly used in Japan.
The file format is also known as LZH.
An LHA file consists of a sequence of elements, each representing a member file or directory. There is no global archive-level header.
There are at least four different formats that an element can have. (Note that this is independent of compression schemes.) In LHA jargon, the formats are known as "header levels", and are usually called "header level 0", "... 1", "... 2", and "... 3".
The format of an element is determined by the byte at offset 20 from the beginning of that element. It is possible for different formats to be used in the same LHA file.
The formats are similar, but irritatingly different. They don't even follow the same principles with respect to how they must be parsed.
Wikipedia has a list of LHA compression methods, as identified by the alphanumeric bytes of the compression method field. It includes the following:
lh0, lh1, lh2, lh4, lh5, lh6, lh7, lh8, lh9, lha, lhb, lhc, lhd, lhe, lhx, lz2, lz3, lz4, lz5, lz7, lz8, lzs, pc1, pm0, pm1, pm2, pms
lhd is not actually a compression scheme, but indicates that the element represents a subdirectory.
For header levels 1 and higher, each member file has an associated list of "extended headers", similar to ZIP's extensible data fields. Each extended header is tagged with a single byte indicating its type. Extended headers are used to store platform-specific metadata, and to extend the format in other ways.
- List of extended headers (from archive.org)
- libarchive: archive_read_support_format_lha.c (look for "EXT_HEADER_CRC")
Header level 0 supports extended data in a more limited way. It allows for just one set of extended header fields (called the "extended area"), the content of which is determined by the initial one-byte "OS type" field.
- Extended area (from archive.org)
'-' 'l' ?? ?? '-' appear at offset 2. This is not a global file signature, but represents the compression scheme of the first member file of the archive.
Note that if you consider PMA to be a form of LHA, then the second of these bytes can also be
- jLHA software: LHA Notes
- Archive format info
- LZH file header format (among other archive types)
- libarchive: archive_read_support_format_lha.c - Has comments with information about the header formats