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authorMartin Ashby <martin@ashbysoft.com>2023-09-14 23:43:44 +0100
committerMartin Ashby <martin@ashbysoft.com>2023-09-14 23:43:44 +0100
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+File: APPNOTE.TXT - .ZIP File Format Specification
+Version: 6.3.10
+Status: FINAL - replaces version 6.3.9
+Revised: Nov 01, 2022
+Copyright (c) 1989 - 2014, 2018, 2019, 2020, 2022 PKWARE Inc., All Rights Reserved.
+
+1.0 Introduction
+---------------
+
+1.1 Purpose
+-----------
+
+ 1.1.1 This specification is intended to define a cross-platform,
+ interoperable file storage and transfer format. Since its
+ first publication in 1989, PKWARE, Inc. ("PKWARE") has remained
+ committed to ensuring the interoperability of the .ZIP file
+ format through periodic publication and maintenance of this
+ specification. We trust that all .ZIP compatible vendors and
+ application developers that use and benefit from this format
+ will share and support this commitment to interoperability.
+
+1.2 Scope
+---------
+
+ 1.2.1 ZIP is one of the most widely used compressed file formats. It is
+ universally used to aggregate, compress, and encrypt files into a single
+ interoperable container. No specific use or application need is
+ defined by this format and no specific implementation guidance is
+ provided. This document provides details on the storage format for
+ creating ZIP files. Information is provided on the records and
+ fields that describe what a ZIP file is.
+
+1.3 Trademarks
+--------------
+
+ 1.3.1 PKWARE, PKZIP, Smartcrypt, SecureZIP, and PKSFX are registered
+ trademarks of PKWARE, Inc. in the United States and elsewhere.
+ PKPatchMaker, Deflate64, and ZIP64 are trademarks of PKWARE, Inc.
+ Other marks referenced within this document appear for identification
+ purposes only and are the property of their respective owners.
+
+
+1.4 Permitted Use
+-----------------
+
+ 1.4.1 This document, "APPNOTE.TXT - .ZIP File Format Specification" is the
+ exclusive property of PKWARE. Use of the information contained in this
+ document is permitted solely for the purpose of creating products,
+ programs and processes that read and write files in the ZIP format
+ subject to the terms and conditions herein.
+
+ 1.4.2 Use of the content of this document within other publications is
+ permitted only through reference to this document. Any reproduction
+ or distribution of this document in whole or in part without prior
+ written permission from PKWARE is strictly prohibited.
+
+ 1.4.3 Certain technological components provided in this document are the
+ patented proprietary technology of PKWARE and as such require a
+ separate, executed license agreement from PKWARE. Applicable
+ components are marked with the following, or similar, statement:
+ 'Refer to the section in this document entitled "Incorporating
+ PKWARE Proprietary Technology into Your Product" for more information'.
+
+1.5 Contacting PKWARE
+---------------------
+
+ 1.5.1 If you have questions on this format, its use, or licensing, or if you
+ wish to report defects, request changes or additions, please contact:
+
+ PKWARE, Inc.
+ 201 E. Pittsburgh Avenue, Suite 400
+ Milwaukee, WI 53204
+ +1-414-289-9788
+ +1-414-289-9789 FAX
+ zipformat@pkware.com
+
+ 1.5.2 Information about this format and a reference copy of this document
+ is publicly available at:
+
+ http://www.pkware.com/appnote
+
+1.6 Disclaimer
+--------------
+
+ 1.6.1 Although PKWARE will attempt to supply current and accurate
+ information relating to its file formats, algorithms, and the
+ subject programs, the possibility of error or omission cannot
+ be eliminated. PKWARE therefore expressly disclaims any warranty
+ that the information contained in the associated materials relating
+ to the subject programs and/or the format of the files created or
+ accessed by the subject programs and/or the algorithms used by
+ the subject programs, or any other matter, is current, correct or
+ accurate as delivered. Any risk of damage due to any possible
+ inaccurate information is assumed by the user of the information.
