7z

From Academic Kids

In computing, 7z is an archive format which allows data compression using a number of compression algorithms. The format was initially implemented by the 7-Zip archiver but the file format is public and 7-Zip's implementation of it is publicly available under the GNU LGPL licence.

The main features of the 7z format are:

Open, modular architecture (which allows any compression, conversion, or encryption method to be used)
High compression ratio (depending on which compression method is used)
Strong AES-256 encryption
Support for large files (up to approximately 16 exabytes)
Unicode file names
Compression of archive headers
Support for solid archives

The format's open architecture allows additional compression methods to be added to the standard. The following are defined in the current version:

LZMA - Improved and optimized version of LZ77 algorithm, using Markov chains/entropy coding and Patricia tries.
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).
BCJ - Converter for 32-bit x86 executables, see LZMA. Compresses target addresses of near jumps and calls.
BCJ2 - Converter for 32-bit x86 executables, see LZMA. Jump, call and conditional jump targets addresses are compressed separately.
Bzip2 - Standard Burrows-Wheeler transform algorithm. Bzip2 uses (faster) Huffman coding, while Bzip is using (stronger) entropy coding.
DEFLATE - Standard LZ77-based algorithm.

7z also 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 (with a large number of iterations, to make a brute-force search for the passphrase more difficult).

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

Other compressors which use PPMII: RAR

Compressors which encode branch targets to improve program file compression ratio:

UPX - for x86 16- and 32-bit binaries.
7-Zip - for x86 32-bit binaries, ARM, ARMThumb, PPC and IA64.
RAR - for x86 32-bit and for IA64 Itanium. Also has preprocessors for true color data, audio data and tables.

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