View source for 7z

{{short description|Family of archive file formats used by 7-Zip}}
{{use dmy dates|date=July 2020}}
{{other uses|7Z (disambiguation)}}
{{Infobox file format
| name         = 7z file format
| icon         = 7zip archive icon.svg
| extension    = .7z
| mime         = application/x-7z-compressed
| uniform type = org.7-zip.7-zip-archive
| owner        = [[Igor Pavlov (programmer)|Igor Pavlov]]<ref name="dobb">{{cite web |url=http://www.ddj.com/architect/184405338 |title=A Few Questions for Igor Pavlov |date=2003-04-30 |publisher=[[Dr. Dobb's]] Data Compression Newsletter |access-date=2009-12-26 |archive-date=28 October 2008 |archive-url=https://web.archive.org/web/20081028091651/http://www.ddj.com/architect/184405338 |url-status=live }}</ref>
| released     = {{Start date and age|1999}}<ref name="History">{{Cite web |url=http://www.7-zip.org/history.txt |title=History of 7-zip changes |access-date=10 June 2010 |archive-date=27 February 2015 |archive-url=https://web.archive.org/web/20150227213935/http://www.7-zip.org/history.txt |url-status=live }}</ref>
| creatorcode  = 
| genre        = [[Data compression]]
| magic        = '7', 'z', 0xBC, 0xAF, 0x27, 0x1C
| max_size = 2<sup>64</sup> bytes (roughly 18 [[exabytes]])
| containerfor = 
| containedby  = 
| extendedfrom = 
| extendedto   = 
| open         = Yes: [[GNU Lesser General Public License]] / [[Public domain]]
| url          = {{URL|7-zip.org}}

}}

'''7z''' is a compressed [[archive file format]] that supports several different [[data compression]], [[encryption]] and pre-processing algorithms.  The 7z format initially appeared as implemented by the [[7-Zip]] archiver.  The 7-Zip program is publicly available under the terms of the [[GNU Lesser General Public License]].  The LZMA SDK 4.62 was placed in the [[public domain]] in December 2008.  The latest stable version of 7-Zip and [[Lempel-Ziv-Markov chain algorithm|LZMA]] SDK is version 25.01.<ref name="History"/>

The 7z file format specification is distributed with 7-Zip's source code since 2015. The specification can be found in plain text format in the "doc" sub-directory of the source code distribution.<ref>LZMA SDK, "DOC" directory, 7zFormat.txt</ref><ref name="lzma2_source_code" />

== Features and enhancements ==
The 7z format provides the following main features:
* [[Open format|Open]], modular architecture that allows any compression, conversion, or encryption method to be stacked.
* High [[data compression ratio|compression ratio]]s (depending on the compression method used).
* [[Advanced Encryption Standard|AES]]-256 bit [[encryption]].
* Zip 2.0 (Legacy) Encryption
* Large file support (up to approximately 16 [[exbibyte]]s, or 2<sup>64</sup> bytes).
* [[Unicode]] file names.
* Support for [[solid compression]], where multiple files of similar type are compressed within a single stream, in order to exploit the combined redundancy inherent in similar files.
* Compression and encryption of archive [[header (computing)|headers]].
* Support for multi-part archives : e.g. xxx.7z.001, xxx.7z.002, ... (see the context menu items ''Split File...'' to create them and ''Combine Files...'' to re-assemble an archive from a set of multi-part component files).
* Support for custom codec plugin DLLs.

The format's [[open architecture]] allows additional future compression methods to be added to the standard.