+ Furthermore, the information relating to the subject programs
+ and/or the file formats created or accessed by the subject
+ programs and/or the algorithms used by the subject programs is
+ subject to change without notice.
+
+2.0 Revisions
+--------------
+
+2.1 Document Status
+--------------------
+
+ 2.1.1 If the STATUS of this file is marked as DRAFT, the content
+ defines proposed revisions to this specification which may consist
+ of changes to the ZIP format itself, or that may consist of other
+ content changes to this document. Versions of this document and
+ the format in DRAFT form may be subject to modification prior to
+ publication STATUS of FINAL. DRAFT versions are published periodically
+ to provide notification to the ZIP community of pending changes and to
+ provide opportunity for review and comment.
+
+ 2.1.2 Versions of this document having a STATUS of FINAL are
+ considered to be in the final form for that version of the document
+ and are not subject to further change until a new, higher version
+ numbered document is published. Newer versions of this format
+ specification are intended to remain interoperable with all prior
+ versions whenever technically possible.
+
+2.2 Change Log
+--------------
+
+ Version Change Description Date
+ ------- ------------------ ----------
+ 5.2 -Single Password Symmetric Encryption 07/16/2003
+ storage
+
+ 6.1.0 -Smartcard compatibility 01/20/2004
+ -Documentation on certificate storage
+
+ 6.2.0 -Introduction of Central Directory 04/26/2004
+ Encryption for encrypting metadata
+ -Added OS X to Version Made By values
+
+ 6.2.1 -Added Extra Field placeholder for 04/01/2005
+ POSZIP using ID 0x4690
+
+ -Clarified size field on
+ "zip64 end of central directory record"
+
+ 6.2.2 -Documented Final Feature Specification 01/06/2006
+ for Strong Encryption
+
+ -Clarifications and typographical
+ corrections
+
+ 6.3.0 -Added tape positioning storage 09/29/2006
+ parameters
+
+ -Expanded list of supported hash algorithms
+
+ -Expanded list of supported compression
+ algorithms
+
+ -Expanded list of supported encryption
+ algorithms
+
+ -Added option for Unicode filename
+ storage
+
+ -Clarifications for consistent use
+ of Data Descriptor records
+
+ -Added additional "Extra Field"
+ definitions
+
+ 6.3.1 -Corrected standard hash values for 04/11/2007
+ SHA-256/384/512
+
+ 6.3.2 -Added compression method 97 09/28/2007
+
+ -Documented InfoZIP "Extra Field"
+ values for UTF-8 file name and
+ file comment storage
+
+ 6.3.3 -Formatting changes to support 09/01/2012
+ easier referencing of this APPNOTE
+ from other documents and standards
+
+ 6.3.4 -Address change 10/01/2014
+
+ 6.3.5 -Documented compression methods 16 11/31/2018
+ and 99 (4.4.5, 4.6.1, 5.11, 5.17,
+ APPENDIX E)
+
+ -Corrected several typographical
+ errors (2.1.2, 3.2, 4.1.1, 10.2)
+
+ -Marked legacy algorithms as no
+ longer suitable for use (4.4.5.1)
+
+ -Added clarity on MS DOS time format
+ (4.4.6)
+
+ -Assign extrafield ID for Timestamps
+ (4.5.2)
+
+ -Field code description correction (A.2)
+
+ -More consistent use of MAY/SHOULD/MUST
+
+ -Expanded 0x0065 record attribute codes (B.2)
+
+ -Initial information on 0x0022 Extra Data
+
+ 6.3.6 -Corrected typographical error 04/26/2019
+ (4.4.1.3)
+
+ 6.3.7 -Added Zstandard compression method ID
+ (4.4.5)
+
+ -Corrected several reported typos
+
+ -Marked intended use for general purpose bit 14
+
+ -Added Data Stream Alignment Extra Data info
+ (4.6.11)
+
+ 6.3.8 -Resolved Zstandard compression method ID conflict
+ (4.4.5)
+
+ -Added additional compression method ID values in use
+
+ 6.3.9 -Corrected a typo in Data Stream Alignment description
+ (4.6.11)
+
+ 6.3.10 -Added several z/OS attribute values for APPENDIX B
+
+ -Added several additional 3rd party Extra Field mappings
+ (thanks to Armijn Hemel @tjaldur.nl for forwarding info
+ on several of the Header ID's)
+
+
+
+3.0 Notations
+-------------
+
+ 3.1 Use of the term MUST or SHALL indicates a required element.
