LHA or LZH is a

freeware Freeware is software, often proprietary, that is distributed at no monetary cost to the end user. There is no agreed-upon set of rights, license, or EULA that defines ''freeware'' unambiguously; every publisher defines its own rules for the free ...

compression utility and associated file format. It was created in 1988 by , a

medical doctor A physician, medical practitioner (British English), medical doctor, or simply doctor is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through the study, diagnosis, prognosis ...

, 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 MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few op ...

5.0 ("load high") command. The original LHA and its

Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

port, LHA32, are no longer in development because Yoshizaki is busy at his day job. Although no longer much used in the west, LHA remained popular in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

until the 2000s. It was used by

id Software id Software LLC () is an American video game developer based in Richardson, Texas. It was founded on February 1, 1991, by four members of the computer company Softdisk: game programmer, programmers John Carmack and John Romero, game designer T ...

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 A permissive software license, sometimes also called BSD-like or BSD-style license, is a free-software license which instead of copyleft protections, carries only minimal restrictions on how the software can be used, modified, and redistributed, ...

, LHA has been ported to many operating systems and is still the main archiving format used on the

Amiga Amiga is a family of personal computers produced by Commodore International, Commodore from 1985 until the company's bankruptcy in 1994, with production by others afterward. The original model is one of a number of mid-1980s computers with 16-b ...

computer, although it competed with

LZX LZX is an LZ77 family Data compression, compression algorithm, a slightly improved version of DEFLATE. imlib: the open source Windows Imaging (WIM) library - Compression algorithm https://wimlib.net/compression.htmlIt is also the name of a arc ...

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 Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct successor to Windows 2000 for high-end and business users a ...

. The Japanese version of

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on July 22, 2009, and became generally available on October 22, ...

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

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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 In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code is Huffman coding, an algorithm developed by ...

instead of

arithmetic coding Arithmetic coding (AC) is a form of entropy encoding used in lossless data compression. Normally, a String (computer science), string of characters is represented using a fixed number of bits per character, as in the American Standard Code for In ...

. LZARI uses Lempel–Ziv–Storer–Szymanski with arithmetic coding. ;lh0 :No compression method is applied to the source data. ;lh1 :This method is introduced in LHarc version 1. :It supports 4 KiB sliding window, with support of maximum 60 bytes of matching length. Dynamic Huffman encoding is used. ;lh2 :lh1 variant. This method supports 8 KiB sliding window, with support of maximum 256 bytes of matching length. Dynamic Huffman encoding is used. ;lh3 :lh2 variant with Static Huffman. ;lh4, lh5, lh6, lh7 :Methods 4, 5, 6, 7 support 4, 8, 32, 64 KiB sliding window respectively, with support of maximum 256 bytes of matching length. Static Huffman encoding is used. lh5 is first introduced in LHarc 2, followed by lh6 in LHA 2.66 (MSDOS), lh7 in LHA 2.67 beta (MSDOS). LHA itself never compresses into lh4. ;lhd :Technically it is not a compression method, but it is used in .LZH archive to indicate that the compressed object is an empty directory.

Joe Jared extensions

Joe Jared extended LZSS to use larger dictionaries. ;lh8, lh9, lha, lhb, lhc, lhe :Dictionary (sliding window) sizes are 64, 128, 256, 512, 1024, 2048 KiB respectively. 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. ;lhx :It uses 128–256 KiB dictionary.

PMarc extensions

These compression methods are created by PMarc, a

CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/Intel 8085, 85-based microcomputers by Gary Kildall of Digital Research, Dig ...

archiver created by Miyo. The archive usually has a .PMA extension. ;pc1 :PopCom compressed executable archive. Details unknown. ;pm0 :No compression method is applied to the source data. ;pm1 :8 KB sliding window, static huffman. Seldom generated, decompressor is reverse-engineered. ;pm2 :lh5 variant, 4K sliding window. ;pms :Used to indicate PMarc self-extracting archive. Should be skipped to reveal the real format.

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 computer science, a binary search tree (BST), also called an ordered or sorted binary tree, is a Rooted tree, rooted binary tree data structure with the key of each internal node being greater than all the keys in the respective node's left ...

in the LZ matching. ;lzs :It supports 2 KiB sliding window, with support of maximum 17 bytes of matching length. ;lz2 :It is similar to lzs, except dictionary size and match length can be changed. ;lz3 :Unknown. ;lz4 :No compression method is applied to the source data. ;lz5 :It supports 4 KiB sliding window, with support of maximum 17 bytes of matching length. ;lz7 ;lz8 :Unknown. 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 Yoshizaki.

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 7-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 Unix time is a date and time representation widely used in computing. It measures time by the number of non-leap seconds that have elapsed since 00:00:00 Coordinated Universal Time, UTC on 1 January 1970, the Unix Epoch (computing), epoc ...

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.

References

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...

LHA decompressor (+UNLHA32, PMArc, LArc extensions)
lzh format
document describing LZH header format. {{DEFAULTSORT:Lha (File Format) Archive formats Data compression software File archivers Amiga 1988 software Japanese inventions