ZIP

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Not to be confused with Zip disk, an unrelated disk cartridge unit.
File Format
Name ZIP
Ontology
Extension(s) .zip
MIME Type(s) application/zip
LoCFDD fdd000354, fdd000355, fdd000362, fdd000361
PRONOM x-fmt/263
UTI com.pkware.zip-archive
Released 1989

ZIP is one of the most popular file compression formats. It was created in 1989 as the native format of the PKZIP program, which was introduced by Phil Katz in the wake of a lawsuit (which he lost) against him by the makers of the then-popular ARC program (and file format) for copyright and trademark infringement in an earlier program PKARC which had been file-compatible with ARC. This resulted in Katz creating a new file format, which rapidly overtook ARC in popularity (to a large extent because of BBS sysops, then the primary users of such compression, resenting the lawsuit). Many programs have been released for a variety of operating systems to compress and decompress ZIP files, and native support for the format is built into several popular operating systems.

ZIP implementations vary in their support for features in the specification from PKWARE[1], particularly features added since version 2 (1993), some of which are protected by patents and require licensing. Many implementations limit the use of compression to the DEFLATE algorithm, introduced with version 2. Extensions incorporated into the specification that have been widely adopted are: long filenames; large files (using a technique known as ZIP64); and filenames in UTF-8. In 2011 work began on an interoperable subset of the latest APPNOTE.TXT with the intention of publication as ISO/IEC 21320-1, Document Container File -- Part 1: Core. As of November 2012, a discussion draft is available[2]. Designed to promote interoperable implementations, the draft ISO/IEC 21320-1 prohibits compression other than using DEFLATE, segmentation or multiple volumes, and features that are subject to patents.

While .zip is the usual file extension, ZIP-formatted files can be found with many other extensions since a number of other file formats use ZIP compression but store their files in application-specific extensions. See Category:ZIP based file formats for a list of such formats.

Contents

See also

Identification

The byte sequence 'P' 'K' 0x05 0x06 (the "end of central directory signature") appears somewhere in the file, usually beginning exactly 22 bytes from the end of the file. However, it will appear earlier if the file contains a "ZIP file comment" (common in the BBS era, but rare today), or for various other reasons. There seems to be no theoretical limit to how far back you may have to search for the signature, but some software limits it to around 64KB, which is the maximum length of a comment.

Most ZIP files (but not self-extracting ZIP files) happen to begin with 'P' 'K' 0x03 0x04. This is not a global file signature, but is the signature that appears once for every compressed file inside the ZIP file. Some ZIP-based formats are designed such that they necessarily begin in this way. But in general, it is even legal for a ZIP file to contain zero files, and such a ZIP file would not contain this signature at all.

Compression

Each file in a ZIP file is compressed using one of a number of compression algorithms. Only compression types 0 (uncompressed) and 8 (DEFLATE) are likely to be seen in modern portable ZIP files. In old ZIP files, types 1 (Shrink) and 6 (Implode) are common.

Code Compression scheme Notes and references
0 Uncompressed
1 Shrink (LZW) Used by PKZIP prior to v2.0.
2–5 Reduce Used by PKZIP v0.x.
6 Implode (Shannon–Fano coding) Used by PKZIP v1.x. See also TTComp archive.
8 DEFLATE Used by PKZIP v2.0+.
9 Deflate64, a.k.a. Enhanced Deflate Defined in ZIP specification v2.1+.
10 PKWARE Data Compression Library Imploding (old IBM TERSE)
12 Bzip2 Defined in ZIP specification v4.6+.
14 LZMA (EFS) Defined in ZIP specification v6.3+.
18 IBM TERSE (new)
19 IBM LZ77 z Architecture (PFS)
96 (JPEG, according to WinZip)
97 WavPack Defined in ZIP specification v6.3.2+.
98 PPMd version I, Rev 1 Defined in ZIP specification v6.3+.
99 (AES encryption, according to WinZip)

Extensible data fields

Each member file of a ZIP file may have one or more extensible data fields (or extra fields), containing arbitrary data. Each field is tagged with a 16-bit identifier. Extra fields are normally used for platform-specific or filesystem-specific metadata, or to work around limitations of the original ZIP format. They are not normally used for application-specific data.

Most of the extra fields in use are documented in the ZIP "APPNOTE" specification, or by the Info-ZIP software (e.g. the proginfo/extrafld.txt file in the Zip program's source distribution).

An example of an extra field is 0x5455, "extended timestamp", which attempts to improve upon ZIP's antiquated MS-DOS date/time format (and somehow manages to come up short, in that it only works until the year 2038, instead of 2099).

Specifications

Metaformat files

Software

Sample files

  • 1608A.ZIP → D1-MAC.ZIP: Example of a file that uses the uncommon "Reduce" compression scheme

References

  1. ↑ http://www.pkware.com/documents/casestudies/APPNOTE.TXT
  2. ↑ http://kikaku.itscj.ipsj.or.jp/sc34/open/1855.pdf

Links

Retrieved from "http://fileformats.archiveteam.org/index.php?title=ZIP&oldid=26057"
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