7z

7z

Filename extension	.7z
Internet media type	application/x-7z-compressed
Developed by	Igor Pavlov
Type of format	Data compression

7z is a compressed archive file format that supports several different data compression, encryption and pre-processing filters. The 7z format initially appeared as implemented by the 7-Zip archiver. Both the 7-Zip program and a library to read the 7z file format are publicly available under the terms of the GNU Lesser General Public License.

The MIME type of 7z is application/x-7z-compressed.

Features and enhancements

The 7z format provides the following main features:

• Open, modular architecture which allows any compression, conversion, or encryption method to be stacked.
• High compression ratios (depending on the compression method used)
• Strong Rijndael/AES-256 encryption.
• Large file support (up to approximately 16 exabytes).
• 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 of archive headers.

The format's open architecture allows additional future compression methods to be added to the standard.

Compression method filters

The following compression methods are currently defined:

• LZMA – A variation of the LZ77 algorithm, using a sliding dictionary up to 1 GB in length for duplicate string elimination. The LZ stage is followed by entropy coding using a Markov chain based range coder and Patricia trees.
• 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/cPPMII) 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 (LZSS actually) 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 defacto 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 or PNG files.

Pre-processing filters (for executable files)

The LZMA SDK comes with the BCJ / BCJ2 preprocessor included, so that later stages are able to achieve greater compression: For x86, ARM, PowerPC (PPC), IA64 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 (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.

• 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.

Similar executable pre-processing technology is included in other software; the RAR compressor features displacement compression for 32-bit x86 executables and IA64 Itanium executables, and the UPX runtime executable file compressor includes support for working with use 16 bit values within DOS binary 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 algorithm. The SHA-256 is executed 256K times which causes a significant delay on slow PCs before compression or extraction starts. This technique is called key strengthening and is used to make a brute-force search for the passphrase more difficult. The 7z format provides the option to encrypt the filenames of a 7z archive.

7z support at present

Listed below is a guide to how currently available archiving software supports the 7z archive format.

Full support

These programs support both the creation of and adding files to 7z archives, not just reading files in them.

Linux

• Peazip
• File Roller
• Ark (GPL) (acts as a wrapper around p7zip)
• p7zip (GPL)
• Karchiver (GPL)

Mac

• 7zX (freeware)
• BetterZip (shareware)
• EZ 7z (freeware)
• p7zip (GPL)
• Apimac Compress Files 3 (shareware)

Windows

• Peazip

• 7-Zip and p7zip (GPL)
• Altap Salamander
• IZArc (no support for Ultra compression or Solid format though)
• PowerArchiver
• QuickZip
• ShellZip
• SimplyZip
• Squeez
• TUGZip
• ZipGenius
• UltimateZip

Extract-only support

These programs support only reading files in existing 7z archives. They do not allow creating new archives, or adding more files to existing archives.

Mac

• The Unarchiver (LGPL)
• Zipeg (freeware) [1]

Windows

• Bread-Zip
• ALZip
• ExtractNow
• WinRAR (read-only support, but 7z creation will most likely be included in the future [2])

See also

• Comparison of archive formats
• List of archive formats
• Free file format
• Open format

External links

• 7z Format — General description about the 7z archive format.