+
+ 3.2 MUST NOT or SHALL NOT indicates an element is prohibited from use.
+
+ 3.3 SHOULD indicates a RECOMMENDED element.
+
+ 3.4 SHOULD NOT indicates an element NOT RECOMMENDED for use.
+
+ 3.5 MAY indicates an OPTIONAL element.
+
+
+4.0 ZIP Files
+-------------
+
+4.1 What is a ZIP file
+----------------------
+
+ 4.1.1 ZIP files MAY be identified by the standard .ZIP file extension
+ although use of a file extension is not required. Use of the
+ extension .ZIPX is also recognized and MAY be used for ZIP files.
+ Other common file extensions using the ZIP format include .JAR, .WAR,
+ .DOCX, .XLSX, .PPTX, .ODT, .ODS, .ODP and others. Programs reading or
+ writing ZIP files SHOULD rely on internal record signatures described
+ in this document to identify files in this format.
+
+ 4.1.2 ZIP files SHOULD contain at least one file and MAY contain
+ multiple files.
+
+ 4.1.3 Data compression MAY be used to reduce the size of files
+ placed into a ZIP file, but is not required. This format supports the
+ use of multiple data compression algorithms. When compression is used,
+ one of the documented compression algorithms MUST be used. Implementors
+ are advised to experiment with their data to determine which of the
+ available algorithms provides the best compression for their needs.
+ Compression method 8 (Deflate) is the method used by default by most
+ ZIP compatible application programs.
+
+
+ 4.1.4 Data encryption MAY be used to protect files within a ZIP file.
+ Keying methods supported for encryption within this format include
+ passwords and public/private keys. Either MAY be used individually
+ or in combination. Encryption MAY be applied to individual files.
+ Additional security MAY be used through the encryption of ZIP file
+ metadata stored within the Central Directory. See the section on the
+ Strong Encryption Specification for information. Refer to the section
+ in this document entitled "Incorporating PKWARE Proprietary Technology
+ into Your Product" for more information.
+
+ 4.1.5 Data integrity MUST be provided for each file using CRC32.
+
+ 4.1.6 Additional data integrity MAY be included through the use of
+ digital signatures. Individual files MAY be signed with one or more
+ digital signatures. The Central Directory, if signed, MUST use a
+ single signature.
+
+ 4.1.7 Files MAY be placed within a ZIP file uncompressed or stored.
+ The term "stored" as used in the context of this document means the file
+ is copied into the ZIP file uncompressed.
+
+ 4.1.8 Each data file placed into a ZIP file MAY be compressed, stored,
+ encrypted or digitally signed independent of how other data files in the
+ same ZIP file are archived.
+
+ 4.1.9 ZIP files MAY be streamed, split into segments (on fixed or on
+ removable media) or "self-extracting". Self-extracting ZIP
+ files MUST include extraction code for a target platform within
+ the ZIP file.
+
+ 4.1.10 Extensibility is provided for platform or application specific
+ needs through extra data fields that MAY be defined for custom
+ purposes. Extra data definitions MUST NOT conflict with existing
+ documented record definitions.
+
+ 4.1.11 Common uses for ZIP MAY also include the use of manifest files.
+ Manifest files store application specific information within a file stored
+ within the ZIP file. This manifest file SHOULD be the first file in the
+ ZIP file. This specification does not provide any information or guidance on
+ the use of manifest files within ZIP files. Refer to the application developer
+ for information on using manifest files and for any additional profile
+ information on using ZIP within an application.
+
+ 4.1.12 ZIP files MAY be placed within other ZIP files.
