7z
7z is an archive format which allows data compression using a number of compression algorithms. The format was initially implemented by the7-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 16000 petabytes)
- Unicode file names
- Compression of archive headers
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
- PPMD - Dmitry Shkarin's PPMdH with small changes
- BCJ - Converter for 32-bit x86 executables
- BCJ2 - Converter for 32-bit x86 executables
- BZip2 - Standard Burrows-Wheeler algorithm
- 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).