7Z Files

What is a .7Z file?

A 7Z file is an archival data format that serves the same function for Igor Pavlov for the 7-Zip file archiver as ZIP does for WinZip or Zip file formats for general purposes. While a document file contains some pre-defined information allowing the opening of the document, there is no set information in a 7Z file which makes it more of a container file, with the files inside determine what opens after extraction.

A 7z file could comprise of many types of folders, files along with the timestamps and the attribute for them. In case of systems supported, 7-Zip would also include alternative data streams, symbolic links as well as integrity data in a 7-Zip 7z archive. There are various places where we would find .7z file formats such as when the user has downloaded some game MODs, disk image files etc.

There is also the issue related to high compression efficiency of the LZMA and LZMA2 compression techniques and generally produces much smaller file archives than the ZIP file for the same set of files especially when they have lots in common. 7z provides support for "solid" archive compression where in many separate files are compressed into one stream; this generally improves the compression ratio although slows update/recovery of separate items. The format identifier used is 37 7A BC AF 27 1C.

How to open a .7Z file

Technical details

The file format starts off at offset zero with the literal signature '37 7a BC AF 27 1C'. This header is followed by version number, start header check sum, start header pointer and next header pointers. The openly accessible spec for the 7z format includes such things as signature header, packed streams, folder(s), coder information including bind pairs and their types, uncompressed sizes and check sums, along with the files properties information.

The latest publicly documented version of the format that's available to read up is 4.65. That would correspond to 7z version, or 7-Zip v.4.

A single container can consist of one or multiple compressed streams of files along with directories, although not necessary all in a form that makes extraction meaningful. Also solid archives exist - a way of having lots of files share one compressed stream, what may enhance ratio. However it makes extraction order depend on decompression order within the solid block, meaning each following file must go after the first.

Available compression algorithms according to 7-z ip format include Copy method, LZMA2, and other such things as PPMd, BZIP2 and so forth. Of them, the ones used predominantly with 7-Zip software is LZMA/LZMA2 since they provide highest ratio. Header-level encryption uses AES 256-bit in this archiver and only after the password is known can the headers be decrypted and therefore filenames get revealed.

The common MIME type is 'application/x-7z-compressed'. Sometimes the file can have MIME field omitted altogether, and generic detector returns something like 'application/octet-stream'. File sizes typical are from some kilobytes (usually config bundles), and go as far up as tens or even hundreds gigabytes (software releases, backup archives etc.).

Format preserves unicode filename(s), time stamps, while permissions etc will stay in case they've been added using some options. Split archive files usually named .7z.001, .7z.002, and so on. The first one contains the 7z signature.

How to convert .7Z files

In case the user prefers the recipient to handle ZIP files due to integrated processing in Windows or macOS or the online service is ZIP friendly, use an external tool to convert .7z to ZIP as the process is not integrated yet:

7z x input.7z -oout 7z a -tzip output.zip ./out/*

The most typical target for converting Unix package formats is TAR.GZ / TAR.XZ where TAR maintains structure of a directory tree and gzip/xz compressors are widely available both serverside and in software releases.

Just extraction (modification, scanning, installation, or adding the files from the .7z archive) means:

7z x input.7z -ooutput_folder

To create an alternative 7z archive from an existing one it would make sense to extract all the data, then archive it again applying a different set of options such as a lower compression level / AES-256 encryption/encrypted headers/solid archives flag off.

GUI tools 7-zip PeaZIP/Keka/Ark/File Roller/WinRAR/WinZip usually do extraction to temporary folder followed by packing into another format. Recompressing image/video/audio formats, like, JPG/MP4/MP3/PDF does save almost no extra space. ZIP/TAR formats do not carry over some additional Windows-related properties, such as streams, ACLS, or creation time. The process also excludes encryption, alternate filenames or comments or passwords, so it is up to you to remove the protection.

Common problems and fixes

An error "Can not open file as archive" usually means an incorrect container file, downloading failure, or renaming of another archive file with extension ".7z". To check a correct archive re-download it and use its hash code or file signing, if the publishing authority signs its distribution files with one. Otherwise 7-zip utility should find header bytes 37 7A BC AF 27 1C located from offset 0 if archive is OK.