+
+4.2 ZIP Metadata
+----------------
+
+ 4.2.1 ZIP files are identified by metadata consisting of defined record types
+ containing the storage information necessary for maintaining the files
+ placed into a ZIP file. Each record type MUST be identified using a header
+ signature that identifies the record type. Signature values begin with the
+ two byte constant marker of 0x4b50, representing the characters "PK".
+
+
+4.3 General Format of a .ZIP file
+---------------------------------
+
+ 4.3.1 A ZIP file MUST contain an "end of central directory record". A ZIP
+ file containing only an "end of central directory record" is considered an
+ empty ZIP file. Files MAY be added or replaced within a ZIP file, or deleted.
+ A ZIP file MUST have only one "end of central directory record". Other
+ records defined in this specification MAY be used as needed to support
+ storage requirements for individual ZIP files.
+
+ 4.3.2 Each file placed into a ZIP file MUST be preceded by a "local
+ file header" record for that file. Each "local file header" MUST be
+ accompanied by a corresponding "central directory header" record within
+ the central directory section of the ZIP file.
+
+ 4.3.3 Files MAY be stored in arbitrary order within a ZIP file. A ZIP
+ file MAY span multiple volumes or it MAY be split into user-defined
+ segment sizes. All values MUST be stored in little-endian byte order unless
+ otherwise specified in this document for a specific data element.
+
+ 4.3.4 Compression MUST NOT be applied to a "local file header", an "encryption
+ header", or an "end of central directory record". Individual "central
+ directory records" MUST NOT be compressed, but the aggregate of all central
+ directory records MAY be compressed.
+
+ 4.3.5 File data MAY be followed by a "data descriptor" for the file. Data
+ descriptors are used to facilitate ZIP file streaming.
+
+
+ 4.3.6 Overall .ZIP file format:
+
+ [local file header 1]
+ [encryption header 1]
+ [file data 1]
+ [data descriptor 1]
+ .
+ .
+ .
+ [local file header n]
+ [encryption header n]
+ [file data n]
+ [data descriptor n]
+ [archive decryption header]
+ [archive extra data record]
+ [central directory header 1]
+ .
+ .
+ .
+ [central directory header n]
+ [zip64 end of central directory record]
+ [zip64 end of central directory locator]
+ [end of central directory record]
+
+
+ 4.3.7 Local file header:
+
+ local file header signature 4 bytes (0x04034b50)
+ version needed to extract 2 bytes
+ general purpose bit flag 2 bytes
+ compression method 2 bytes
+ last mod file time 2 bytes
+ last mod file date 2 bytes
+ crc-32 4 bytes
+ compressed size 4 bytes
+ uncompressed size 4 bytes
+ file name length 2 bytes
+ extra field length 2 bytes
+
+ file name (variable size)
+ extra field (variable size)
+
+ 4.3.8 File data
+
+ Immediately following the local header for a file
+ SHOULD be placed the compressed or stored data for the file.
+ If the file is encrypted, the encryption header for the file
+ SHOULD be placed after the local header and before the file
+ data. The series of [local file header][encryption header]
+ [file data][data descriptor] repeats for each file in the
+ .ZIP archive.
+
+ Zero-byte files, directories, and other file types that
+ contain no content MUST NOT include file data.
+
+ 4.3.9 Data descriptor:
+
+ crc-32 4 bytes
+ compressed size 4 bytes
+ uncompressed size 4 bytes
+
+ 4.3.9.1 This descriptor MUST exist if bit 3 of the general
+ purpose bit flag is set (see below). It is byte aligned
+ and immediately follows the last byte of compressed data.
+ This descriptor SHOULD be used only when it was not possible to
+ seek in the output .ZIP file, e.g., when the output .ZIP file
+ was standard output or a non-seekable device. For ZIP64(tm) format
+ archives, the compressed and uncompressed sizes are 8 bytes each.
+
+ 4.3.9.2 When compressing files, compressed and uncompressed sizes
+ SHOULD be stored in ZIP64 format (as 8 byte values) when a
+ file's size exceeds 0xFFFFFFFF. However ZIP64 format MAY be
+ used regardless of the size of a file. When extracting, if
+ the zip64 extended information extra field is present for
+ the file the compressed and uncompressed sizes will be 8
+ byte values.
