7z is a compressed archive file format that supports multiple data compression, encryption, and different pre-processing algorithms. Format 7z originally appeared as implemented by 7-Zip archiver. The 7-Zip program is publicly available under the terms of the GNU General Public License. The LZMA SDK 4.62 is placed in the public domain in December 2008. The latest stable version of the 7-Zip and LZMA SDK is version 16.
The 7z file format specification is distributed with 7-Zip source code. Specifications can be found in plain text format in the 'doc' subdirectory of the source code distribution.
Video 7z
Features and enhancements
Format 7z provides the following key features:
- Open, a modular architecture that enables any compression, conversion, or encryption method to be stacked.
- High compression ratio (depending on the compression method used)
- AES-256 Encryption.
- Large file support (up to about 16 exbibytes, or 2 64 bytes).
- Unicode filename
- Support for compressed solids, where some similar files are compressed in a single stream, to exploit the combined redundancy attached to similar files.
- Compression and archive header encryption.
- Support for multi-part archives: e.g. xxx.7z.001, xxx.7z.002,... (see the context menu item Split Files... to create and Merge File... to reassemble archive of a collection of multi-component component files)
- Support for custom DLL codec plugins.
Open format architecture allows additional compression methods in the future to be added to the standard.
Compression method
The following compression methods are currently defined:
- LZMAÃ, - Variations of the LZ77 algorithm, using a sliding dictionary up to 4 GB in length for the elimination of duplicate strings. The LZ stage is followed by entropy coding using the Markov chain-based binary tree and tree.
- LZMA2Ã, - a modified version of LZMA provides better multithreading support and fewer non-compressed data expansions.
- Bzip2Ã, - The standard Burrows-Wheeler transformation algorithm. Bzip2 uses two reversible transformations; BWT, then Move forward with Huffman coding for symbol reduction (actual compression element).
- PPMdÃ, - Dmitry Shkarin 2002 PPMdH (PPMII/cPPMII) with minor changes: PPMII is an improved version of PPM 1984 compression algorithm (predicted by partial matching).
- DEFLATEÃ, - The standard algorithm is based on 32 kB LZ77 and Huffman coding. Deflate is found in several file formats including ZIP, gzip, PNG and PDF. 7-Zip contains DEFLATE encoder from scratch that often defeat de standard zlib version de facto in compression size, but at the expense of CPU usage.
A set of recompression tools called AdvanceCOMP contains a copy of the DEFLATE encoder from the 7-Zip implementation; this utility can often be used to compress the size of an existing gzip, ZIP, PNG, or MNG file.
Pre-processing filter
The LZMA SDK comes with a preprocessed BCJ and BCJ2 preprocessor, so that the next stage can achieve greater compression: For x86, ARM, PowerPC (PPC), IA-64 Itanium, and ARM Thumb processors, normal spike targets before compression by changing the relative position into an absolute value. For x86, this means that close jumps, call and conditional jumps (but not short jumps and conditional jumps) are changed from machine language "1655 byte jump" style notation to normalization "jump to address 5554" style notation; all jumped to 5554, perhaps a common subroutine, thus encoded identically, making it easier to compress.
- BCJÃ, - Converter for 32-bit x86 executable. Normalize target address from close jump and call from relative distance to absolute destination.
- BCJ2Ã, - Pre-processor for 32-bit x86 executable. BCJ2 is an increase in BCJ, adding an additional x86 jump/call instruction processing. Close jumps, close calls, close conditional jump targets are separated and compressed separately in other streams.
- Delta encoding - delta filter, basic preprocessor for multimedia data.
Similar pre-processing technologies are included in other software; RAR compressor displacement compression features for 32-bit x86 executable and IA-64 executable, and an executable UPX runtime file compressor including support for working with 16-bit values ​​in binary DOS files.
Encryption
Format 7z supports encryption with the AES algorithm with 256-bit keys. The key is generated from the user-provided passphrase using an algorithm based on the SHA-256 hash function. SHA-256 runs 2 18 (262144) times, which causes significant delay on slow PC before compression or extraction begins. This technique is called key stretching and is used to make rugged force searches for more difficult passphrases. GPU-based hardware attacks currently limit the effectiveness of this particular button stretch method, so it's still important to choose a strong password. Format 7z provides the option to encrypt the file name of the 7z archive.
Limitations
Format 7z does not store filesystem permissions (such as UNIX or UNL NTFS owner/group permissions), and is therefore not suitable for backup/archive purposes. A solution on a UNIX-like system for this is to convert data to bitstream tar before compressing with 7z. But it should be noted that GNU tar (common in many UNIX environments) can also compress the original LZMA algorithm, without using 7z, and in this case the file extension suggested for the archive is ".tar.lzma" (or just ".tlz" ), and not ".tar.7z". On the other hand, it is important to note, that tar does not store the filesystem encoding, which means that the filenames that compressed tar can become unreadable if decompressed on different computers. It is also possible to use LZMA2 by running it via the xz tool. The latest version of GNU tar supports the -J switch, which runs TAR via XZ. The file extension is ".tar.xz" or ".txz". This compression method has been adopted with many distributions for packaging, such as Arch, Debian (deb), Fedora (rpm) and Slackware.
Format 7z does not allow the extraction of some "corrupted files" - ie (for example) if someone has the first segment of a set of 7z files, 7z can not give start files in the archive - it must wait until all segments are downloaded. Format 7z also has no recovery record, making it vulnerable to data degradation. For comparison, zip files also do not have recovery features. Instead the proprietary RAR format allows recovery as well as the extraction of damaged files and stretched files.
Maps 7z
See also
- Comparison of archive formats
- List of archive formats
- Free file format
- Open the format
References
Further reading
- Salomon, David (2007). Data compression: full reference . Jumper. p.Ã, 241. ISBNÃ, 1-84628-602-6.
External links
- Official website
- 7z on SourceForge.net
Source of the article : Wikipedia
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