ZIP (file format)


ZIP (file format)
ZIP
Filename extension .zip
.zipx (newer compression algorithms)
Internet media type application/zip
Uniform Type Identifier com.pkware.zip-archive
Magic number none, though PK\003\004 , PK\005\006 (empty archive), or PK\007\008 (spanned archive) are common.
Developed by Phil Katz, PKWARE
Type of format Data compression
Extended to JAR (EAR, RAR (Java), WAR)
Office Open XML (Microsoft)
OpenDocument (ODF)
XPI (Mozilla extensions)
Standard(s) APPNOTE from PKWARE

The ZIP file format is a data compression and archive format. A ZIP file contains one or more files that have been compressed, to reduce file size, or stored as is. The ZIP file format permits a number of compression algorithms.

The format was originally created in 1989 by Phil Katz, and was first implemented in PKWARE's PKZIP utility,[1] as a replacement for the previous ARC compression format by Thom Henderson. The ZIP format is now supported by many software utilities other than PKZIP (see List of file archivers). Microsoft has included built-in ZIP support (under the name "compressed folders") in versions of its Windows operating system since 1998. Apple has included built-in ZIP support in Mac OS X 10.3 (via BOMArchiveHelper, now Archive Utility) and later, along with other compression formats.

ZIP files generally use the file extensions ".zip" or ".ZIP" and the MIME media type application/zip, although the ZIP file format has also been used by many programs, usually under a different name.

Contents

History

The name "zip" (meaning "speed") was suggested by Katz's friend, Robert Mahoney. They wanted to imply that their product would be faster than ARC and other compression formats of the time.

The earliest known version of .ZIP File Format Specification was first published as part of PKZIP 0.9 package under the file APPNOTE.TXT.

The ZIP file format was released into the public domain making it an open format: The ZIP file format is given freely into the public domain and can be claimed neither legally nor morally by any individual, entity or company.[1] The PKWARE company has also stated the base format is in the public domain: Because the format has been dedicated to the public domain, it is possible for other people to write programs which can read .ZIP files. This was a direct result of lawsuits in 1988. [2][3] The same understanding is present in the UNIX/LINUX documentation for the FOSS Info-ZIP version which acknowledges: to Phil Katz for placing in the public domain the zip file format, compression format, and .ZIP filename extension, and for accepting minor changes to the file format.

Even so, more recent versions of APPNOTE.TXT at the PKWARE site note ... However, the use or implementation in a product of certain technological aspects set forth in the current APPNOTE, including those with regard to strong encryption, patching, or extended tape operations requires a license from PKWARE. Please contact PKWARE with regard to acquiring a license.

Origins

The ZIP file format was created by Phil Katz of PKWARE. He created the format after his company had lawsuits filed against him by Systems Enhancement Associates (SEA) claiming that his archiving products were derivatives of SEA's ARC archiving system.

Similarly named formats

There are numerous other standards and formats using "zip" as part of their name. Phil Katz stated that he wanted to allow the "zip" name for any archive type.[citation needed] For example, ZIP is distinct from gzip, and the latter is defined in an IETF RFC (RFC 1952). Both ZIP and gzip primarily use the DEFLATE algorithm for compression. Likewise, the ZLIB format (IETF RFC 1950) also uses the DEFLATE compression algorithm, but specifies different headers for error and consistency checking. Other common, similarly named formats and programs with different native formats include 7-Zip, bzip2, and rzip.

Version history

The .ZIP File Format Specification has its own version number, which does not necessarily correspond to the version numbers for the PKZIP tool, especially with PKZIP 6 or later. At various times, PKWARE has added preliminary features that allow PKZIP products to extract archives using advanced features, but PKZIP products that create such archives are not made available until the next major release. Other companies or organizations support the PKWARE specifications at their own pace.

A summary of key advances in various versions of the PKWARE specification:

  • 2.0: File entries can be compressed with DEFLATE.
  • 4.5: Documented 64-bit ZIP format.
  • 5.0: DES, Triple DES, RC2, RC4 supported for encryption
  • 5.2: RC2-64 supported for Encryption.
  • 6.1: Documented certificate storage.
  • 6.2.0: Documented Central Directory Encryption.
  • 6.3.0: Documented Unicode (UTF-8) filename storage. Expanded list of supported hash, compression, encryption algorithms.
  • 6.3.1: Corrected standard hash values for SHA-256/384/512.
  • 6.3.2: Documented compression method 97 (WavPack).

