7z is a compressed archive file format that supports several different data
7z is a compressed archive file format that supports several different data

compression, encryption and pre-processing algorithms. The 7z format
compression, encryption and pre-processing algorithms. The 7z format

initially appeared as implemented by the 7-Zip archiver. The 7-Zip program is
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.
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
The LZMA SDK 4.62 was placed in the public domain in December 2008. The

latest stable version of 7-Zip and LZMA SDK is version 9.20.
latest stable version of 7-Zip and LZMA SDK is version 9.20.

The official 7z file format specification is distributed with
The official 7z file format specification is distributed with

7-Zip's source code. The specification can be found in plain text format in the
7-Zip's source code. The specification can be found in plain text format in the

'doc' sub-directory of the source code distribution.
'doc' sub-directory of the source code distribution.

Features and enhancements The 7z format provides the following
Features and enhancements The 7z format provides the following

main features: Open, modular architecture which allows
main features: Open, modular architecture which allows

any compression, conversion, or encryption method to be stacked.
any compression, conversion, or encryption method to be stacked.

High compression ratios Rijndael/AES-256 encryption.
High compression ratios Rijndael/AES-256 encryption.

Large file support. Unicode file names
Large file support. Unicode file names

Support for solid compression, where multiple files of like type are
Support for solid compression, where multiple files of like type are

compressed within a single stream, in order to exploit the combined redundancy
compressed within a single stream, in order to exploit the combined redundancy

inherent in similar files. Compression and encryption of archive
inherent in similar files. Compression and encryption of archive

headers. Support for multi-part archives : e.g.
headers. Support for multi-part archives : e.g.

xxx.7z.001, xxx.7z.002, ... The format's open architecture allows
xxx.7z.001, xxx.7z.002, ... The format's open architecture allows

additional future compression methods to be added to the standard.
additional future compression methods to be added to the standard.

= Compression methods = The following compression methods are
= Compression methods = The following compression methods are

currently defined: LZMA – A variation of the LZ77
currently defined: LZMA – A variation of the LZ77

algorithm, using a sliding dictionary up to 4 GB in length for duplicate string
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
elimination. The LZ stage is followed by entropy coding using a Markov

chain-based range coder and binary trees.
chain-based range coder and binary trees.

LZMA2 – modified version of LZMA providing better multithreading support
LZMA2 – modified version of LZMA providing better multithreading support

and less expansion of incompressible data.
and less expansion of incompressible data.

Bzip2 – The standard Burrows–Wheeler transform algorithm. Bzip2 uses two
Bzip2 – The standard Burrows–Wheeler transform algorithm. Bzip2 uses two

reversible transformations; BWT, then Move to front with Huffman coding for
reversible transformations; BWT, then Move to front with Huffman coding for

symbol reduction. PPMd – Dmitry Shkarin's 2002 PPMdH with
symbol reduction. PPMd – Dmitry Shkarin's 2002 PPMdH with

small changes: PPMII is an improved version of the 1984 PPM compression
small changes: PPMII is an improved version of the 1984 PPM compression

algorithm. DEFLATE – Standard algorithm based on 32
algorithm. DEFLATE – Standard algorithm based on 32

kB LZ77 and Huffman coding. Deflate is found in several file formats including
kB LZ77 and Huffman coding. Deflate is found in several file formats including

ZIP, gzip, PNG and PDF. 7-Zip contains a from-scratch DEFLATE encoder that
ZIP, gzip, 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
frequently beats the de facto standard zlib version in compression size, but at

the expense of CPU usage. A suite of recompression tools called
the expense of CPU usage. A suite of recompression tools called

AdvanceCOMP contains a copy of the DEFLATE encoder from the 7-Zip
AdvanceCOMP contains a copy of the DEFLATE encoder from the 7-Zip

implementation; these utilities can often be used to further compress the
implementation; these utilities can often be used to further compress the

size of existing gzip, ZIP, PNG, or MNG files.
size of existing gzip, ZIP, PNG, or MNG files.

= Pre-processing filters = The LZMA SDK comes with the BCJ / BCJ2
= Pre-processing filters = The LZMA SDK comes with the BCJ / BCJ2

preprocessor included, so that later stages are able to achieve greater
preprocessor included, so that later stages are able to achieve greater

compression: For x86, ARM, PowerPC, IA-64 Itanium, and ARM Thumb processors,
compression: For x86, ARM, PowerPC, IA-64 Itanium, and ARM Thumb processors,

jump targets are normalized before compression by changing relative
jump targets are normalized before compression by changing relative

position into absolute values. For x86, this means that near jumps, calls and
position into absolute values. For x86, this means that near jumps, calls and

conditional jumps are converted from the machine language "jump 1655 bytes
conditional jumps are converted from the machine language "jump 1655 bytes

backwards" style notation to normalized "jump to address 5554" style notation;
backwards" style notation to normalized "jump to address 5554" style notation;

all jumps to 5554, perhaps a common subroutine, are thus encoded
all jumps to 5554, perhaps a common subroutine, are thus encoded

identically, making them more compressible.
identically, making them more compressible.