=== Compression methods ===
The following compression methods are currently defined:

* [[LZMA]]&nbsp;– A variation of the [[LZ77 and LZ78|LZ77]] algorithm, using a sliding dictionary up to 4 GB in length for duplicate string elimination.  The LZ stage is followed by [[entropy coding]] using a [[Markov chain]]-based [[range encoding|range coder]] and [[binary tree]]s.
* [[Lempel–Ziv–Markov chain algorithm#LZMA2 format|LZMA2]]&nbsp;– modified version of LZMA providing better multithreading support and less expansion of incompressible data.<ref name="lzma2_source_code">{{cite web |last=Collin |first=Lasse |title=lzma_.lzma |url=http://jpf91.github.io/lzmad/api/lzma_lzma.html |url-status=live |archive-url=https://web.archive.org/web/20100208075245/https://www.google.com/codesearch/p?hl=en |archive-date=8 February 2010 |access-date=2010-01-03 |work=liblzma bindings |quote=Compared to LZMA1, LZMA2 adds support for LZMA_SYNC_FLUSH, uncompressed chunks (smaller expansion when trying to compress uncompressible data), possibility to change lc/lp/pb in the middle of encoding, and some other internal improvements.}}</ref>
* [[Bzip2]]&nbsp;– The standard [[Burrows–Wheeler transform]] algorithm.  Bzip2 uses two reversible transformations; BWT, then [[Move to front]] with [[Huffman coding]] for symbol reduction (the actual compression element).
<!-- [[Bzip]] used (stronger, but patented) [[arithmetic coding]].  No point mentioning this since 7z doesn't use it! -->
* [[Prediction by Partial Matching|PPMd]]&nbsp;– Dmitry Shkarin's 2002 PPMdH (PPMII (Prediction by Partial matching with Information Inheritance) and cPPMII (complicated PPMII)) with small changes: PPMII is an improved version of the 1984 [[PPM compression algorithm]] (prediction by partial matching).
* [[DEFLATE]]&nbsp;– Standard algorithm based on 32 kB [[LZ77 and LZ78|LZ77]] and [[Huffman coding]].  Deflate is found in several file formats including [[ZIP (file format)|ZIP]], [[gzip]], [[Portable Network Graphics|PNG]] and [[PDF]].  7-Zip contains a from-scratch DEFLATE encoder that frequently beats the ''de facto'' standard [[zlib]] version in compression size, but at the expense of CPU usage.

A suite of recompression tools called AdvanceCOMP contains a copy of the DEFLATE encoder from the 7-Zip implementation; these utilities can often be used to further compress the size of existing [[gzip]], [[ZIP (file format)|ZIP]], [[Portable Network Graphics|PNG]], or [[Multiple-image Network Graphics|MNG]] files.

=== Pre-processing filters ===
The LZMA SDK comes with the [[BCJ (algorithm)|BCJ]] and [[BCJ2]] preprocessors included, so that later stages are able to achieve greater compression: For [[x86]], [[ARM architecture|ARM]], [[PowerPC]] (PPC), IA-64 [[Itanium]], and [[ARM Thumb]] processors, jump targets are "normalized"<ref name="lzma2_source_code" /> before compression by changing relative position into absolute values. For x86, this means that near jumps, calls and conditional jumps (but not short jumps and conditional jumps) are converted from the machine language "jump 1655 bytes backwards" style notation to normalized "jump to address 5554" style notation; all jumps to 5554, perhaps a common subroutine, are thus encoded identically, making them more compressible.

*[[BCJ (algorithm)|BCJ]]&nbsp;– Converter for 32-bit x86 executables. Normalises target addresses of near jumps and calls from relative distances to absolute destinations.
*[[BCJ2]]&nbsp;– Pre-processor for x86-64 executables. BCJ2 is an improvement on BCJ, adding additional x86 jump/call instruction processing.  Near jump, near call, conditional near jump targets are split out and compressed separately in another stream.
*BCJ-type filters for ARM64, ARM32, ARM-Thumb, PowerPC, SPARC.
*[[Delta encoding]]&nbsp;– delta filter, basic preprocessor for multimedia data.
*Swap2/Swap4&nbsp;– endianess-swap filter.