WinZip, starting with version 12.1, uses the extension .zipx for ZIP files that use compression methods newer than DEFLATE; specifically, methods BZip, LZMA, PPMd, Jpeg and Wavpack – the last 2 are applied to appropriate file types when "Best method" compression is selected.[2][3]

Standardization

In April 2010 ISO/IEC JTC 1 initiated a ballot to determine whether a project should be initiated to create an ISO/IEC International Standard format compatible with ZIP.[4] The proposed project, entitled Document Packaging envisages a ZIP-compatible 'minimal compressed archive format' suitable for use with a number of existing standards including OpenDocument, Office Open XML and EPUB.

The current ZIP format may not meet the requirements for open format: it is not developed through a publicly visible, community driven process, nor by an open industry organization and affirmed and maintained by a standards body. Parts of the current ZIP format do not meet the requirements for a free file format: not encumbered by any copyrights, patents, trademarks or other restrictions so that anyone may use it at no monetary cost for any desired purpose. [4] Recent material on the PKWARE site identify that only the extension and format specification are public domain [5], though the format specification has a copyright notice.

Technical information

ZIP is a simple archive format that stores multiple files. ZIP allows contained files to be compressed using many different methods, as well as simply storing a file without compressing it. Each file is stored separately, allowing different files in the same archive to be compressed using different methods.

A directory is placed at the end of a ZIP file. This identifies what files are in the ZIP and identifies where in the ZIP that file is located. This allows ZIP readers to load the list of files without reading the entire ZIP archive. ZIP archives can also include extra data that is not related to the ZIP archive. This allows for ZIP archives to be made into self-extracting archives, applications that decompress their contained data, by including the program code in a ZIP archive and marking the file as executable (i.e., with the .exe extension). On the other hand, it also allows for a innocuous file, such as a GIF image file, to hide malicious code by making the file a ZIP archive.

The ZIP format uses a 32-bit CRC algorithm and includes two copies of the directory structure of the archive to provide greater protection against data loss.

Structure

A ZIP file is identified by the presence of a central directory which is located at the end of the structure in order to allow the appending of new files. The central directory stores a list of the names of the entries (files or directories) stored in the ZIP file, along with other metadata about the entry, and an offset into the ZIP file, pointing to the actual entry data. This allows a file listing of the archive to be performed relatively quickly, as the entire archive does not have to be read to see the list of files. The entries in the ZIP file also include this information for redundancy.

The order of the file entries in the directory need not coincide with the order of file entries in the archive.

Each entry is introduced by a local header with information about the file such as the comment, file size and file name, followed by optional "Extra" data fields, and then the possibly compressed, possibly encrypted file data. The "Extra" data fields are the key to the extensibility of the ZIP format. "Extra" fields are exploited to support the ZIP64 format, WinZip-compatible AES encryption, file attributes, and higher-resolution NTFS or Unix file timestamps. Other extensions are possible via the "Extra" field. ZIP tools are required by the specification to ignore Extra fields they do not recognize.

ZIPformat.jpg

The ZIP format uses specific 4-byte "signatures" to denote the various structures in the file. Each file entry is marked by a specific signature. The beginning of the central directory is indicated with a different signature, and each entry in the central directory is marked with yet another particular 4-byte signature.

There is no BOF or EOF marker in the ZIP specification. Often the first thing in a ZIP file is a ZIP entry, which can be identified easily by its signature. But it is not necessarily the case that a ZIP file begins with a ZIP entry, and is not required by the ZIP specification.

Tools that correctly read ZIP archives must scan for the signatures of the various fields, the ZIP central directory. They must NOT scan for entries because only the directory specifies where a file chunk starts. Scanning could lead to false positives, as the format allows for other data to be between chunks.[citation needed]

The ZIP specification also supports spreading archives across multiple filesystem files. Originally intended for storage of large zip files across multiple 1.44 MB floppy disks, this feature is now used for sending zip archives in parts over email, or over other transports or removable media.

The FAT filesystem of DOS has a timestamp resolution of only two seconds; ZIP file records mimic this. As a result, the built-in timestamp resolution of files in a ZIP archive is only two seconds, though extra fields can be used to store more accurate timestamps.

In September 2007, PKZIP released a revision of the ZIP specification that contains a provision to store file names using UTF-8, finally adding Unicode compatibility to ZIP.[5]

File headers

All multi-byte values in the header are stored in little-endian byte order. All length fields count the length in bytes.

