View source for LHA (file format)

{{Short description|Data compression file format}}
{{More footnotes needed|date=July 2016}}
{{Infobox software
| name = LHA
| screenshot = LHarc usage message.jpg
| other_names = LHarc, LHx, LH
| author = Haruyasu Yoshizaki
| latest release version = 2.13
| latest release date = {{Start date and age|1991|07|20|df=yes}}
| latest preview version = 2.55b
| latest preview date = {{Start date and age|1992|11|24|df=yes}}
| programming language = [[Assembly language]], [[C (programming language)|C]]
| operating system = [[DOS]]
| replaced_by = LHA32
| license = [[Permissive software license|Permissive license]]
| website = https://www.vector.co.jp/vpack/browse/person/an000224.html
}}
{{Infobox file format
| name = LZH
| extension = .lzh, .lha
| mime = application/x-lzh-compressed
| owner = Haruyasu Yoshizaki (''Yoshi'')
| uniform type = public.archive.lha
| type code = "<code>LHA␣</code>"&nbsp;(L-H-A-SPACE)
| genre = [[Data compression]]
| extendedfrom = LArc
| extendedto = 
}}

'''LHA''' or '''LZH''' is a [[freeware]] [[Data compression|compression]] utility and associated file format. It was created in 1988 by {{nihongo|Haruyasu Yoshizaki|[[:ja:吉崎栄泰|吉崎栄泰]]|Yoshizaki Haruyasu}}, a [[medical 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 {{mono|LH}} ("load high") command. The original LHA and its [[Microsoft Windows|Windows]] port, LHA32, are no longer in development because Yoshizaki is busy at his day job.<ref>{{Cite web|date=1999-04-28|title=LHA World by Dr.Haruyasu Yoshizaki|url=http://www.clione.ne.jp/lha/index_e.html|url-status=dead|archive-url=https://web.archive.org/web/19990428174148/http://www.clione.ne.jp/lha/index_e.html|archive-date=1999-04-28|access-date=2021-01-12}}</ref>

Although no longer much used in the west, LHA remained popular in [[Japan]] until the 2000s.<ref>{{Cite web|last=吉澤|first=亨史|date=2010-06-07|title=「LZH」の開発中止--企業などは使用しないよう作者が注意喚起|url=https://japan.cnet.com/article/20414735/|access-date=2021-01-12|website=CNET Japan|language=ja}}</ref> It was used by [[id Software]] to compress installation files for their earlier games, including ''[[Doom (1993 video game)|Doom]]'' and ''[[Quake (video game)|Quake]]''. Because some versions of LHA have been distributed with source code under the [[Permissive software license|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]].<ref>{{cite web|url=http://www.microsoft.com/globaldev/outreach/dnloads/LZH_add-on.mspx |title=Microsoft Compressed (LZH) Folder Add-on|website=[[Microsoft]] |access-date=2007-10-05 |url-status=dead|archive-url=https://web.archive.org/web/20070819165530/http://www.microsoft.com/globaldev/outreach/dnloads/LZH_add-on.mspx |archive-date=2007-08-19}}</ref> The Japanese version of [[Windows 7]] ships with the LZH folder add-on built-in.<ref>{{cite web|url=http://support.microsoft.com/kb/971613 |title=Windows 7 で (LZH の圧縮に Microsoft) フォルダーのアドインをインストールできません |website=Support.microsoft.com |access-date=2016-07-17}}</ref> 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. {{tt|-lz1-}}. 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 {{nihongo|[[Haruhiko Okumura]]|奥村晴彦|Okumura Haruhiko}}, but uses [[Huffman coding]] instead of [[arithmetic coding]]. 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&nbsp;[[KiB]] [[sliding window]], with support of maximum 60 bytes of matching length. Dynamic Huffman encoding is used.
;lh2
:lh1 variant. This method supports 8&nbsp;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.<ref>Jared (1998). [https://web.archive.org/web/20021005080911/http://www.osirusoft.com/joejared/lzhformat.html lzhformat.html]</ref>