BCJ – Converter for 32-bit x86 executables. Normalise target addresses
BCJ – Converter for 32-bit x86 executables. Normalise target addresses

of near jumps and calls from relative distances to absolute destinations.
of near jumps and calls from relative distances to absolute destinations.

BCJ2 – Pre-processor for 32-bit x86 executables. BCJ2 is an improvement on
BCJ2 – Pre-processor for 32-bit x86 executables. BCJ2 is an improvement on

BCJ, adding additional x86 jump/call instruction processing. Near jump, near
BCJ, adding additional x86 jump/call instruction processing. Near jump, near

call, conditional near jump targets are split out and compressed separately in
call, conditional near jump targets are split out and compressed separately in

another stream. Delta encoding – delta filter, basic
another stream. Delta encoding – delta filter, basic

preprocessor for multimedia data. Similar executable pre-processing
preprocessor for multimedia data. Similar executable pre-processing

technology is included in other software; the RAR compressor features
technology is included in other software; the RAR compressor features

displacement compression for 32-bit x86 executables and IA-64 executables, and
displacement compression for 32-bit x86 executables and IA-64 executables, and

the UPX runtime executable file compressor includes support for working
the UPX runtime executable file compressor includes support for working

with 16-bit values within DOS binary files.
with 16-bit values within DOS binary files.

= Encryption = The 7z format supports encryption with
= Encryption = The 7z format supports encryption with

the AES algorithm with a 256-bit key. The key is generated from a
the 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
user-supplied passphrase using an algorithm based on the SHA-256 hash

function. The SHA-256 is executed 218 times which causes a significant delay
function. The SHA-256 is executed 218 times which causes a significant delay

on slow PCs before compression or extraction starts. This technique is
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
called key stretching and is used to make a brute-force search for the

passphrase more difficult. Current GPU-based, and custom hardware attacks
passphrase more difficult. Current GPU-based, and custom hardware attacks

limit the effectiveness of this particular method of key stretching, so
limit the effectiveness of this particular method of key stretching, so

it is still important to choose a strong password. The 7z format provides the
it is still important to choose a strong password. The 7z format provides the

option to encrypt the filenames of a 7z archive.
option to encrypt the filenames of a 7z archive.

= Limitations = The 7z format does not store filesystem
= Limitations = The 7z format does not store filesystem

permissions, and hence can be inappropriate for backup/archival
permissions, and hence can be inappropriate for backup/archival

purposes. A workaround on UNIX-like systems for this is to convert data to a
purposes. A workaround on UNIX-like systems for this is to convert data to a

tar bitstream before compressing with 7z. But it is worth noting that GNU tar
tar bitstream before compressing with 7z. But it is worth noting that GNU tar

can also compress with the LZMA algorithm natively, without the use of
can also compress with the LZMA algorithm natively, without the use of

7z, and that in this case the suggested file extension for the archive is
7z, and that in this case the suggested file extension for the archive is

".tar.lzma", and not ".tar.7z". On the other hand, it is important to note,
".tar.lzma", and not ".tar.7z". On the other hand, it is important to note,

that tar does not save the filesystem encoding, which means that tar
that tar does not save the filesystem encoding, which means that tar

compressed filenames can become unreadable if decompressed on a
compressed filenames can become unreadable if decompressed on a

different computer. It is also possible to use LZMA2 by running it through the
different computer. It is also possible to use LZMA2 by running it through the

xz tool. Recent versions of GNU tar support the -J switch which runs TAR
xz tool. Recent versions of GNU tar support the -J switch which runs TAR

through XZ. The file extension is ".tar.xz" or ".txz". This method of
through XZ. The file extension is ".tar.xz" or ".txz". This method of

compression has been adopted with many distributions for packaging, such as
compression has been adopted with many distributions for packaging, such as

Arch, Debian, Fedora and Slackware. The 7z format does not allow extraction
Arch, Debian, Fedora and Slackware. The 7z format does not allow extraction

of some "broken files"—that is if one has the first segment of a series of 7z
of some "broken files"—that is 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
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,
until all segments are downloaded. The 7z format also lacks recovery records,

which might be a problem when limited file corruption has occurred. By way of
which might be a problem when limited file corruption has occurred. By way of

comparison, zip files also lack a recovery feature. In contrast the
comparison, zip files also lack a recovery feature. In contrast the

proprietary rar format permits recoveries as well as the extraction of
proprietary rar format permits recoveries as well as the extraction of

broken files and file spanning. See also
broken files and file spanning. See also

Comparison of archive formats List of archive formats
Comparison of archive formats List of archive formats

Free file format Open format
Free file format Open format

References Further reading
References Further reading

Salomon, David. Data compression: the complete reference. Springer. p. 241.
Salomon, David. Data compression: the complete reference. Springer. p. 241.

ISBN 1-84628-602-6. External links
ISBN 1-84628-602-6. External links

7z Format – General description about the 7z archive format.
7z Format – General description about the 7z archive format.

7-Zip Official Website 7-Zip on Sourceforge
7-Zip Official Website 7-Zip on Sourceforge

7-Zip Command-Line Manual 7-Zip Extractor
7-Zip Command-Line Manual 7-Zip Extractor