7z

Source: Wikipedia, the free encyclopedia.

For other uses, see 7Z (disambiguation).
7z file format
Filename extension
.7z
Internet media type
application/x-7z-compressed
exabytes)
Developed byIgor Pavlov[1]
Initial release1999; 25 years ago (1999)[2]
Type of formatData compression
Open format?Yes: GNU Lesser General Public License / Public domain
Website7-zip.org

7z is a compressed

LZMA SDK is version 23.01.[2]

The official, informal 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.[3] There have been additional third-party attempts at writing more concrete documentation based on the released code.[4]

Features and enhancements

The 7z format provides the following main features:

  • Open
    , modular architecture that allows any compression, conversion, or encryption method to be stacked.
  • High compression ratios (depending on the compression method used).
  • AES-256 bit encryption.
  • Zip 2.0 (Legacy) Encryption
  • Large file support (up to approximately 16
    exbibytes
    , or 264 bytes).
  • Unicode file names.
  • Support for solid compression, where multiple files of like type are compressed within a single stream, in order to exploit the combined redundancy inherent in similar files.
  • Compression and encryption of archive 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:

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

PNG, or MNG
files.

Pre-processing filters

The LZMA SDK comes with the

ARM Thumb processors, jump targets are 'normalized' [5]
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 – Converter for 32-bit x86 executables. Normalise target addresses of near jumps and calls from relative distances to absolute destinations.
  • BCJ2– Pre-processor for 32-bit x86 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.
  • Delta encoding – delta filter, basic preprocessor for multimedia data.

Similar executable pre-processing technology is included in other software; the RAR compressor features displacement compression for 32-bit x86 executables and IA-64 executables, and the UPX runtime executable file compressor includes support for working with 16-bit values within DOS binary files.

Encryption

The 7z format supports

SHA-256 hash function. The SHA-256 is executed 219 (524288) times,[6] 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,[7]
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

tar bitstream before compressing with 7z. But GNU tar (common in many UNIX environments) can also compress with the LZMA2 algorithm ("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.)[8]
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

zip files also lack a recovery feature while the rar
format has one.

See also

References

  1. Dr. Dobb's
    Data Compression Newsletter. 30 April 2003. Retrieved 26 December 2009.
  2. ^ a b History of 7-zip changes
  3. ^ LZMA SDK, "DOC" directory, 7zFormat.txt
  4. ^ ".7z format specification — py7zr – 7-zip archive library". py7zr.readthedocs.io.
  5. ^ a b Collin, Lasse. "lzma_.lzma". liblzma bindings. Archived from the original on 8 February 2010. Retrieved 3 January 2010. 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.
  6. ^ 7-zip source code
  7. ^ Colin Percival. scrypt. As presented in "Stronger Key Derivation via Sequential Memory-Hard Functions". presented at BSDCan'09, May 2009.
  8. ^ "GNU tar 1.34: 8.1 Using Less Space through Compression".

Further reading

  • Salomon, David (2007). Data compression: the complete reference. Springer. p. 241.
    ISBN 978-1-84628-602-5
    .

External links
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