+
+ 4.3.9.3 Although not originally assigned a signature, the value
+ 0x08074b50 has commonly been adopted as a signature value
+ for the data descriptor record. Implementers SHOULD be
+ aware that ZIP files MAY be encountered with or without this
+ signature marking data descriptors and SHOULD account for
+ either case when reading ZIP files to ensure compatibility.
+
+ 4.3.9.4 When writing ZIP files, implementors SHOULD include the
+ signature value marking the data descriptor record. When
+ the signature is used, the fields currently defined for
+ the data descriptor record will immediately follow the
+ signature.
+
+ 4.3.9.5 An extensible data descriptor will be released in a
+ future version of this APPNOTE. This new record is intended to
+ resolve conflicts with the use of this record going forward,
+ and to provide better support for streamed file processing.
+
+ 4.3.9.6 When the Central Directory Encryption method is used,
+ the data descriptor record is not required, but MAY be used.
+ If present, and bit 3 of the general purpose bit field is set to
+ indicate its presence, the values in fields of the data descriptor
+ record MUST be set to binary zeros. See the section on the Strong
+ Encryption Specification for information. Refer to the section in
+ this document entitled "Incorporating PKWARE Proprietary Technology
+ into Your Product" for more information.
+
+
+ 4.3.10 Archive decryption header:
+
+ 4.3.10.1 The Archive Decryption Header is introduced in version 6.2
+ of the ZIP format specification. This record exists in support
+ of the Central Directory Encryption Feature implemented as part of
+ the Strong Encryption Specification as described in this document.
+ When the Central Directory Structure is encrypted, this decryption
+ header MUST precede the encrypted data segment.
+
+ 4.3.10.2 The encrypted data segment SHALL consist of the Archive
+ extra data record (if present) and the encrypted Central Directory
+ Structure data. The format of this data record is identical to the
+ Decryption header record preceding compressed file data. If the
+ central directory structure is encrypted, the location of the start of
+ this data record is determined using the Start of Central Directory
+ field in the Zip64 End of Central Directory record. See the
+ section on the Strong Encryption Specification for information
+ on the fields used in the Archive Decryption Header record.
+ Refer to the section in this document entitled "Incorporating
+ PKWARE Proprietary Technology into Your Product" for more information.
+
+
+ 4.3.11 Archive extra data record:
+
+ archive extra data signature 4 bytes (0x08064b50)
+ extra field length 4 bytes
+ extra field data (variable size)
+
+ 4.3.11.1 The Archive Extra Data Record is introduced in version 6.2
+ of the ZIP format specification. This record MAY be used in support
+ of the Central Directory Encryption Feature implemented as part of
+ the Strong Encryption Specification as described in this document.
+ When present, this record MUST immediately precede the central
+ directory data structure.
+
+ 4.3.11.2 The size of this data record SHALL be included in the
+ Size of the Central Directory field in the End of Central
+ Directory record. If the central directory structure is compressed,
+ but not encrypted, the location of the start of this data record is
+ determined using the Start of Central Directory field in the Zip64
+ End of Central Directory record. Refer to the section in this document
+ entitled "Incorporating PKWARE Proprietary Technology into Your
+ Product" for more information.
+
+ 4.3.12 Central directory structure:
+
+ [central directory header 1]
+ .
+ .
+ .
+ [central directory header n]
+ [digital signature]
+
+ File header:
+
+ central file header signature 4 bytes (0x02014b50)
+ version made by 2 bytes
+ version needed to extract 2 bytes
+ general purpose bit flag 2 bytes
+ compression method 2 bytes
+ last mod file time 2 bytes
+ last mod file date 2 bytes
+ crc-32 4 bytes
+ compressed size 4 bytes
+ uncompressed size 4 bytes
+ file name length 2 bytes
+ extra field length 2 bytes
+ file comment length 2 bytes
+ disk number start 2 bytes
+ internal file attributes 2 bytes
+ external file attributes 4 bytes
+ relative offset of local header 4 bytes
+
+ file name (variable size)
+ extra field (variable size)
+ file comment (variable size)
+
+ 4.3.13 Digital signature:
+
+ header signature 4 bytes (0x05054b50)
+ size of data 2 bytes
+ signature data (variable size)
+
+ With the introduction of the Central Directory Encryption
+ feature in version 6.2 of this specification, the Central
+ Directory Structure MAY be stored both compressed and encrypted.