ZIP local file header
Offset Bytes Description[5]
 0 4 Local file header signature = 0x04034b50 (read as a little-endian number)
 4 2 Version needed to extract (minimum)
 6 2 General purpose bit flag
 8 2 Compression method
10 2 File last modification time
12 2 File last modification date
14 4 CRC-32
18 4 Compressed size
22 4 Uncompressed size
26 2 File name length (n)
28 2 Extra field length (m)
30 n File name
30+n m Extra field

The extra field contains a variety of optional data such as OS-specific attributes. It is divided into chunks, each with a 16-bit ID code and a 16-bit length.

This is immediately followed by the compressed data.

If bit 3 (0x08) of the general-purpose flags field is set, then the CRC-32 and file sizes are not known when the header is written. The fields in the local header are filled with zero, and the CRC-32 and size are appended in a 12-byte structure immediately after the compressed data:

ZIP data descriptor
Offset Bytes Description[5]
 0 4 Local file header signature = 0x08074b50
 4 4 CRC-32
 8 4 Compressed size
 12 4 Uncompressed size

The central directory entry is an expanded form of the local header:

ZIP central directory file header
Offset Bytes Description[5]
 0 4 Central directory file header signature = 0x02014b50
 4 2 Version made by
 6 2 Version needed to extract (minimum)
 8 2 General purpose bit flag
10 2 Compression method
12 2 File last modification time
14 2 File last modification date
16 4 CRC-32
20 4 Compressed size
24 4 Uncompressed size
28 2 File name length (n)
30 2 Extra field length (m)
32 2 File comment length (k)
34 2 Disk number where file starts
36 2 Internal file attributes
38 4 External file attributes
42 4 Relative offset of local file header. This is the number of bytes between the start of the first disk on which the file occurs, and the start of the local file header. This allows software reading the central directory to locate the position of the file inside the ZIP file.
46 n File name
46+n m Extra field
46+n+m k File comment

After all the central directory entries comes the end of central directory record, which marks the end of the ZIP file:

ZIP end of central directory record
Offset Bytes Description[5]
 0 4 End of central directory signature = 0x06054b50
 4 2 Number of this disk
 6 2 Disk where central directory starts
 8 2 Number of central directory records on this disk
10 2 Total number of central directory records
12 4 Size of central directory (bytes)
16 4 Offset of start of central directory, relative to start of archive
20 2 ZIP file comment length (n)
22 n ZIP file comment

This ordering allows a ZIP file to be created in one pass, but it is usually decompressed by first reading the central directory at the end.

Compression methods

Current .ZIP File Format Specification documented the details of following methods: stored (no compression), Shrunk, Reduced (methods 1-4), Imploded, Tokenizing, Deflated, Deflate64, BZIP2, LZMA (EFS), WavPack, PPMd. The most commonly used compression method is DEFLATE, which is described in IETF RFC 1951.

Compression methods mentioned, but not documented in detail in the specification include: PKWARE Data Compression Library (DCL) Imploding (old IBM TERSE), IBM TERSE (new), IBM LZ77 z Architecture (PFS).

Encryption

ZIP supports a simple password-based symmetric encryption system which is documented in the ZIP specification, and known to be seriously flawed. In particular it is vulnerable to known-plaintext attacks which are in some cases made worse by poor implementations of random number generators.[6]

New features including new compression and encryption (e.g. AES) methods have been documented in the .ZIP File Format Specification since version 5.2. A WinZip-developed AES-based standard is used also by 7-Zip, XCeed, and DotNetZip, but some vendors use other formats.[7] PKWARE SecureZIP also supports RC2, RC4, DES, Triple DES encryption methods, Digital Certificate-based encryption and authentication (X.509), and archive header encryption.[8]

ZIP64

The original ZIP format had a 4 GiB limit on various things (uncompressed size of a file, compressed size of a file and total size of the archive), as well as a limit of 65535 entries in a ZIP archive. In version 4.5 of the specification (which is not the same as v4.5 of any particular tool), PKWARE introduced the "ZIP64" format extensions to get around these limitations increasing the limitation to 16 EiB (264 bytes). ZIP64 support is emerging. For example, the File Explorer in Windows XP does not support ZIP64, but the Explorer in Windows Vista does. Likewise, some libraries, such as DotNetZip and IO::Compress::Zip in Perl, support ZIP64. Java's built-in java.util.zip does support ZIP64 from version Java 7.[9]

Advantages and disadvantages

Compressing files separately, as is done in ZIP files, allows for random access: individual files can be retrieved without reading through other data. It may allow better overall compression by using different algorithms for different files. Even when confining the possibility to DEFLATE compression, the use of different compression dictionaries for each file may result in a smaller archive overall.

