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.