Overview
Zip refers to the zip file format. The ZIP file format is a popular data compression and archival format. A ZIP file contains one or more files that have been compressed, to reduce their file size, or stored as-is. ZIP files generally use the file extensions ".zip" or ".ZIP" and the MIME media type application/zip. Some software uses the ZIP file format as a wrapper for a large number of small items in a specific structure. Generally when this is done a different file extension is used. Examples of this usage are Java JAR files, id Software .pk3/.pk4 files, package files for StepMania and Winamp/Windows Media Player skins, XPInstall, as well as OpenDocument and Office Open XML office formats. Both OpenDocument and Office Open XML formats use the JAR (file format) internally, so files can be easily uncompressed and compressed using tools for ZIP files.
Technical Information
ZIP is a fairly simple archive format that compresses every file separately. Compressing files separately allows for individual files to be retrieved without reading through other data; in theory, it may allow better compression by using different algorithms for different files. However a caveat to this is that archives containing a large number of small files end up significantly larger than if they were compressed as a single file.
The specification for ZIP indicates that files can be stored either uncompressed or using a variety of compression algorithms. However, in practice, ZIP is almost always used with Katz's DEFLATE algorithm, except when files being added are already compressed or are resistant to compression.
ZIP supports a simple password-based symmetric encryption system which is known to be seriously flawed. In particular it is vulnerable to known-plaintext attacks which are in some cases made worse by poor implementations of random number generators.[1] It also supports spreading archives across multiple removable disks (generally floppy disks, but it could also be used with other removable media).New features including new compression and encryption methods have been added to ZIP in more recent times, but these are not supported by many tools and are not in wide use.The original zip format had a number of limits (uncompressed size of a file, compressed size of a file and total size of the zipfile) at 4GB. In version 4.5 of the specification pkware introduced the "zip64" extensions to get around these limitations.
See Also
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