Parchive

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:

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. 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):

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.

Versions

Versions 1 and 2 of the file format are incompatible. (However, many clients support both.)

Parity Volume Set Specification 1.0

For Par1, the files f1, f2, ..., fn, the Parchive consists of an index file (f.par), which is CRC type file with no recovery blocks, and a number of "parity volumes" (f.p01, f.p02, etc.). Given all of the original files except for one (for example, f2), it is possible to create the missing f2 given all of the other original files and any one of the parity volumes. Alternatively, it is possible to recreate two missing files from any two of the parity volumes and so forth.

Par1 supports up to 256 recovery files. Each recovery file must be the size of the largest input file.

Parity Volume Set Specification 2.0

Par2 files generally use this naming/extension system: filename.vol000+01.PAR2, filename.vol001+02.PAR2, filename.vol003+04.PAR2, filename.vol007+06.PAR2, etc. The +01, +02, etc. in the filename indicates how many blocks it contains, and the vol000, vol001, vol003 etc. indicates the number of the first recovery block within the PAR2 file. If an index file of a download states that 4 blocks are missing, the easiest way to repair the files would be by downloading filename.vol003+04.PAR2. However, due to the redundancy, filename.vol007+06.PAR2 is also acceptable. There is also an index file filename.PAR2, it is identical in function to the small index file used in PAR1.

Par2 supports up to 65536 (2¹⁶) recovery blocks (however, par2cmdline, the official PAR2 implementation, it limited to 32767 blocks at once). Input files are split into multiple equal-sized blocks so that recovery files do not need to be the size of the largest input file.

Although Unicode is mentioned in the PAR2 specification as an option, most PAR2 implementations do not support unicode.

Directory support is included in the PAR2 specification, but most or all implementations do not support it.

Parity Volume Set Specification 3.0

Par3 is a planned improvement over Par2. The authors intend to fix problems related to creating or repairing when the block count or block size is very high. Par3 also adds support for including directories (file folders) in a parchive and Unicode characters in file names. In addition, the authors plan to enable the Par3 algorithm to identify files that have been moved or renamed.

Windows

Mac OS X

Linux

FreeBSD

POSIX

Software for POSIX conforming operating systems: