RAR (file format)

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RAR ( R oshal AR chive)
Rar-logo.svg
RAR Logo (WinRAR)
File extension : .rar
MIME type : application / vnd.rar application / x-rar-compressed application / octet-stream
Magic number : 5261.7221.1A07.00 hex
Rare! \ X1A \ a \ 0

( ASCII-C notation )

Developed by: Yevgeny Lazarevich Roshal
Type: Data compression
Container for: any files
Website : rarlab.com

RAR is a data compression algorithm , software, and file format used to reduce the storage space required by files for archiving and transmission. RAR has a good compression rate depending on the file format and the number of files. Named after its inventor, the format, Eugene Roshal , the name stands for R oshal AR chive. The archive files usually have the ending ; of MIME type is . .rarapplication/x-rar-compressed

Technical characteristics

Solid compression

RAR supports the technology of progressive or solid archiving , in which files selected for archiving are not compressed individually, but all together. This roughly corresponds to a previous generation of a tarball with subsequent compression by gzip . Before compression, RAR sorts the data according to similar patterns or file types. The advantage of progressive compression is that redundancies between several files are eliminated, which has the greatest effect with many smaller files of the same type (e.g. source code). Overall, a significantly better compression rate is achieved. However, this approach has two disadvantages: Individual files cannot be extracted from an archive without decoding the entire data stream, which also contains unwanted files. In addition, a damaged archive leads to the loss of all archived files (not just the file in question, since there are no more individual files in a compressed state).

Recovery records

In order to minimize the risk of data loss due to defective archives, RAR supports so-called recovery records - in contrast to other compression formats such as ZIP, TAR or gzip . With additional error correction data (similar to Reed-Solomon codes ) faulty RAR archives can be repaired to a limited extent. The amount of correction data can be freely specified in percent when compressing.

In addition , so-called mirror archives (reconstruction archives) can be created for multi-part archives, called multiple volumes in RAR , each of which is as large as the largest of the individual partial archives. For example, if there is a ten-part archive and two mirror archives and any two partial archives have been damaged, these can be completely reconstructed using the two mirror archives. This procedure even works if a partial archive is not only damaged but does not exist. However, as with RAID n , there must always be at least as many reconstruction archives as there are damaged or missing archives so that this method can be used.

Encryption

RAR supports encryption of the compressed data using the Advanced Encryption Standard (AES-128 for RAR 4.x, AES-256 from RAR 5.0) and the encryption of file names, so that you cannot infer the content of an archive without knowing the password . It can also alternate data streams of a file system (eg NTFS , HFS , HPFS are saved) what this format qualified in corresponding systems as a backup ready.

If a large file, for example a film, is distributed over several archives, the file names in the archives are often identical and, with the exception of the first partial archive or related first parts, cannot be unpacked individually. The entire file is automatically merged after unpacking the first archive, provided that all packages are available.

distribution

Since the decompression algorithm was published by the developers and can be used without license fees, RAR archives can be decompressed by numerous other packing programs. The compression algorithm, however, is not accessible, so officially only the manufacturer's programs WinRAR , RAR for DOS , RAR for Linux , UnRarX for Macintosh etc. support the creation of RAR files. However, version 2 of the RAR format was reproduced by the authors of the SpeedCommander and Squeez programs so that they can also generate RAR archives. Complete compatibility could not be guaranteed and the version 2 documented at the time is now out of date.

Despite stronger compression and innovative functions (including division into several sub-files), RAR was only able to establish itself in isolated cases over the popular ZIP format. One main reason is that, unlike ZIP, RAR is a proprietary file format. In addition, RAR compresses significantly better than ZIP, but there are now similarly powerful but free formats such as 7z .

history

The first version of RAR, RAR 0.1, was published in March 1993, although version 1.3, which appeared in the fall, was not widely used until the fall. The corresponding version 1.3 of the file format did not yet have a magic number . At the beginning of 2000, work began on a free decoder (UniquE RAR File Library, unrarlib) based on a version of the official unrar from Rarlabs, which, with the permission of Yevgeny Lazarevich Roschal, was also allowed to be made available under the GPL . This resulted in a free unrar that can unpack archives according to all versions of the RAR format up to 2.x. With version 2.9 of WinRAR from the end of September 2001 a new generation (format version 3) of the RAR format was introduced, which among other things enables the use of better compression methods ( LZSS , PPMd ) and with the possibility of embedded error correction data (or recovery parts in multi-part archives) Previously customary additional use of PAR is no longer necessary. Since July 2010, version 2.5 of The Unarchiver has also included free ( LGPL ) unpacking software for archives according to version 3 of the RAR format, which can also handle multi-part, encrypted and self-extracting archives.

See also

Web links

swell

  1. Encryption ( en ) winrar-france.fr. Retrieved on April 5, 2019: "RAR archives are encrypted with [...] AES-128 standard for RAR 4.x and AES-256 in CBC mode for RAR 5.0 archives."
  2. compression.ru
  3. unrarlib.org
  4. gna.org ( Memento of the original from May 17, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / gna.org
  5. fsf.org
  6. Version history of The Unarchiver from http://wakaba.c3.cx/s/apps/unarchiver.html
  7. code.google.com with the release date of version 2.5 of The Unarchiver