PAX (Pre-Allocate eXtension) is an open-source compressed archive format developed by Microsoft as a modern alternative to existing formats like ZIP, RAR and tar. It was designed to address limitations and improve upon the compression, performance, security, and functionality of archive handling on modern systems and devices.
The key differentiating features of the PAX format include enhanced compression using modern algorithms, efficient random access to files within archives, native multi-threading support, extensible metadata, built-in encryption and integrity checking, and a documented open specification to encourage wide adoption and interoperability.
PAX archives use the file extension .pax and have a multi-part internal structure consisting of a header, central directory, compressed data blocks, and a footer. This allows key information like the archive contents, compression parameters, and integrity hashes to be stored separately from the actual compressed file data.
The PAX header starts with a 4-byte magic number (50 41 58 00 in hex) for identification. It then contains fields for the PAX version, compression method, encryption method, hash method, block size, number of parallel compression threads, and various flags. The header ends with extensible XML metadata providing details about the archive.
Following the header is the PAX central directory. This contains an entry for each compressed file/folder in the archive, storing the full path, attributes, sizes, block offsets and hashes. Having this in one place allows efficiently listing archive contents and random access to individual files without scanning through compressed data.
The bulk of a PAX archive is a series of compressed data blocks. Each block has a small header indicating the uncompressed and compressed size, followed by a chunk of file data compressed with the configured algorithm. Blocks default to 1 MB in size but this is tunable in the archive header.
Compressed data blocks are optionally encrypted if an encryption method is specified. PAX supports modern encryption schemes like AES-256. The archive password is used to derive a key that encrypts each block independently, allowing efficient random access. For authentication, PAX hashes passwords with a secure KDF.
For compression, PAX supports a variety of modern general-purpose codecs optimized for fast decompression: LZMA, LZ4, Brotli, Zstandard, etc. It also allows preprocessors for further size reduction on specific filetypes (e.g. Delta encoding on EXEs/DLLs, E8E9 encoding on x86 code). Codecs and preprocessors are applied in a pipeline.
To enable efficient multi-threaded compression, files are partitioned into independently compressed blocks that can be processed by parallel codec instances. The PAX compressor scales automatically to use all available CPU cores. Similar partitioning allows parallel decompression for faster extraction.
PAX provides data integrity and tamper detection by storing hashes of the original and compressed data. Archives carry a header hash to detect truncation. The central directory is also hashed to prevent tampering with file metadata. Bit rot in compressed data is caught by hashing each block.
At the end of a PAX archive is the footer. This contains a copy of the header fields, the offset/size of the central directory, and a whole-archive hash. The footer is a fixed size and always at the end of the file, allowing easy location and verification of PAX archives.
PAX archives can be efficiently updated by modifying the central directory and appending changed data blocks, versus rewriting entire archives like ZIP. Whole files can be inserted, removed or replaced by updating metadata and adding/removing the relevant blocks. Archives can also be quickly appended to.
To mitigate zip-slip vulnerabilities, PAX requires explicit paths (no ../ traversal) and prevents writing outside the extraction root. Lengthy ZIP metadata fields that enabled denial-of-service are restricted. Compression bombs are mitigated via limits on compression ratio and memory usage.
File timestamps in PAX archives use a standard 64-bit format covering a wide range of dates with 1-second precision. Attributes for POSIX permissions and Windows ACLs are supported. PAX can store NTFS alternate data streams and resource forks. Symlinks and hardlinks are also representable.
The open-source PAX SDK provides simple APIs for creating, extracting, updating and verifying PAX archives programmatically. It handles all the low-level details of the PAX format. The SDK is available in multiple languages including C, C++, C#, Java, Python, JavaScript, Go, and Rust.
In summary, the PAX archive format builds upon the foundation of proven formats like ZIP while introducing modern features and optimizations - efficient compression, multi-threading, random access, security, and an open specification. This makes PAX ideal for a wide range of archival scenarios on today's systems.
File compression reduces redundancy so the same information takes fewer bits. The upper bound on how far you can go is governed by information theory: for lossless compression, the limit is the entropy of the source (see Shannon’s source coding theorem and his original 1948 paper “A Mathematical Theory of Communication”). For lossy compression, the trade-off between rate and quality is captured by rate–distortion theory.
Most compressors have two stages. First, a model predicts or exposes structure in the data. Second, a coder turns those predictions into near-optimal bit patterns. A classic modeling family is Lempel–Ziv: LZ77 (1977) and LZ78 (1978) detect repeated substrings and emit references instead of raw bytes. On the coding side, Huffman coding (see the original paper 1952) assigns shorter codes to more likely symbols. Arithmetic coding and range coding are finer-grained alternatives that squeeze closer to the entropy limit, while modern Asymmetric Numeral Systems (ANS) achieves similar compression with fast table-driven implementations.
DEFLATE (used by gzip, zlib, and ZIP) combines LZ77 with Huffman coding. Its specs are public: DEFLATE RFC 1951, zlib wrapper RFC 1950, and gzip file format RFC 1952. Gzip is framed for streaming and explicitly does not attempt to provide random access. PNG images standardize DEFLATE as their only compression method (with a max 32 KiB window), per the PNG spec “Compression method 0… deflate/inflate… at most 32768 bytes” and W3C/ISO PNG 2nd Edition.
