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 similar 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: •
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. •
Bzip2 – 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). •
PPMd – 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 – 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 (PPC), IA-64
Itanium, and
ARM Thumb processors, jump targets are "normalized" 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 (such as
UNIX owner/group permissions or
NTFS ACLs), 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 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.) On the other hand, 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
parchives, or within
filesystems with robust
error-correction. By way of comparison,
zip files also lack a recovery feature while the
rar format has one. == See also ==