7-Zip is a popular file archiver and compression tool that utilizes its own archive format, known as the 7z format. Developed by Igor Pavlov, the 7z format was designed to provide high compression ratios, strong encryption, and support for multiple compression methods. This technical explainer will delve into the details of the 7z archive format, its structure, and the various compression and encryption techniques it employs.
The 7z format is a container that can store multiple files and directories, along with their metadata, in a single archive file. It supports solid compression, which allows multiple files to be compressed together, resulting in better overall compression ratios. The format also includes features such as header compression, multi-threading, and the ability to split archives into multiple volumes.
The structure of a 7z archive consists of three main parts: the signature header, the header, and the compressed data blocks. The signature header is a 6-byte sequence that identifies the file as a 7z archive. It always starts with the bytes '7z\xBC\xAF\x27\x1C'. The header follows the signature and contains essential information about the archive, such as the version, the number of files, and the compression methods used.
The header is divided into several subparts, including the archive properties, the stream information, and the file information. The archive properties store general information about the archive, such as the number of files and the creation time. The stream information contains details about the compressed data blocks, such as their size and the compression methods used. The file information stores metadata for each file in the archive, including the file name, size, and attributes.
One of the key features of the 7z format is its support for multiple compression methods. The most common methods used in 7z archives are LZMA (Lempel-Ziv-Markov chain Algorithm) and LZMA2. LZMA is a high-performance compression algorithm that offers excellent compression ratios, especially for text and executable files. LZMA2 is an improved version of LZMA that offers better multi-threading support and faster decompression speeds.
In addition to LZMA and LZMA2, the 7z format also supports other compression methods, such as BZip2, PPMd, and Delta. BZip2 is a general-purpose compression algorithm that provides good compression ratios for a wide range of file types. PPMd is a statistical compression method that works well for text files and can achieve very high compression ratios. Delta compression is used to store differences between similar files, which can significantly reduce the size of the archive when storing multiple versions of the same file.
The 7z format also includes strong encryption capabilities to protect the contents of the archive. It supports the AES-256 encryption algorithm, which is considered one of the most secure encryption methods available. When an archive is encrypted, all file names, metadata, and compressed data blocks are protected, making it virtually impossible for unauthorized users to access the contents of the archive without the correct password.
To ensure data integrity, the 7z format uses a combination of cyclic redundancy check (CRC) and SHA-256 hash values. Each compressed data block has a CRC value that is used to detect and correct errors during decompression. Additionally, the archive header and the file metadata are protected by SHA-256 hash values, which can be used to verify the integrity of the archive and its contents.
The 7z format also supports the creation of self-extracting archives (SFX). An SFX archive is an executable file that includes the compressed data and the necessary extraction code. When run, the SFX archive automatically extracts the contents to a specified location, without the need for any additional software. This feature makes it easy to distribute compressed files to users who may not have a compatible extraction tool installed.
One of the advantages of the 7z format is its open architecture, which allows developers to create compatible tools and libraries. The 7-Zip software itself is open-source, and its source code is available under the GNU Lesser General Public License (LGPL). This has led to the development of various third-party tools and plugins that can create, extract, and manipulate 7z archives.
In conclusion, the 7z archive format is a powerful and versatile compression container that offers high compression ratios, strong encryption, and support for multiple compression methods. Its advanced features, such as solid compression, multi-threading, and self-extracting archives, make it an attractive choice for both individual users and enterprise environments. As the format continues to evolve and improve, it is likely to remain a popular choice for file compression and archiving.
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.