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 is a process that reduces the size of data files for efficient storage or transmission. It uses various algorithms to condense data by identifying and eliminating redundancy, which can often substantially decrease the size of the data without losing the original information.
There are two main types of file compression: lossless and lossy. Lossless compression allows the original data to be perfectly reconstructed from the compressed data, which is ideal for files where every bit of data is important, like text or database files. Common examples include ZIP and RAR file formats. On the other hand, lossy compression eliminates less important data to reduce file size more significantly, often used in audio, video, and image files. JPEGs and MP3s are examples where some data loss does not substantially degrade the perceptual quality of the content.
File compression is beneficial in a multitude of ways. It conserves storage space on devices and servers, lowering costs and improving efficiency. It also speeds up file transfer times over networks, including the internet, which is especially valuable for large files. Moreover, compressed files can be grouped together into one archive file, assisting in organization and easier transportation of multiple files.
However, file compression does have some drawbacks. The compression and decompression process requires computational resources, which could slow down system performance, particularly for larger files. Also, in the case of lossy compression, some original data is lost during compression, and the resultant quality may not be acceptable for all uses, especially professional applications that demand high quality.
File compression is a critical tool in today's digital world. It enhances efficiency, saves storage space and decreases download and upload times. Nonetheless, it comes with its own set of drawbacks in terms of system performance and risk of quality degradation. Therefore, it is essential to be mindful of these factors to choose the right compression technique for specific data needs.
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.