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 LZMA SDK is version 19.00. The 7z file format specification is distributed with 7-Zip's source code. The specification can be found in plain text format in the 'doc' sub-directory of the source code distribution.
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.
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.
Support for multi-part archives : e.g. xxx.7z.001, xxx.7z.002,....
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 – A variation of the 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 coder and binary trees.
LZMA2 – modified version of LZMA providing better multithreading support and less expansion of incompressible data.
PPMd – Dmitry Shkarin's 2002 PPMdH with small changes: PPMII is an improved version of the 1984 PPM compression algorithm.
DEFLATE – Standard algorithm based on 32 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 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, PNG, or MNG files.
Pre-processing filters
The LZMA SDK comes with the BCJ and BCJ2 preprocessors included, so that later stages are able to achieve greater compression: For x86, ARM, PowerPC, IA-64 Itanium, and ARM Thumb processors, jump targets are 'normalized' before compression by changing relative position into absolute values. For x86, this means that near jumps, calls 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 DOSbinary files.
Encryption
The 7z format supports encryption with 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 function. The SHA-256 is executed 218 times, 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, 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, and hence can be inappropriate for backup/archival 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 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 ".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 compressed filenames can become unreadable if decompressed on a 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 through XZ. The file extension is ".tar.xz" or ".txz". This method of compression has been adopted with many distributions for packaging, such as 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 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 parchives, or within filesystems with robust error-correction. By way of comparison, zip files also lack a recovery feature.