===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]] 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.<ref>{{cite web |title=fragglet/lhasa |url=https://github.com/fragglet/lhasa/blob/master/lib/pm1_decoder.c |website=GitHub |date=7 July 2022 |language=en}}</ref>
;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".<ref>{{cite web|url=http://www.lzh-zip.com/extension/ext31.html |title=圧縮データの拡張子 ".LZS" &#124; 圧縮・解凍ソフトのガイド |website=Lzh-zip.com |access-date=2016-07-17}}</ref> The program seems to have come before LZH. It uses a [[binary search tree]] in the LZ matching.<ref>{{cite web |title=Data Compression Algorithms of LARC and LHarc |url=http://archive.gamedev.net/archive/reference/articles/article295.html |website=GameDev.net}}</ref>

;lzs
:It supports 2&nbsp;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&nbsp;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.<ref>{{cite web|url=http://oku.edu.mie-u.ac.jp/~okumura/compression/history.html|title=History of Data Compression in Japan|website=Oku.edu.mie-u.ac.jp|access-date=12 July 2016}}</ref>

===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.<ref name="aminet.net">{{cite web|url=http://aminet.net/package/util/arc/lha138pch|title=Aminet - util/arc/lha138pch.lha|website=Aminet.net|access-date=12 July 2016}}</ref><ref name="aminet2.net">{{cite web|url=http://aminet.net/package/util/arc/lha_68k|title=Aminet - util/arc/lha_68k.lha|website=Aminet.net|access-date=12 July 2016}}</ref>

The newer Level 2 and 3 headers use a 32-bit [[Unix time]] instead. It suffers from the [[Year 2038 problem]].<ref>Nifty's LHA Format Notes, Other data formats.</ref>

===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.<ref>{{cite web |title=LZH書庫のヘッダー処理における脆弱性について(2010年版) |url=https://micco.mars.jp/vul/2010/mhvi20100425.htm |website=micco.mars.jp}}</ref>

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.

==See also==
{{Portal|Amiga}}
* [[Lhasa (computing)]]
* [[List of archive formats]]
* [[LZX]]

==References==
{{Reflist}}

==External links==
*[http://oku.edu.mie-u.ac.jp/~okumura/compression/history.html A history of data compression in Japan]
*[https://web.archive.org/web/20080724142842/http://homepage1.nifty.com/dangan/en/Content/Program/Java/jLHA/Notes/Notes.html LHA Notes] Document about LHA.
*[https://web.archive.org/web/20070322103559/http://homepage1.nifty.com/dangan/en/Content/Program/Java/jLHA/jLHA.html jLHA] – LHA library for Java
**[http://sourceforge.net/projects/jlhafrontend/ jLHA front-end]
*[http://www2m.biglobe.ne.jp/~dolphin/lha/lha.htm LHA for Unix] {{Webarchive|url=https://web.archive.org/web/20070528141733/http://www2m.biglobe.ne.jp/~dolphin/lha/lha.htm |date=2007-05-28 }} {{in lang|ja}}
*[http://www.fadden.com/techmisc/hdc/lesson11.htm Hacking Data Compression  Lesson 11  LZH, LZARI, and LZB]
*[http://www.nsrl.nist.gov/perl/NSRLmagic NSRL Magic File] contains PMarc info
*[http://www.ponsoftware.com/en/ Explzh] Current Windows 7 archiver for LZH/LHA. (Besides LZH it supports RAR, Zip, 7Z, ACE, Tar, Cab & others)
*[https://fragglet.github.io/lhasa/ lhasa] a cross-platform, [[Open-source software|open source]] LHA decompressor (+UNLHA32, PMArc, LArc extensions)
*[https://web.archive.org/web/20021005080911/http://www.osirusoft.com/joejared/lzhformat.html lzh format] document describing LZH header format.

{{Archive formats}}
{{Compression software}}
{{AmigaOS}}

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[[Category:Archive formats]]
[[Category:Data compression software]]
[[Category:File archivers]]
[[Category:Amiga]]
[[Category:1988 software]]
[[Category:Japanese inventions]]