+ Although not required, it is assumed when encrypting the
+ Central Directory Structure, that it will be compressed
+ for greater storage efficiency. Information on the
+ Central Directory Encryption feature can be found in the section
+ describing the Strong Encryption Specification. The Digital
+ Signature record will be neither compressed nor encrypted.
+
+ 4.3.14 Zip64 end of central directory record
+
+ zip64 end of central dir
+ signature 4 bytes (0x06064b50)
+ size of zip64 end of central
+ directory record 8 bytes
+ version made by 2 bytes
+ version needed to extract 2 bytes
+ number of this disk 4 bytes
+ number of the disk with the
+ start of the central directory 4 bytes
+ total number of entries in the
+ central directory on this disk 8 bytes
+ total number of entries in the
+ central directory 8 bytes
+ size of the central directory 8 bytes
+ offset of start of central
+ directory with respect to
+ the starting disk number 8 bytes
+ zip64 extensible data sector (variable size)
+
+ 4.3.14.1 The value stored into the "size of zip64 end of central
+ directory record" SHOULD be the size of the remaining
+ record and SHOULD NOT include the leading 12 bytes.
+
+ Size = SizeOfFixedFields + SizeOfVariableData - 12.
+
+ 4.3.14.2 The above record structure defines Version 1 of the
+ zip64 end of central directory record. Version 1 was
+ implemented in versions of this specification preceding
+ 6.2 in support of the ZIP64 large file feature. The
+ introduction of the Central Directory Encryption feature
+ implemented in version 6.2 as part of the Strong Encryption
+ Specification defines Version 2 of this record structure.
+ Refer to the section describing the Strong Encryption
+ Specification for details on the version 2 format for
+ this record. Refer to the section in this document entitled
+ "Incorporating PKWARE Proprietary Technology into Your Product"
+ for more information applicable to use of Version 2 of this
+ record.
+
+ 4.3.14.3 Special purpose data MAY reside in the zip64 extensible
+ data sector field following either a V1 or V2 version of this
+ record. To ensure identification of this special purpose data
+ it MUST include an identifying header block consisting of the
+ following:
+
+ Header ID - 2 bytes
+ Data Size - 4 bytes
+
+ The Header ID field indicates the type of data that is in the
+ data block that follows.
+
+ Data Size identifies the number of bytes that follow for this
+ data block type.
+
+ 4.3.14.4 Multiple special purpose data blocks MAY be present.
+ Each MUST be preceded by a Header ID and Data Size field. Current
+ mappings of Header ID values supported in this field are as
+ defined in APPENDIX C.
+
+ 4.3.15 Zip64 end of central directory locator
+
+ zip64 end of central dir locator
+ signature 4 bytes (0x07064b50)
+ number of the disk with the
+ start of the zip64 end of
+ central directory 4 bytes
+ relative offset of the zip64
+ end of central directory record 8 bytes
+ total number of disks 4 bytes
+
+ 4.3.16 End of central directory record:
+
+ end of central dir signature 4 bytes (0x06054b50)
+ number of this disk 2 bytes
+ number of the disk with the
+ start of the central directory 2 bytes
+ total number of entries in the
+ central directory on this disk 2 bytes
+ total number of entries in
+ the central directory 2 bytes
+ size of the central directory 4 bytes
+ offset of start of central
+ directory with respect to
+ the starting disk number 4 bytes
+ .ZIP file comment length 2 bytes
+ .ZIP file comment (variable size)
+
+4.4 Explanation of fields
+--------------------------
+
+ 4.4.1 General notes on fields
+
+ 4.4.1.1 All fields unless otherwise noted are unsigned and stored
+ in Intel low-byte:high-byte, low-word:high-word order.