Zstandard (zstd): a newer general-purpose compressor designed for high ratios with very fast decompression. The format is documented in RFC 8878 (also HTML mirror) and the reference spec on GitHub. Like gzip, the basic frame doesn’t aim for random access. One of zstd’s superpowers is dictionaries: small samples from your corpus that dramatically improve compression on many tiny or similar files (see python-zstandard dictionary docs and Nigel Tao’s worked example). Implementations accept both “unstructured” and “structured” dictionaries (discussion).
Brotli: optimized for web content (e.g., WOFF2 fonts, HTTP). It mixes a static dictionary with a DEFLATE-like LZ+entropy core. The spec is RFC 7932, which also notes a sliding window of 2WBITS−16 with WBITS in [10, 24] (1 KiB−16 B up to 16 MiB−16 B) and that it does not attempt random access. Brotli often beats gzip on web text while decoding quickly.
ZIP container: ZIP is a file archive that can store entries with various compression methods (deflate, store, zstd, etc.). The de facto standard is PKWARE’s APPNOTE (see APPNOTE portal, a hosted copy, and LC overviews ZIP File Format (PKWARE) / ZIP 6.3.3).
LZ4 targets raw speed with modest ratios. See its project page (“extremely fast compression”) and frame format. It’s ideal for in-memory caches, telemetry, or hot paths where decompression must be near RAM speed.
XZ / LZMA push for density (great ratios) with relatively slow compression. XZ is a container; the heavy lifting is typically LZMA/LZMA2 (LZ77-like modeling + range coding). See .xz file format, the LZMA spec (Pavlov), and Linux kernel notes on XZ Embedded. XZ usually out-compresses gzip and often competes with high-ratio modern codecs, but with slower encode times.
bzip2 applies the Burrows–Wheeler Transform (BWT), move-to-front, RLE, and Huffman coding. It’s typically smaller than gzip but slower; see the official manual and man pages (Linux).
“Window size” matters. DEFLATE references can only look back 32 KiB (RFC 1951 and PNG’s 32 KiB cap noted here). Brotli’s window ranges from about 1 KiB to 16 MiB (RFC 7932). Zstd tunes window and search depth by level (RFC 8878). Basic gzip/zstd/brotli streams are designed for sequential decoding; the base formats don’t promise random access, though containers (e.g., tar indexes, chunked framing, or format-specific indexes) can layer it on.
The formats above are lossless: you can reconstruct exact bytes. Media codecs are often lossy: they discard imperceptible detail to hit lower bitrates. In images, classic JPEG (DCT, quantization, entropy coding) is standardized in ITU-T T.81 / ISO/IEC 10918-1. In audio, MP3 (MPEG-1 Layer III) and AAC (MPEG-2/4) rely on perceptual models and MDCT transforms (see ISO/IEC 11172-3, ISO/IEC 13818-7, and an MDCT overview here). Lossy and lossless can coexist (e.g., PNG for UI assets; Web codecs for images/video/audio).
Theory: Shannon 1948 · Rate–distortion · Coding: Huffman 1952 · Arithmetic coding · Range coding · ANS. Formats: DEFLATE · zlib · gzip · Zstandard · Brotli · LZ4 frame · XZ format. BWT stack: Burrows–Wheeler (1994) · bzip2 manual. Media: JPEG T.81 · MP3 ISO/IEC 11172-3 · AAC ISO/IEC 13818-7 · MDCT.
Bottom line: choose a compressor that matches your data and constraints, measure on real inputs, and don’t forget the gains from dictionaries and smart framing. With the right pairing, you can get smaller files, faster transfers, and snappier apps — without sacrificing correctness or portability.
File compression is a process that reduces the size of a file or files, typically to save storage space or speed up transmission over a network.
File compression works by identifying and removing redundancy in the data. It uses algorithms to encode the original data in a smaller space.
The two primary types of file compression are lossless and lossy compression. Lossless compression allows the original file to be perfectly restored, while lossy compression enables more significant size reduction at the cost of some loss in data quality.
A popular example of a file compression tool is WinZip, which supports multiple compression formats including ZIP and RAR.
With lossless compression, the quality remains unchanged. However, with lossy compression, there can be a noticeable decrease in quality since it eliminates less-important data to reduce file size more significantly.
Yes, file compression is safe in terms of data integrity, especially with lossless compression. However, like any files, compressed files can be targeted by malware or viruses, so it's always important to have reputable security software in place.
Almost all types of files can be compressed, including text files, images, audio, video, and software files. However, the level of compression achievable can significantly vary between file types.
A ZIP file is a type of file format that uses lossless compression to reduce the size of one or more files. Multiple files in a ZIP file are effectively bundled together into a single file, which also makes sharing easier.
Technically, yes, although the additional size reduction might be minimal or even counterproductive. Compressing an already compressed file might sometimes increase its size due to metadata added by the compression algorithm.
To decompress a file, you typically need a decompression or unzipping tool, like WinZip or 7-Zip. These tools can extract the original files from the compressed format.