Sustainability of Digital Formats: Planning for Library of Congress Collections

7z is an archive file format that supports a variety of compression methods and encryption. 7z was initially the default format for 7-Zip, developed by Igor Pavlov in 1999 that was used to compress groups of files. There is no formal format specification for 7z but the LZMA compression specification serves as an "unofficial" specification with information on the file and byte structure. There are other specification options from third-party sources including but not limited to py7zr, a 7z python library that also documents the file's key characteristics and structure.  According to 7-zip.org, the official homepage for the 7-Zip file archiving program, the 7z file format has the following main features:

• An open architecture, which allows the support for a variety of compression methods. (See below)
• High compression ratio (dependent on the compression method used)
• AES-256 encryption
• Large file size support, up to 1600 terabytes
• Unicode file names
• Archive header compression
• Support for solid compression, where multiple files of a similar type are compressed within a single stream.

7z files are compatible with several compression methods because of its open architecture. More compression options may follow in time. The following are a list of compression methods, as stated by 7-zip.org that are integrated with the 7z file format:

• LZMA, an optimized version of the LZ77 compression algorithm. LZMA is the default compression method for 7z files.
• LZMA2, an improved version of the LZMA compression method
• PPMD
• BCJ, converter for 32 bit x86 executables
• BCJ2, converter for 32 bit x86 executables
• Bzip2, BWT algorithm
• Deflate, another LZ77 based compression algorithm

Per 7-zip.org's 7z history, beginning in 2008, 7z supported UTF-8 encoding for filenames. 7z archives now have 3 distinct modes:

• If there are no required symbols, then the default mode uses UTF-8.
• mcu-switch: if there are non-ASCII symbols then 7-Zip uses UTF-8 encoding.
• mcl-switch: 7-Zip uses local code page.

The file structure for 7z files begins with a “signature header”. The header can contain information about the version number where version numbers consist of two bytes. A major version consists of the bytes "0x00" and a minor version has the bytes "0x04". As explained in the LZMA 7z file format specification, the data block is placed after the signature header known as "Packed Streams." The signature header should consist of 32 bytes with the first six bytes of any given 7z file consisting of the same bytes as follows, {'7', 'z', 0xBC, 0xAF, 0x27, 0x1C}. "Packed Streams" is optional as the data block may be empty. From LZMA's GitHub, 7z files use little endian encoding where the least significant bytes are stored first. The majority of the file structure is a sequence of "Property IDs" which dictate the type of information contained within the 7z file. The py7zr documentation lists all of the associated IDs for each type of property including:

• Header
• Fileinfo
• Substreams info
• Folder

The "Files Information" or "Fileinfo" Property ID can contain information about the files within the archive including: number of files, names, sizes, timestamps, and select attributes such as permissions.

The file structure also contains Codec IDs which correspond to specific compression methods (such as the ones listed above). Some of those Codec IDs are defined as follows:

• LZMA - 0x030101
• BCJ - 0x04
• LZMA2 - 0x21
• Bzip2 - 0x040202
• Deflate - 0x040108

Additional compression Codec IDs can be found in the py7zr documentation as well as LZMA's GitHub, which provides information about many of the aforementioned compression methods. The 7z file format also supports solid compression, where several files of a similar type are compressed within a single stream to avoid redundancies.

7z files utilize AES (Advanced Encryption Standard) encryption. As stated in its definitive standard, the encryption uses a cypher that converts the data to an unintelligible form called ciphertext while the decryption converts the data back to its original format, or plaintext. The AES encryption method is able to use cryptographic keys of 128, 192, and 256 bits. The 7z format also supports file naming encryption.

Peazip (another compression program similar to 7zip, Winzip, among others) GitHub released a compression benchmark that compared a variety of compression software programs including Peazip, Winzip, WinRar, and 7-Zip. Most of the focus of this benchmark pinpoints the strengths and weaknesses of the particular compression programs, but it should be noted that the results summary highlighted that 7z files provided better compression but slower speeds than RAR files while decompression speeds were comparable to ZIP and RAR files making it a good candidate for content distribution.

For more information on the history of 7z and the 7-Zip application, see Notes.

A note on terminology. In computing and in many standards specifications, 7z, ZIP, RAR, tar and the like are classified as archive files. This site is using the term aggregate instead of archive when defining quality and functional factors because the latter term archive has broader community use beyond the definitions of these formats. The term aggregate is used here instead to convey the basic function of bringing disparate parts together into a single collective object but also with the added features of compression, potential for encryption, error detection and more. See Quality and Functionality Factors for more information.

Below is a non-comprehensive list of applications that can open 7z files. This list can be found at 7-zip.org

Windows:

• WinRAR
• PowerArchiver
• Peazip

MAC OS X:

• Unarchiver
• Commander One
• Keka

Linux:

• p7zip

7z files can be created using the 7-Zip graphic interface as well as via command line operations using both UNIX and LINUX systems. Examples of documentation for command line operations to create 7z files can be found at 7ziphelp, which provides tutorials on using different 7-Zip application functions.

There is also considerable support from a variety of software programs to convert 7z files to other archive file formats. Lifewire’s entry for 7z files highlights software programs such as Zamzar that can convert 7z files to other archive formats such as ZIP, RAR, ISO_image, and tar files.

The 7z file format provides no additional metadata support beyond what is needed to support unpacking the archive and extracting the component items into a file system.

Accessibility Features

No specific features in the file format. Features to support accessibility would be found in the bundled and compressed files (such as embedded captions and subtitles in audiovisual content, tagged and structured text in textual documents, and alt text for images). Aggregate files can also contain separate files for transcripts, timed text or captions as part of the bundled package. See Relationships to other formats for details.

7z files can also utilize AES (Advanced Encryption Standard) encryption. The bit key is generated from a passphrase supplied by users that is based on the SHA-256 hash function. As stated in its definitive standard, the encryption uses a cypher that converts the data to an unintelligible form called ciphertext while the decryption converts the data back to its original format, or plaintext. The AES encryption method is able to use cryptographic keys of 128, 192, and 256 bits. The 7z format also supports file naming encryption.

There are a variety of documentation sources such as the University of Victory University Systems provide information for users on how to use the 7-Zip program to encrypt 7z archive files.

7-Zip.org provides an extensive history of the archive program and file format on 7-Zip's site. The original beta version was first released on January 2, 1999. The 7z file format was designated the default format for the 7z.exe and 7za.exe applications. Further support for additional compression programs such as bzip2, gzip, tar, and zip was developed in late 2001. 7-Zip’s history log chronicles ongoing bug fixes, software improvements, and additional functionality with the latest entry on December 26, 2021.