+
+ 4.4.1.2 String fields are not null terminated, since the length
+ is given explicitly.
+
+ 4.4.1.3 The entries in the central directory MAY NOT necessarily
+ be in the same order that files appear in the .ZIP file.
+
+ 4.4.1.4 If one of the fields in the end of central directory
+ record is too small to hold required data, the field SHOULD be
+ set to -1 (0xFFFF or 0xFFFFFFFF) and the ZIP64 format record
+ SHOULD be created.
+
+ 4.4.1.5 The end of central directory record and the Zip64 end
+ of central directory locator record MUST reside on the same
+ disk when splitting or spanning an archive.
+
+ 4.4.2 version made by (2 bytes)
+
+ 4.4.2.1 The upper byte indicates the compatibility of the file
+ attribute information. If the external file attributes
+ are compatible with MS-DOS and can be read by PKZIP for
+ DOS version 2.04g then this value will be zero. If these
+ attributes are not compatible, then this value will
+ identify the host system on which the attributes are
+ compatible. Software can use this information to determine
+ the line record format for text files etc.
+
+ 4.4.2.2 The current mappings are:
+
+ 0 - MS-DOS and OS/2 (FAT / VFAT / FAT32 file systems)
+ 1 - Amiga 2 - OpenVMS
+ 3 - UNIX 4 - VM/CMS
+ 5 - Atari ST 6 - OS/2 H.P.F.S.
+ 7 - Macintosh 8 - Z-System
+ 9 - CP/M 10 - Windows NTFS
+ 11 - MVS (OS/390 - Z/OS) 12 - VSE
+ 13 - Acorn Risc 14 - VFAT
+ 15 - alternate MVS 16 - BeOS
+ 17 - Tandem 18 - OS/400
+ 19 - OS X (Darwin) 20 thru 255 - unused
+
+ 4.4.2.3 The lower byte indicates the ZIP specification version
+ (the version of this document) supported by the software
+ used to encode the file. The value/10 indicates the major
+ version number, and the value mod 10 is the minor version
+ number.
+
+ 4.4.3 version needed to extract (2 bytes)
+
+ 4.4.3.1 The minimum supported ZIP specification version needed
+ to extract the file, mapped as above. This value is based on
+ the specific format features a ZIP program MUST support to
+ be able to extract the file. If multiple features are
+ applied to a file, the minimum version MUST be set to the
+ feature having the highest value. New features or feature
+ changes affecting the published format specification will be
+ implemented using higher version numbers than the last
+ published value to avoid conflict.
+
+ 4.4.3.2 Current minimum feature versions are as defined below:
+
+ 1.0 - Default value
+ 1.1 - File is a volume label
+ 2.0 - File is a folder (directory)
+ 2.0 - File is compressed using Deflate compression
+ 2.0 - File is encrypted using traditional PKWARE encryption
+ 2.1 - File is compressed using Deflate64(tm)
+ 2.5 - File is compressed using PKWARE DCL Implode
+ 2.7 - File is a patch data set
+ 4.5 - File uses ZIP64 format extensions
+ 4.6 - File is compressed using BZIP2 compression*
+ 5.0 - File is encrypted using DES
+ 5.0 - File is encrypted using 3DES
+ 5.0 - File is encrypted using original RC2 encryption
+ 5.0 - File is encrypted using RC4 encryption
+ 5.1 - File is encrypted using AES encryption
+ 5.1 - File is encrypted using corrected RC2 encryption**
+ 5.2 - File is encrypted using corrected RC2-64 encryption**
+ 6.1 - File is encrypted using non-OAEP key wrapping***
+ 6.2 - Central directory encryption
+ 6.3 - File is compressed using LZMA
+ 6.3 - File is compressed using PPMd+
+ 6.3 - File is encrypted using Blowfish
+ 6.3 - File is encrypted using Twofish
+
+ 4.4.3.3 Notes on version needed to extract
+
+ * Early 7.x (pre-7.2) versions of PKZIP incorrectly set the
+ version needed to extract for BZIP2 compression to be 50
+ when it SHOULD have been 46.
