ZIP (file format)

Introduction, History, and Standardization of ZIP file format
– ZIP is an archive file format that supports lossless data compression.
– ZIP files may contain compressed files or directories.
– The format was created in 1989 as a replacement for the ARC compression format.
– The ZIP file format was designed by Phil Katz and Gary Conway.
– The format proliferated widely on the internet during the 1990s.
– ISO/IEC 21320-1 Document Container File — Part 1: Core was published in 2015, stating that document container files are conforming Zip files.
– The standard restricts certain features and prohibits encryption, digital signatures, and patched data features.

Version history and features of ZIP file format
– The ZIP File Format Specification has its own version number.
– Key advances in various versions include support for Deflate64 compression, AES encryption, Unicode filename storage, and expanded compression algorithms.
– The latest version is 6.3.10, released in 2022, which added z/OS attribute values and additional 3rd party Extra Field mappings.

ZIP file structure and internal layout
– ZIP files allow for random-access processing without compressing or decompressing the entire archive.
– A directory is placed at the end of a ZIP file, identifying the files and their locations.
– ZIP archives can include extra data unrelated to the archive itself, allowing for self-extracting archives.
– The order of file entries in the central directory does not have to coincide with the order in the archive.
– ZIP files are identified by the presence of an end of central directory record.
– Local file headers provide redundancy by including the same information as the central directory.
– ZIP files can be updated by appending new files and an updated central directory.

ZIP file signatures, headers, and timestamps
– Each entry in a ZIP archive is introduced by a local file header with file information and optional extra data fields.
– Extra data fields are used to support ZIP64 format, encryption, file attributes, and higher-resolution timestamps.
– The ZIP format uses specific 4-byte signatures to denote various structures in the file.
– ZIP archives can be spread across multiple file-system files for storage or transmission purposes.
– The timestamp resolution of files in a ZIP archive is limited to two seconds, matching the FAT filesystem of DOS.

Compression methods, encryption, ZIP64 support, and compatibility
– The ZIP format supports various compression methods, with DEFLATE being the most common.
– ZIP supports a password-based symmetric encryption system known as ZipCrypto.
– ZIP64 format extensions were introduced to overcome the 4GB limit of the original ZIP format.
– Support for ZIP64 varies across different software and operating systems.
– Some extension libraries and tools support ZIP64, while others may not.