ZIP

Revision as of 12:16, 16 November 2023

ZIP is one of the most popular file compression/archiving formats.

Disambiguation

There are many other things, related or not, whose names use the word "Zip".

• Zip disk, an unrelated disk cartridge unit
• "ZIP compression" can sometimes refer to DEFLATE, or something based on it (zlib, Gzip).

See also the "See also" section, elsewhere on this page.

Discussion

ZIP was created in 1989 as the native format of the PKZIP program, which was introduced by Phil Katz (with co-creator Gary Conway) 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 (see "APPNOTE" in the Specifications section below), particularly features added since version 2 (1993). 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 include large files (using a technique known as ZIP64), and filenames in UTF-8.

An interoperable subset of ZIP has been defined, and published as ISO/IEC 21320-1: Document Container File (refer to the Specifications section below). Designed to promote interoperable implementations, it prohibits various features, including compression other than DEFLATE, multiple volumes, and various encryption-related features.

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.

Format details

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.

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).

Known extensible data fields:

Multi-part archives

The ZIP format has supported archives consisting of multiple files from day one, though this feature does not appear to have been utilized by PKZIP until floppy disk spanning features were added in the v2.xx series.

The first fragment of a multi-part archive usually begins with signature bytes 'P' 'K' 0x07 0x08. This signature can be present even for single-part archives, if the disk spanning feature was enabled but turned out not to be needed.

The APPNOTE documentation (at least v4.5+) also mentions signature 'P' 'K' 0x30 0x30, calling it the "temporary spanning marker", but more research is needed to understand when it is used.

Fragments of a multi-part archive that are neither first nor last may not have any identifying signatures.

Only the last fragment necessarily contains the usual 'P' 'K' 0x05 0x06 signature.

Self-extracting archives

Refer to Self-extracting ZIP.

Character encoding

In general there is no official file name encoding for ZIP files, and non ASCII filenames are not generally well supported. The original implementation specified IBM Code Page 437 for filenames, but as many characters cannot be expressed in that encoding, the filename bytes have often been interpreted using the current system codepage (implementation dependent behaviour). There is a flag to specify UTF-8 as the encoding, but it is not supported in all major clients (e.g. Windows Explorer).

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.

Refer to the #Multi-part archives section, elsewhere on this page, for additional relevant information.

That Phil Katz guy has thus managed to get his initials at the start of a large number of files on many millions of computers and devices, given how many file formats are based on ZIP (even if they use different extensions). He died in 2000, but this memorial to him will live on indefinitely.

See also

• Self-extracting ZIP
• Zipx
• SOF (Spectrum Software) - Variant format
• DEFLATE
• Category:ZIP based file formats
• Category:PKWARE

Specifications

• APPNOTEs - The format documentation from PKWARE is traditionally in a file named APPNOTE.TXT.
  • APPNOTE from PKWARE (latest version of formal spec)
  • APPNOTE Archives from PKWARE (selected versions all the way back to 1.0)
  • Documentation from Info-ZIP (Includes Info-ZIP variants on APPNOTE.TXT dated from 1996 to 2004, specifications used as the basis for various open-source tools)
  • LoC archive
  • An early version of APPNOTE (apparently from PKZIP v1.10)
  • APPNOTE.ZIP - Possibly the first v2.x APPNOTE
  • APPNOTE v6.1.0, from archive.org
  • Bundled with PKZIP software through v1.93 - refer to the #Software section below.
• IANA registration for application/zip in July 1993 (corresponds to version 2 of APPNOTE.TXT)
• ISO/IEC 21320-1: Document Container File (see also fdd000361)
• Archive format info, including ZIP (from 1989, when ZIP was newly released)
• ZIP file header format (among other archive types)
• TorrentZip
• Info-ZIP's "extra fields" documentation

Metaformat files

• Synalysis grammar file (for Hexinator / Synalize It!; more details)

Software

• Info-ZIP: Zip, UnZip
• 7-Zip
• zlib - The zlib library does not support ZIP format, but it is distributed with "minizip" code that supports most ZIP files.
• libzip - Uses zlib.
• libarchive - Uses zlib.
• zziplib
  • Archive::ZZip: Perl bindings for zziplib
• miniz
• PKZIP
  • For DOS (main line): 0.80-beta · 0.90 · 0.92 · 1.01 · 1.02 · 1.10 · 1.93-alpha · 2.04c · 2.04e · 2.04g · 2.50
  • For Windows console: 2.50 · 4.00
  • Various versions at old-dos.ru
  • v1.10 DOS "export" version (no encryption)
• Konvertor
• Deark (for analysis, or converting old compression methods)
• PCDEZIP - Source code and DOS binary to decompress Deflate and all older methods (1993)
• XAD framework

Sample files

• https://github.com/corkami/pocs/tree/master/zip
• Examples that use the uncommon "Reduce" compression scheme: VISA_CRD.ZIP, 1608A.ZIP → D1-MAC.ZIP
• https://telparia.com/fileFormatSamples/archive/zip/

Links

• Wikipedia: ZIP (file format)
• Wikipedia: PKZIP
• Zip files all the way down (creating an infinitely-regressed ZIP file)
• ZIP101 an archive walkthrough
• Serve deepzoom images from a zip archive with openseadragon
• How are zlib, gzip and Zip related? What do they have in common and how are they different? - Response to StackOverflow question by zlib/gzip co-creator Mark Adler
• Does Microsoft OneDrive export large ZIP files that are corrupt? - Discusses an issue where large ZIP files generated by Microsoft OneDrive result in read errors when they are opened with tools like Info-Zip and 7-Zip
• ZIP is Broken, Except it’s Not, Except it Is
• Zip Files: History, Explanation and Implementation and The Legacy Zip Compression Methods
• Zip - How not to design a file format.
• Ten thousand security pitfalls: The ZIP file format by Gynvael Coldwind (2018)