This approach is less well-suited, in general, to archival of a large number of small files. In the ZIP archive format, the metadata for each entry—the information about each individual entry—is not compressed. This limits the maximum achievable compression ratio, especially as the size of the individual entries diminishes and approaches the size of the metadata for the entry.

An alternate approach is used in a compressed tar archive (.tar.gz, or .tgz), in which the file data and metadata are compressed as a unit using gzip. The downside of this approach is the loss of random access. The same approach can be used with ZIP: creating first a ZIP archive in which the individual files are uncompressed (STORE method), and then compressing the first ZIP file into another ZIP file which contains the first, will emulate solid archives. As in the case of compressed tar archives, random access is not possible.

Combining ZIP with other file formats

The ZIP file format allows for a comment containing any data to occur at the end of the file after the central directory.[5] Also, because the central directory specifies the offset of each file in the archive with respect to the start, it is possible in practice for the first file entry to start at an offset other than zero.

This allows arbitrary data to occur in the file both before and after the ZIP archive data, and for the archive to still be read by a ZIP application. A side-effect of this is that it is possible to author a file that is both a working ZIP archive and another format, provided that the other format tolerates arbitrary data at its end, beginning, or middle. Self-extracting archives (SFX), of the form supported by WinZip and DotNetZip, take advantage of this—they are .exe files that conform to the PKZIP AppNote.txt specification and can be read by compliant zip tools or libraries.

This property of the ZIP format, and of the JAR format which is a variant of ZIP, can be exploited to hide harmful Java classes inside a seemingly harmless file, such as a GIF image uploaded to the web. This so-called GIFAR exploit has been demonstrated as an effective attack against web applications such as Facebook.[10]

Implementation

There are numerous ZIP tools available, and numerous ZIP libraries for various programming environments; licenses used include commercial and open source. For instance, WinZip is one well-known ZIP tool running on Windows and WinRAR, IZarc, Info-zip, 7-Zip, PeaZip and DotNetZip are other tools, available on various platforms. Some of those tools have library or programmatic interfaces.

Some development libraries licensed under open source agreement are the GNU gzip project and Info-ZIP. For Java: Java Platform, Standard Edition contains the package "java.util.zip" to handle standard zip files; the Zip64File library specifically supports large files (larger than 4 GB) and treats ZIP files using random access; and the Apache Ant tool contains a more complete implementation released under the Apache Software License.

For .NET applications, there is a no-cost open-source library called DotNetZip[11] available in source and binary form under the Microsoft Public License.[12] It supports many ZIP features, including passwords for traditional ZIP encryption or WinZip-compatible AES encryption, Unicode, ZIP64, zip comments, spanned archives, and self-extracting archives. The Microsoft .NET 3.5 runtime library includes a class System.IO.Packaging.Package[13] that supports the ZIP format. It is primarily designed for document formats using the ISO/IEC international standard Open Packaging Conventions.

The Info-ZIP implementations of the ZIP format adds support for Unix filesystem features, such as user and group IDs, file permissions, and support for symbolic links. The Apache Ant implementation is aware of these to the extent that it can create files with predefined Unix permissions. The Info-ZIP implementations also know how to use the error correction capabilities built into the ZIP compression format. Some programs (such as IZArc) do not and will choke on a file that has errors.

The Info-ZIP Windows tools also support NTFS filesystem permissions, and will make an attempt to translate from NTFS permissions to Unix permissions or vice-versa when extracting files. This can result in potentially unintended combinations, e.g. .exe files being created on NTFS volumes with executable permission denied.

ZIP file limitations

The minimum size of a ZIP file is 22 bytes.

The maximum size for both the archive file and the individual files inside it is 4,294,967,295 bytes (232−1 bytes, or 4 GiB) for standard ZIP, and 18,446,744,073,709,551,615 bytes (264−1 bytes, or 16 EiB) for ZIP64.[14]

Windows compressed folders

Versions of Microsoft Windows have included support for ZIP compression in Explorer since the Plus! pack was released for Windows 98.[15] Microsoft calls this feature "Compressed Folders". Not all ZIP features are supported by the Windows Compressed Folders capability. For example, AES Encryption, split or spanned archives, and Unicode entry encoding are not known to be readable or writable by the Compressed Folders feature in Windows XP or Windows Vista. A common symptom of these limitations is the message

Compressed (zipped) Folders Error
The Compressed (zipped) Folder is invalid or corrupted.

when trying to open an unsupported archive. The solution is to use a third-party zip utility rather than the built-in Windows Explorer function to open the archive.

