Sustainability of Digital Formats
 Planning for Library of Congress Collections

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 original version of the format was developed by Phil Katz (hence the "PK" in PKWARE). Like the earlier versions of ZIP_PK, ZIP_6_0_2, published in April 2004, combines data compression, file management, and data encryption within 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. Of the extensions to ZIP since its introduction that are most significant for open, interoperable functionality, version 6.2.0 supports:

• 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.

Of the functional extensions other than new compression methods and enhanced encryption, version 6.2.0 lacks one extension that has proved important for worldwide adoption. It does not support the use of UNICODE in filenames. See ZIP_PK for detail on other chronological versions.

ZIP_6_2_0 has been incorporated as the package or container for digital formats that represent a single document or other logical unit but comprise multiple files. See Notes below for details on use by OOXML (Office Open XML) and ODF (Open Document Format).

Version 6.2.0 of ZIP is documented in APPNOTE.TXT, Version 6.2.0 (April 2004).

ZIP_6_2_0 has been incorporated as the package or container for several digital formats that represent a single document or other logical unit but comprise multiple files. These include:

• Office Open XML (OOXML, standardized as ECMA 376 and ISO/IEC 29500)
  The Open Packaging Conventions (OPC, Part 2 of ISO/IEC 29500, OOXML) are based on version 6.2.0. Compression is restricted to DEFLATE; digital signatures are not supported. Details on the use of ZIP in OPC are in section 10 and Annex C of ISO/IEC 29500-2:2012.
• Office Document Format (ODF), standardized as ISO/IEC 26300
  Version 1.2 of ODF (Open Document Format for Office Applications) from OASIS incorporates version 6.2.0. The only compression algorithm allowed is DEFLATE (RFC 1951). ODF defines its own encryption mechanism. Version 1.0 of ODF (as published as ISO/IEC 26300:2006) referred to the APPNOTE.TXT version dated 1997-03-11 from Info-ZIP, because no appropriate version was made available publicly by PKWARE at the time because the version 6.2.0 referred to by the original ODF specification from OASIS had been superseded. Now that PKWARE maintains an archive of documentation to facilitate persistent references, Version 1.2 of ODF again refers to version 6.2.0.