RAR (file format)
RAR 2.50 for DOS and OS/2
File extension: .rar
MIME type: application/x-rar-compressed application/octet-stream
Developed by: Eugene Roshal
Type of format: Data compression
In computing, RAR is a proprietary file format for data compression and archiving, developed by Eugene Roshal (hence the name RAR: Roshal ARchive). Some people mistakenly believe that RAR stands for "Russian ARchiver".
The file extension is .rar. The MIME-Type is application/x-rar-compressed.
History
Roshal created the RAR file format, and developed programs for packing and unpacking RAR files, originally for DOS, which were later ported to other platforms. The main Windows version of the archiver, known as WinRAR, is distributed as shareware, but Roshal has released the decoder's source code under a license that allows free distribution and modification, on the condition that it is not used to build a compatible encoder. Although the encoding method is proprietary, compatible programs for decompression are available for several platforms, such as the open-source 7-Zip. Due to widespread confusion, there do not seem to be any open source implementations to decompress newer than version 2.0 RAR archives (7-zip uses a proprietary plugin under "unRAR license" for decompression).
Comparison to other compression algorithms
RAR compression operations are typically much slower than compressing the same data with early compression algorithms like ZIP and gzip, but a much better rate of compression is achieved whenever the data can still be compressed further. Bzip2, which came after ZIP and gzip, compresses more efficiently at a cost of computational time compared to them, but is in turn less efficient and computationally cheaper than RAR.
7z's LZMA algorithm is quite similar to RAR in providing extremely high compression efficiency at the cost of compute time to compress and decompress. Both provide among the highest compression efficiency of any popular scheme, with the question of which algorithm is the more efficient compression scheme strongly depending on the files being compressed. Both formats are still being actively developed.
Archiver features
Apart from the rate of compression, RAR has several other original features:
* It is able to handle efficiently split volumes. Before the advent of RAR the most notable such format was ARJ. It is unnecessary to use split volumes for this purpose alone since just binary splitting the files will work fine, and they can be reassembled with cat or binary copy. Multi-volume files have wide use though, mainly because they are generally easier to handle, especially when the file is spanning multiple disks. Built-in support for multi-volume files enable the unpacking program to simply prompt the user for the next disk, without any hassle of manually copying and then rejoining the pieces, or for extracting a file from a single piece without needing all pieces.
* RAR archives can be of a solid format, in which all of the compressed files are treated as a single data block. Most currently used compression formats (with the exception of the older ZIP) allow solid structuring.
* It features strong encryption capabilities. Older versions of the file format used a proprietary algorithm; newer versions use the AES encryption algorithm, which is considered very strong by today's standards. The only known ways to recover an encrypted file are via dictionary or brute force attacks, which are usually infeasible with non-dictionary passphrases starting from 8 characters.
* Variable amounts of redundancy (“recovery record”) can be added to an archive, making it more resistant to corruption. Even if parts of an archive are damaged, it is possible to fully recover the stored data if a large enough recovery record exists.
* For audio, the compression ratio is typically between 50%–90% dependent on the complexity of the source audio and the predictability of the incoming waveform.[citation needed] While RAR does not compress PCM audio as well as WavPack, Monkey's Audio or FLAC, this feature allows entire projects to be backed up and compressed properly without having to losslessly compress the audio files in advance.
* Versions of RAR up to 2.50 closely resemble earlier versions of Norton Commander.
* (Win)RAR also has the capability of storing NTFS streams and security information within the archive – information that is usually lost on compression.
* Under OS/2 RAR also handles extended attributes.
* RAR files can be embedded in other file types, probably the most common being JPEG. Image handling programs, browsers, and other utilities usually ignore any additional data after the end of the image, while RAR ignores anything before the RAR header. The procedure to create such a file is to append a RAR file to a JPEG. (e.g.: in DOS/Windows command-line: copy /b image1.jpg+something.rar image2.jpg, in UNIX: cat image1.jpg something.rar >> image2.jpg). [1]
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