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Parchive (a portmanteau of parity archive, and formally known as Parity Volume Set Specification〔(Re: Correction to Parchive on Wikipedia ), reply #3, by Yutaka Sawada: "Their formal title are "Parity Volume Set Specification 1.0" and "Parity Volume Set Specification 2.0." 〕) is an erasure code system that produces par files for checksum verification of data integrity, with the capability to perform data recovery operations that can repair or regenerate corrupted or missing data. Parchive was originally written to solve the problem of reliable file sharing on Usenet,〔(【引用サイトリンク】 title = Parchive: Parity Archive Volume Set )〕 but it is now commonly used for protecting any kind of data from data corruption, bit rot, and accidental or malicious damage. Despite the name, Parchive uses more advanced techniques that do not utilize simplistic parity methods of error detection and correction. The original SourceForge Parchive project has been inactive since November 9, 2010.〔(【引用サイトリンク】 title = Parchive: Parity Archive Tool )〕 As of 2014, Par1 is obsolete, Par2 is mature for widespread use, and Par3 is an experimental version being developed by MultiPar author Yutaka Sawada.〔(possibility of new PAR3 file )〕〔(Question about your usage of PAR3 )〕〔(Risk of undetectable intended modification )〕〔(PAR3 specification proposal not finished as of April 2011 )〕 == History == Parchive was intended to increase the reliability of transferring files via Usenet newsgroups. Usenet was originally designed for informal conversations, and the underlying protocol, NNTP was not designed to transmit arbitrary binary data. Another limitation, which was acceptable for conversations but not for files, was that messages were normally fairly short in length and limited to 7-bit ASCII text. Various techniques were devised to send files over Usenet, such as uuencoding and Base64. Later Usenet software allowed 8 bit Extended ASCII, which permitted new techniques like yEnc. Large files were broken up to reduce the effect of a corrupted download, but the unreliable nature of Usenet remained. With the introduction of Parchive, parity files could be created that were then uploaded along with the original data files. If any of the data files were damaged or lost while being propagated between Usenet servers, users could download parity files and use them to reconstruct the damaged or missing files. Parchive included the construction of small index files ( *.par in version 1 and *.par2 in version 2) that do not contain any recovery data. These indexes contain file hashes that can be used to quickly identify the target files and verify their integrity. Because the index files were so small, they minimized the amount of extra data that had to be downloaded from Usenet to verify that the data files were all present and undamaged, or to determine how many parity volumes were required to repair any damage or reconstruct any missing files. They were most useful in version 1 where the parity volumes were much larger than the short index files. These larger parity volumes contain the actual recovery data along with a duplicate copy of the information in the index files (which allows them to be used on their own to verify the integrity of the data files if there is no small index file available). In July 2001, Tobias Rieper and Stefan Wehlus proposed the Parity Volume Set specification, and with the assistance of other project members, version 1.0 of the specification was published in October 2001.〔 〕 Par1 used Reed-Solomon error correction to create new recovery files. Any of the recovery files can be used to rebuild a missing file from an incomplete download. Version 1 became widely used on Usenet, but it did suffer some limitations: * It was restricted to handle at most 255 files. * The recovery files had to be the size of the largest input file, so it did not work well when the input files were of various sizes. (This limited its usefulness when not paired with the proprietary RAR compression tool.) * The recovery algorithm had a bug, due to a flaw in the academic paper on which it was based. * It was strongly tied to Usenet and it was felt that a more general tool might have a wider audience. In January 2002, Howard Fukada proposed that a new Par2 specification should be devised with the significant changes that data verification and repair should work on blocks of data rather than whole files, and that the algorithm should switch to using 16 bit numbers rather than the 8 bit numbers that PAR 1 used. Michael Nahas and Peter Clements took up these ideas in July 2002, with additional input from Paul Nettle and Ryan Gallagher (who both wrote Par1 clients). Version 2.0 of the Parchive specification was published by Michael Nahas in September 2002. Peter Clements then went on to write the first two Par2 implementations, QuickPar and par2cmdline. Abandoned since 2004, Paul Houle created phpar2 to supersede par2cmdline, but phpar2 is now unmaintained as of 2014. Yutaka Sawada created MultiPar to supersede QuickPar. Sawada maintains par2cmdline to use as MultiPar's PAR engine backend. On May 10, 2014, Sawada reported a hash collision security problem in par2cmdline (the backend for MultiPar):〔(v1.2.5.3 is public )〕 I'm not sure this problem can be used for DoS attack against automated Par2 usage. If someone has a skill to forge CRC-32, it is possible to make a set of source file and Par2 file, which freeze a Par2 client for several hours. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parchive」の詳細全文を読む スポンサード リンク
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