The errors such as "Data error", "CRC failed" indicate some kind of a corruption or incompleteness in compressed stream inside archive. Also, these errors might occur as well when trying to open an encrypted archive with 7-Zip using an incorrect password. Try the archive again using proper password before downloading the new clean version and testing its integrity by running 7z t file.7z. In case of errors of type "Unexpected end of data" this might indicate cutting off or an incomplete archive consisting of more than one part. In case of the multipart archives, make sure that all parts are placed into the same directory. Usually, these names include something like .7z.001 and .7z.002.

"Headers Error" can occur for 7z files with corrupted headers inside. When the header encryption is enabled, these errors occur when you use improper password to access the 7z file, when files names aren't decrypted yet. The next kind of error is "Unsupported Method". It occurs, when an extractor doesn't recognize compression, filter, and/or encryption used. Check your 7-zip or libarchive libraries updates or use the newest build from their developers.

In case when wrong software opens 7z archives, try fixing this issue using Open With dialog box, reset default programs or reinstalling of the appropriate utility, which enables shell extensions for .7z archives on installation. If it still doesn't work in Windows environment, very long path names or reserved names might interfere the process. Enable long-path support. If files extracted successfully still cannot be opened, it means that 7z unpacking process finished without issues but inner format isn't known by system and application needed is missing.

Security considerations

An archive in the 7Z format is a wrapper for your files, yet potentially there can be dangerous executable .exe, scripts, setup installers, documents of MS Office Suite with macros and similar malware inside it that gets hazardous once unpacked. There is no support of macro language and no autorun functionality associated directly with 7z file format, hence the major threat comes with payload and from exploits or vulnerabilities of programs which are utilized for its extraction. Security flaws have appeared already in various archive management utilities before, and it is crucial to update programs 7-zip and WinRAR, and mobile applications such as ZArchiver and Documents by Readdle prior any untrustworthy 7Z archives unpacking operations. In the case of unexpected 7Z attachment in your mail consider that compression and encryption can get through preview and filename scanning. Namely header encryption could keep your filenames secret until one enters proper password for it.

History and versions

The creator of 7-Zip and the .7z archive file format was Igor Pavlov who released the first version of this application publicly in year 1999 through the Windows platform. The use of 7-Zip accelerated due to a compression algorithm called LZMA which had been found performing better ratios compared to the many ZIP processing algorithms available before it. This was especially when it came to compressing computer software and other coding scripts. 7z file format with version 4.65 that is defined by the use of 7-Zip 4.x software and is used by many 7-Zip compliant software today was the commonly cited version of this format. The second generation of the development of 7-Zip has resulted in a newer variant known as lzma2 that improved performance in compressing data in multiple thread systems than LZMA and it did so in more situations than before. 7-Zip application as a reference is freely available open source licensed as LGPL software though some of its components might have some different types of licenses.

Related formats

In terms of ubiquity, ZIP (.zip) is the closest universal rival to .7z, though. While most operating systems have tools to view ZIPs without having separate software to do that, old-style ZIPs tend to produce bigger file archives than those archived through the LZMA2 algorithm using 7-Zip. On the other hand, the RAR (.rar) archive type allows great compressing opportunities, recovery data records, and multipart sets. In general, making RAR-based archive requires using WinRAR or another licensed RAR-capable tool.

When talking about UNIX systems, TAR (.tar) stands apart since it separates archiving from compression and mostly operates with .tar.gz, .tgz, .tar.xz, and .txz. Being quite quick and wide used on single steams, the GZ(.gz), however, needs to be complemented by TAR. XZ(.xz) also has LZMA2 support; however, it operates only on single files, while in case of .7z we will get multiple files and directories in one bundle. Another format, BZ2 (.bz2), that has good support among Unix systems but it tends to be somewhat less rapid in comparison with current LZMA2 and ZStandard algorithms. And speaking of ZST (.zst), which is used with Zstandard, is often faster at performing many actions while .7z achieves high results on reaching the maximal ratio of compressibility.