Sustainability of Digital Formats: Planning for Library of Congress Collections

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ZIP File Format, Version 6.3.3 (PKWARE)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name ZIP File Format, Version 6.3.3 (PKWARE)

ZIP_6_3_3 is the version of the ZIP format from PKWARE (ZIP_PK) as specified in APPNOTE.TXT, Version 6.3.3. The ZIP format is designed for cross-platform data exchange and efficient data storage for a set of related files. ZIP_PK is a de facto industry standard, developed, maintained, and openly documented by PKWARE. The ZIP format combines data compression, file management, and data encryption to store multiple files in a portable archive format. A ZIP file is a package containing one or more files, usually compressed and sometimes encrypted. The basic structure consists of a sequence of chunks comprising a "local file header" followed by the file data (after compression and/or encryption) followed by a chunk known as the "central directory," which lists the files in the package along with key metadata to support their extraction, decryption, etc.

The original version of the format was developed by Phil Katz (hence the "PK" in PKWARE). Since the first specification was published in 1990, PKWARE has updated the format as supported in its products and issued new versions of the specification in a document called APPNOTE.TXT. The formats defined by versions 6.3.2 (September 2007) and 6.3.3 (September 2012) of APPNOTE.TXT are technically identical. Version 6.3.3 of the APPNOTE.TXT states that the changes from version 6.3.2 are "formatting changes to support easier referencing of this APPNOTE from other documents and standards."

Of the extensions to ZIP since its introduction that are most significant for open, interoperable functionality, version 6.3.2 and 6.3.3 support:

  • the widely used Deflate data compression algorithm;
  • support for large files, raising the 65535 limit on the number of files in a ZIP file and allowing file sizes beyond the previous 4 gigabyte limit.
  • support for Unicode characters, using UTF-8 encoding, in filenames. Previously only characters legal in filenames in MS-DOS were permitted.

See ZIP_PK for detail on other chronological versions.

ZIP_6_3_3 was used as the basis for a ISO/IEC 21320-1, an international standard for a container based on the ZIP format for digital documents that comprise multiple files. See ZIP_21320_1.

Production phase May be used at any lifecycle phase for bundling/packaging files together for exchange, storage, or distribution.
Relationship to other formats
    Subtype of ZIP_PK, ZIP File Format (PKWARE)

Local use Explanation of format description terms

LC experience or existing holdings See ZIP_PK.
LC preference See ZIP_PK.

Sustainability factors Explanation of format description terms

Disclosure The ZIP_PK format has been developed and maintained by PKWARE, Inc. since 1998. The format specification is proprietary, but the most recent version has always been openly disclosed as the .ZIP Application Note with a file name of APPNOTE.TXT.

Version 6.3.3 of ZIP is documented in APPNOTE.TXT, Version 6.3.3 (September 2012).

Adoption See ZIP_PK.
    Licensing and patents See ZIP_PK.
Transparency See ZIP_PK.
Self-documentation See ZIP_PK.
External dependencies See ZIP_PK.
Technical protection considerations See ZIP_PK.

Quality and functionality factors Explanation of format description terms

Bundling/compression Separate functionality factors for comparing formats that are used to bundle and or compress files have not been developed. From the perspective of digital preservation, consideration of the sustainability factors above is more important than the degree of compression.
Beyond normal See ZIP_PK.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension zip
Other extensions are used for particular applications that use the ZIP format as a container.
Internet Media Type application/zip
Other Internet Media Types are used for particular applications that use the ZIP format as a container.
File signature See related format.  See ZIP_PK.

Notes Explanation of format description terms

General See ZIP_PK.
History See ZIP_PK.

Format specifications Explanation of format description terms

Useful references


Last Updated: 05/22/2020