File extension

.zlib

ZLIB Compressed Data Format Specification version 3.3

The zlib format is a compact wrapper around a DEFLATE stream, adding a two-byte header and an Adler-32 checksum; it is the compressed-data layer embedded inside PNG, PDF, HTTP, and many other formats.

Known meanings

Formats and reference material for .zlib

ZLIB Compressed Data Format Specification version 3.3

ZLIB Compressed Data Format Specification version 3.3

  • archive
  • Network Working Group
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Details

Format notes

zlib Data Format

zlib is a compressed-data format rather than a file-archive format. It wraps a DEFLATE-compressed stream (the same LZ77-plus-Huffman algorithm used by gzip and inside ZIP) with a minimal two-byte header describing the compression method and window size, and appends an Adler-32 checksum of the uncompressed data for integrity verification. There is no file name, timestamp, or multi-file structure — that framing is what distinguishes zlib from gzip.

Because it is small and self-checking, the zlib format is used as the compression layer inside many other formats and protocols rather than as a standalone file type. PNG image data, PDF stream objects, many network protocols, Git object storage, and countless application data files carry zlib-wrapped DEFLATE streams. The widely used zlib software library implements this format along with raw DEFLATE and the gzip wrapper.

Relationship To DEFLATE And gzip

Three closely related things are easy to confuse: raw DEFLATE (RFC 1951, no framing), the zlib wrapper (RFC 1950, header plus Adler-32), and the gzip wrapper (RFC 1952, magic bytes plus CRC-32 and size). They share the same compression core but differ in framing and checksum, so a decoder must be told, or must detect, which framing is present.

Security Notes

As with any compressed data, software inflating untrusted zlib streams should bound output size and memory to resist decompression bombs, and should verify the Adler-32 checksum where end-to-end integrity matters.

Further Reading

  • RFC 1950 (zlib format): https://datatracker.ietf.org/doc/html/rfc1950
  • zlib library home page: https://www.zlib.net/