=== Encryption ===
The 7z format supports [[encryption]] with the [[Advanced Encryption Standard|AES]] algorithm with a 256-bit key.  The key is generated from a user-supplied [[passphrase]] using an algorithm based on the [[SHA-256]] hash function. The SHA-256 is executed 2<sup>19</sup> (524288) times,<ref>{{Cite web |url=https://sourceforge.net/projects/sevenzip/?source=directory |title=7-zip source code |access-date=23 March 2018 |archive-date=22 March 2019 |archive-url=https://web.archive.org/web/20190322095659/https://sourceforge.net/projects/sevenzip/?source=directory |url-status=live }}</ref> which causes a significant delay on slow PCs before compression or extraction starts. This technique is called [[key stretching]] and is used to make a [[brute-force search]] for the passphrase more difficult.  Current GPU-based, and custom hardware attacks limit the effectiveness of this particular method of key stretching,<ref name="percival2009">[[Colin Percival]].
[http://www.tarsnap.com/scrypt.html scrypt] {{Webarchive|url=https://web.archive.org/web/20190528073159/https://www.tarsnap.com/scrypt.html |date=28 May 2019 }}.
As presented in
[http://www.tarsnap.com/scrypt/scrypt.pdf "Stronger Key Derivation via Sequential Memory-Hard Functions"] {{Webarchive|url=https://web.archive.org/web/20190414144147/http://www.tarsnap.com/scrypt/scrypt.pdf |date=14 April 2019 }}.
presented at BSDCan'09, May 2009.</ref> so it is still important to choose a strong password.
The 7z format provides the option to encrypt the filenames of a 7z archive.

=== Limitations ===
The 7z format does not store [[filesystem permissions]] (such as [[UNIX]] owner/group permissions or [[NTFS]] [[Access control list|ACL]]s), and hence can be inappropriate for backup/archival purposes. A workaround on UNIX-like systems for this is to convert data to a [[Tar (file format)|tar bitstream]] before compressing with 7z. But GNU tar (common in many UNIX environments) can also compress with the LZMA2 algorithm ("[[XZ Utils|xz]]") natively, without the use of 7z, using the "-J" switch. The resulting file extension is ".tar.xz" or ".txz" and not ".tar.7z". This method of compression has been adopted with many distributions for packaging, such as Arch, Debian (deb), Fedora (rpm) and Slackware. (The older "lzma" format is less efficient.)<ref>{{Cite web|url=https://www.gnu.org/software/tar/manual/html_section/Compression.html|title=GNU tar 1.34: 8.1 Using Less Space through Compression|access-date=17 March 2015|archive-date=2 April 2015|archive-url=https://web.archive.org/web/20150402103628/https://www.gnu.org/software/tar/manual/html_section/Compression.html|url-status=live}}</ref> On the other hand, it is important to note, that tar does not save the filesystem encoding, which means that tar compressed filenames can become unreadable if decompressed on a different computer. 

The 7z format does not allow extraction of some "broken files"—that is (for example) if one has the first segment of a series of 7z files, 7z cannot give the start of the files within the archive—it must wait until all segments are downloaded. The 7z format also lacks recovery records, making it vulnerable to [[data degradation]] unless used in conjunction with external solutions, like [[Parchive|parchives]], or within [[File system|filesystems]] with robust [[Error correction code|error-correction]]. By way of comparison, [[zip (file format)|zip]] files also lack a recovery feature while the [[RAR_(file_format)|rar]] format has one.

== See also ==
*[[7-Zip]]
*[[Comparison of archive formats]]
*[[List of archive formats]]
*[[Open file format]]

==References==
{{Reflist}}

==Further reading==
* {{cite book |title=Data compression: the complete reference |last=Salomon |first=David |year=2007 |publisher=Springer |isbn=978-1-84628-602-5 |page=241}}

== External links ==
<!-- Per [[WP:ELMINOFFICIAL]], choose one official website only -->
* {{Official Website|www.7-zip.org}}
* {{sourceforge|sevenzip}}

{{Archive formats}}

[[Category:Computer-related introductions in 1999]]
[[Category:Archive formats]]
[[Category:Russian inventions]]