ZIP (file format)

[citation needed] Apple has included built-in ZIP support in Mac OS X 10.3 (via BOMArchiveHelper, now Archive Utility) and later.

Most free operating systems have built in support for ZIP in similar manners to Windows and macOS.

[3] The name "zip" (meaning "move at high speed") was suggested by Katz's friend, Robert Mahoney.

A summary of key advances in various versions of the PKWARE specification: WinZip, starting with version 12.1, uses the extension .zipx for ZIP files that use compression methods newer than DEFLATE; specifically, methods BZip, LZMA, PPMd, Jpeg and Wavpack.

[30] The proposed project, entitled Document Packaging, envisaged a ZIP-compatible 'minimal compressed archive format' suitable for use with a number of existing standards including OpenDocument, Office Open XML and EPUB.

The .ZIP format uses a 32-bit CRC algorithm and includes two copies of each entry metadata to provide greater protection against data loss.

The CRC-32 algorithm was contributed by David Schwaderer and can be found in his book "C Programmers Guide to NetBIOS" published by Howard W. Sams & Co. Inc.[32] A ZIP file is correctly identified by the presence of an end of central directory record which is located at the end of the archive structure in order to allow the easy appending of new files.

However, this is not necessarily the case, as this is not required by the ZIP specification - most notably, a self-extracting archive will begin with an executable file header.

The FAT filesystem of DOS has a timestamp resolution of only two seconds; ZIP file records mimic this.

A ZIP64 local file extra field record, for example, has the signature 0x0001 and a length of 16 bytes (or more) so that two 64-bit values (the uncompressed and compressed sizes) may follow.

If the bit at offset 3 (0x08) of the general-purpose flags field is set, then the CRC-32 and file sizes are not known when the header is written.

The equivalent fields in the local header (or in the Zip64 extended information extra field in the case of archives in Zip64 format) are filled with zero, and the CRC-32 and size are appended in a 12-byte structure (optionally preceded by a 4-byte signature) immediately after the compressed data: The central directory file header entry is an expanded form of the local header: After all the central directory entries comes the end of central directory (EOCD) record, which marks the end of the ZIP file: This ordering allows a ZIP file to be created in one pass, but the central directory is also placed at the end of the file in order to facilitate easy removal of files from multiple-part (e.g. "multiple floppy-disk") archives, as previously discussed.

The .ZIP File Format Specification documents the following compression methods: Store (no compression), Shrink (LZW), Reduce (levels 1–4; LZ77 + probabilistic), Implode, Deflate, Deflate64, bzip2, LZMA, Zstandard, WavPack, PPMd, and a LZ77 variant provided by IBM z/OS CMPSC instruction.

The DCL Implode is undocumented partially due to its proprietary nature held by IBM, but Mark Adler has nevertheless provided a decompressor called "blast" alongside zlib.

In particular, it is vulnerable to known-plaintext attacks, which are in some cases made worse by poor implementations of random-number generators.

[37] New features including new compression and encryption (e.g. AES) methods have been documented in the ZIP File Format Specification since version 5.2.

To signal that the actual values are stored in ZIP64 extra fields, they are set to 0xFFFF or 0xFFFFFFFF in the corresponding LOC or CDFH record.

[citation needed] Likewise, some extension libraries support ZIP64, such as DotNetZip, QuaZIP[41] and IO::Compress::Zip in Perl.

Self-extracting archives (SFX), of the form supported by WinZip, take advantage of this, in that they are executable (.exe) files that conform to the PKZIP AppNote.txt specification, and can be read by compliant zip tools or libraries.

ZIP readers that are not aware of that extension can read a SOZip-enabled file normally and ignore the extended features that support efficient seek capability.

.ZIP file format includes an extra field facility within file headers, which can be used to store extra data not defined by existing ZIP specifications, and which allow compliant archivers that do not recognize the fields to safely skip them.

WinZip technical consultant Kevin Kearney and StuffIt product manager Mathew Covington accused PKWARE of withholding SES, but PKZIP chief technology officer Jim Peterson claimed that certificate-based encryption was still incomplete.

In another controversial move, PKWare applied for a patent on 16 July 2003 describing a method for combining ZIP and strong encryption to create a secure file.

[53] PKWARE eventually released version 5.2 of the .ZIP File Format Specification to the public, which documented SES.

The Free Software project 7-Zip also supports AES, but not SES in ZIP files (as does its POSIX port p7zip).

WinZip, WinRAR, Info-ZIP, ZipGenius, 7-Zip, PeaZip and B1 Free Archiver are well-known .ZIP tools, available on various platforms.

The Apache Ant implementation is aware of these to the extent that it can create files with predefined Unix permissions.

This can result in potentially unintended combinations, e.g. .exe files being created on NTFS volumes with executable permission denied.

Likewise, the ZLIB format (IETF RFC 1950) also uses the DEFLATE compression algorithm, but specifies different headers for error and consistency checking.

These zip bombs unzip to extremely large sizes, overwhelming the capacity of the computer they are decompressed on.