Strong encryption controversy

When WinZip 9.0 public beta was released in 2003, WinZip introduced its own AES-256 encryption, using a different file format, along with the documentation for the new specification.[16] The encryption standards themselves were not proprietary, but PKWARE had not updated APPNOTE.TXT to include Strong Encryption Specification (SES) since 2001, which had been used by PKZIP versions 5.0 and 6.0. WinZip technical consultant Kevin Kearney and StuffIt product manager Mathew Covington accused PKWARE of withholding SES, but PKZIP chief technology officer Jim Peterson claimed that Certificate-based encryption was still incomplete. However, the latest publicly available APPNOTE.TXT at the time was version 4.5 (available on PKWARE's FTP site), which not only omitted SES, but also omitted Deflate64, DCL Implode, BZip2 compression methods used by .ZIP files created by contemporary PKZIP products.

To overcome this shortcoming, contemporary products such as PentaZip 'implemented' strong ZIP encryption by encrypting ZIP archives into a different file format.[17]

In another controversial move, PKWare applied for a patent on 2003-07-16 describing a method for combining .ZIP and strong encryption to create a secure .ZIP file.[18]

In the end, PKWARE and WinZip agreed to support each other's products. On 2004-01-21, PKWARE announced the support of WinZip-based AES compression format.[19] In a later version of WinZip beta, it was able to support SES-based ZIP files.[20] PKWARE eventually released version 5.2 of .ZIP File Format Specification to public, which documented SES. The Free Software project 7-Zip also supports AES in ZIP files (as does its POSIX port p7zip).

See also

References

  1. ^ "Phillip Katz, Computer Software Pioneer, 37". The New York Times. Monday, May 1, 2000. http://www.nytimes.com/2000/05/01/us/phillip-katz-computer-software-pioneer-37.html. Retrieved 2009-06-14. 
  2. ^ "Additional Compression Methods Specification". WinZip. Mansfield, CT: WinZip Computing, S.L. 2009-05-19. http://www.winzip.com/comp_info.htm. Retrieved 2009-05-24. 
  3. ^ "What is a Zipx File?". Winzip: Knowledgebase. Mansfield, CT: WinZip Computing, S.L. August 13, 2010. http://kb.winzip.com/kb/entry/7/. Retrieved August 17, 2010. 
  4. ^ http://www.itscj.ipsj.or.jp/sc34/open/1414.pdf
  5. ^ a b c d e f http://www.pkware.com/documents/casestudies/APPNOTE.TXT
  6. ^ Stay, Michael. "ZIP Attacks with Reduced Known Plaintext". http://math.ucr.edu/~mike/zipattacks.pdf
  7. ^ AES Encryption Information: Encryption Specification AE-1 and AE-2
  8. ^ Application Note on the .ZIP file format
  9. ^ Shen, Xueming (2009-04-17). "ZIP64, The Format for > 4G Zipfile, Is Now Supported". Xueming Shen's Blog. Sun Microsystems. http://blogs.sun.com/xuemingshen/entry/zip64_support_for_4g_zipfile. Retrieved 2010-09-27. 
  10. ^ A photo that can steal your online credentials
  11. ^ http://www.codeplex.com/DotNetZip
  12. ^ http://www.codeplex.com/DotNetZip/license
  13. ^ http://msdn.microsoft.com/en-us/library/system.io.packaging.package.aspx
  14. ^ Limits of ZIP file: Standard versus ZIP64.
  15. ^ http://en.wikipedia.org/wiki/Windows_Millennium
  16. ^ WinZip - AES Encryption Information
  17. ^ The .zip standard splinters | InfoWorld | News | 2003-06-10 | By Lincoln Spector, PC World.com
  18. ^ PKWare seeks patent for .zip file format | InfoWorld | News | 2003-07-25 | By Robert McMillan, IDG News Service
  19. ^ Software makers patch Zip tiff - CNET News.com
  20. ^ http://www.theregister.co.uk/2004/01/21/zip_file_encryption_compromise_thrashed/

External links


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