ZIP (file format): archive and compression standard

Structure and key characteristics

At the file level a ZIP archive uses repeated records: each file entry has a local file header followed by compressed data, and the archive ends with a central directory that indexes entries for quick access. Every compressed entry carries a CRC-32 checksum and metadata such as file name, modification time, and compression method. The most common compression method found in ZIP archives is DEFLATE, which combines LZ77-style dictionary compression with Huffman coding.

ZIP is flexible in allowing multiple compression algorithms and features. Common methods include:

• Store (no compression) — for already compressed or small items.
• DEFLATE — the de facto standard in most ZIP files.
• BZIP2, LZMA, PPMd — supported by some tools but less widely interoperable.

Extensions, limits and security

Original ZIP structures used 32-bit fields for sizes and counts, which imposed limits on archive size and number of entries. The ZIP64 extensions lift those limits and are supported by modern tools for very large archives. For safety and verification, ZIP relies on CRC-32 checksums and optional extra fields in headers.

Encryption and integrity deserve special attention: the traditional "ZipCrypto" method is simple and widely implemented but cryptographically weak. Stronger options, such as AES-based extensions used by some vendors, improve security but are not universally supported. When encryption is required, verify which algorithm and toolchain are used to ensure compatibility and adequate protection.

ZIP's ubiquity makes it a common choice for software distribution, backups, file exchange and packaging resources in applications. It differs from other archive/compression models (for example, tar combined with gzip or bzip2, or newer container formats like 7z) in that it natively supports random access to files within the archive and a range of optional features. For broader reading on compression concepts see lossless compression and general compression algorithms.

Practical tools that create and extract ZIP archives include many built-in OS utilities and third-party programs; when creating archives intended for wide distribution, prefer DEFLATE and avoid obscure extensions unless recipients support them.