Design
Zip files are archives that store multiple files. Zip allows contained files to be compressed using many different methods, as well as simply storing a file without compressing it. Each file is stored separately, allowing different files in the same archive to be compressed using different methods. Because the files in a Zip archive are compressed individually it is possible to extract them, or add new ones, without applying compression or decompression to the entire archive. This contrasts with the format of compressed tar files, for which such random-access processing is not easily possible.
A directory is placed at the end of a zip file. This identifies what files are in the zip and identifies where in the zip that file is located. This allows zip readers to load the list of files without reading the entire zip archive. Zip archives can also include extra data that is not related to the zip archive. This allows for zip archives to be made into self-extracting archives, applications that decompress their contained data, by including the program code in a zip archive and marking the file as executable. On the other hand, it also allows for an innocuous file, such as a GIF image file, to hide malicious code by making the file a zip archive.
The zip format uses a 32-bit CRC algorithm and includes two copies of the directory structure of the archive to provide greater protection against data loss.
Read more about this topic: Zip (file Format)
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