LHA (file format)

LHA or LZH is a freeware compression utility and associated file format. It was created in 1988 by a doctor , and originally named LHarc. A complete rewrite of LHarc, tentatively named "LHx", was eventually released as "LH". It was then renamed to "LHA" to avoid conflicting with the then-new MS-DOS 5.0 ("load high") command. The original LHA and its Windows port, LHA32, are no longer in development because Yoshizaki is busy at work.
Although no longer much used in the west, LHA remained popular in Japan until the 2000s. It was used by id Software to compress installation files for their earlier games, including "Doom" and "Quake". Because some versions of LHA have been distributed with source code under the permissive license, LHA has been ported to many operating systems and is still the main archiving format used on the Amiga computer, although it competed with LZX in the mid 1990s. This was due to Aminet, the world's largest archive of Amiga-related software and files, standardising on Stefan Boberg's implementation of LHA for the Amiga.
Microsoft released the Microsoft Compressed (LZH) Folder Add-on, which was designed for the Japanese version of Windows XP. The Japanese version of Windows 7 ships with the LZH folder add-on built-in. Users of non-Japanese versions of Windows 7 Enterprise and Ultimate can also install the LZH folder add-on by installing the optional Japanese language pack from Windows Update.
Compression methods.
In an LZH archive, the compression method is stored as a five-byte text string, e.g. . These are the third through seventh bytes of the file.
Canonical LZH.
LHarc compresses files using an algorithm from Yoshizaki's earlier LZHUF product, which was modified from LZARI developed by , but uses Huffman coding instead of arithmetic coding. LZARI uses Lempel–Ziv–Storer–Szymanski with arithmetic coding.
Joe Jared extensions.
Joe Jared extended LZSS to use larger dictionaries.
Jared ported LZH to Atari. The fact that lh8 is the same as lh7 was an oversight. Files using larger numbered methods may as well not exist, as Jared only considers them planned features.
UNLHA32 extensions.
UNLHA32.DLL uses its own method for testing purposes.
PMarc extensions.
These compression methods are created by PMarc, a CP/M archiver created by Miyo. The archive usually has a .PMA extension.
LArc extensions.
LArc uses the same file format as .LZH, but was written by Kazuhiko Miki, Haruhiko Okumura and Ken Masuyama, with extension name ".LZS". The program seems to have come before LZH. It uses a binary search tree in the LZ matching.
Common implementations appear to only support lzs, lz5, plus the storage-only lz4.
Issues.
LHICE/ICE.
There are copies of LHICE marked as version 1.14. According to Okumura, LHICE is not written by Yoshi.
Y2K11 bug.
Because of a bug, DOS time stamps from Level 0 and 1 headers after the year 2011 will be set to 1980, meaning that some utilities need to be patched. This is caused by a bug that interprets the unsigned 8-bit year number bitfield as a 5-bit number. The maximum year should be 2107 instead.
The newer Level 2 and 3 headers use a 32-bit Unix time instead. It suffers from the Year 2038 problem.
Header size.
According to Micco, the author of a popular LHA library UNLHA32.DLL, many LHA implementations do not check for the length of LHA file headers when reading the archive. Two problems could emerge from this scenario: a buffer-overrun may occur for naive implementations assuming a 4KB max size from the original specification; antivirus software may skip over files with such large headers and fail to scan for a virus. A similar problem exists with ARJ. Micco reported this problem to Japanese authorities, but they do not consider it a valid vulnerability.
Micco went so far to conclude the development of UNLHA32 and advise people to give up on the format. Nevertheless, they came back in 2017 to fix a DLL hijacking issue.