RAR (file format)

Overview

The RAR (Roshal Archive) file format is a proprietary data compression format developed by Eugene Roshal. It is named after its creator's surname (Roshal ARchive). The RAR format supports data compression, error recovery, and file spanning. It was first released in 1993 and has been widely used for its high compression ratio and flexibility.

A close-up view of a RAR file icon on a computer screen.

History

The RAR file format was developed by Eugene Roshal, a Russian software engineer. Roshal began developing the format in 1991, and the first version of the RAR format was released in fall 1993. The format was initially designed for use with Roshal's file archiver, "RAR," but it has since become a standard format used by many other file compression and archiving software.

Technical Details

RAR files are binary files that contain one or more files compressed using RAR compression. The format supports a wide range of compression algorithms, including LZSS and PPM. It also supports a variety of features not found in other file formats, such as strong AES-256 encryption, file comments, and the ability to handle files larger than 4 GB.

The RAR format uses a proprietary compression algorithm that is known for its high compression ratios. This algorithm, combined with the format's support for large files and strong encryption, makes RAR a popular choice for compressing and archiving large files and sensitive data.

Compression Algorithm

The RAR format uses a proprietary compression algorithm that is based on the LZSS (Lempel-Ziv-Storer-Szymanski) and PPM (Prediction by Partial Matching) algorithms. This algorithm is known for its high compression ratios, especially when compressing large files.

The LZSS algorithm is a lossless data compression algorithm that replaces repeated occurrences of data with references to a single copy. The PPM algorithm, on the other hand, is a statistical compression technique that predicts the probability of a given symbol (or sequence of symbols) based on the previous symbols in the sequence.

The RAR format's compression algorithm combines these two techniques to achieve high compression ratios. It first uses the LZSS algorithm to eliminate repeated data, and then uses the PPM algorithm to further compress the data by predicting the probability of different symbols.

Features

The RAR format supports a wide range of features that make it a versatile and powerful file format. These features include:

Data compression: The RAR format supports a proprietary compression algorithm that provides high compression ratios, especially for large files.

Error recovery: The format includes a built-in error recovery mechanism that can repair corrupted or damaged files. This feature is particularly useful for recovering data from physical media that may have been damaged or degraded over time.

File spanning: The RAR format supports file spanning, which allows large files to be split into smaller parts. This feature is useful for storing large files on media with limited storage capacity, or for transmitting large files over networks with bandwidth limitations.

Encryption: The RAR format supports strong AES-256 encryption, which can be used to secure sensitive data.

File comments: The format allows for the inclusion of file comments, which can be used to provide additional information about the files contained in the archive.

Large file support: The RAR format can handle files larger than 4 GB, making it suitable for compressing and archiving large files.

Usage

The RAR format is widely used for data compression, file archiving, and error recovery. It is commonly used to compress large files or groups of files, to create backups of data, and to transmit large files over networks.

The format is supported by a variety of software applications, including WinRAR, 7-Zip, and PeaZip. These applications can be used to create, extract, and manage RAR files.