+
+ ** Refer to the section on Strong Encryption Specification
+ for additional information regarding RC2 corrections.
+
+ *** Certificate encryption using non-OAEP key wrapping is the
+ intended mode of operation for all versions beginning with 6.1.
+ Support for OAEP key wrapping MUST only be used for
+ backward compatibility when sending ZIP files to be opened by
+ versions of PKZIP older than 6.1 (5.0 or 6.0).
+
+ + Files compressed using PPMd MUST set the version
+ needed to extract field to 6.3, however, not all ZIP
+ programs enforce this and MAY be unable to decompress
+ data files compressed using PPMd if this value is set.
+
+ When using ZIP64 extensions, the corresponding value in the
+ zip64 end of central directory record MUST also be set.
+ This field SHOULD be set appropriately to indicate whether
+ Version 1 or Version 2 format is in use.
+
+
+ 4.4.4 general purpose bit flag: (2 bytes)
+
+ Bit 0: If set, indicates that the file is encrypted.
+
+ (For Method 6 - Imploding)
+ Bit 1: If the compression method used was type 6,
+ Imploding, then this bit, if set, indicates
+ an 8K sliding dictionary was used. If clear,
+ then a 4K sliding dictionary was used.
+
+ Bit 2: If the compression method used was type 6,
+ Imploding, then this bit, if set, indicates
+ 3 Shannon-Fano trees were used to encode the
+ sliding dictionary output. If clear, then 2
+ Shannon-Fano trees were used.
+
+ (For Methods 8 and 9 - Deflating)
+ Bit 2 Bit 1
+ 0 0 Normal (-en) compression option was used.
+ 0 1 Maximum (-exx/-ex) compression option was used.
+ 1 0 Fast (-ef) compression option was used.
+ 1 1 Super Fast (-es) compression option was used.
+
+ (For Method 14 - LZMA)
+ Bit 1: If the compression method used was type 14,
+ LZMA, then this bit, if set, indicates
+ an end-of-stream (EOS) marker is used to
+ mark the end of the compressed data stream.
+ If clear, then an EOS marker is not present
+ and the compressed data size must be known
+ to extract.
+
+ Note: Bits 1 and 2 are undefined if the compression
+ method is any other.
+
+ Bit 3: If this bit is set, the fields crc-32, compressed
+ size and uncompressed size are set to zero in the
+ local header. The correct values are put in the
+ data descriptor immediately following the compressed
+ data. (Note: PKZIP version 2.04g for DOS only
+ recognizes this bit for method 8 compression, newer
+ versions of PKZIP recognize this bit for any
+ compression method.)
+
+ Bit 4: Reserved for use with method 8, for enhanced
+ deflating.
+
+ Bit 5: If this bit is set, this indicates that the file is
+ compressed patched data. (Note: Requires PKZIP
+ version 2.70 or greater)
+
+ Bit 6: Strong encryption. If this bit is set, you MUST
+ set the version needed to extract value to at least
+ 50 and you MUST also set bit 0. If AES encryption
+ is used, the version needed to extract value MUST
+ be at least 51. See the section describing the Strong
+ Encryption Specification for details. Refer to the
+ section in this document entitled "Incorporating PKWARE
+ Proprietary Technology into Your Product" for more
+ information.
+
+ Bit 7: Currently unused.
+
+ Bit 8: Currently unused.
+
+ Bit 9: Currently unused.
+
+ Bit 10: Currently unused.
+
+ Bit 11: Language encoding flag (EFS). If this bit is set,
+ the filename and comment fields for this file
+ MUST be encoded using UTF-8. (see APPENDIX D)
+
+ Bit 12: Reserved by PKWARE for enhanced compression.
+
+ Bit 13: Set when encrypting the Central Directory to indicate
+ selected data values in the Local Header are masked to
+ hide their actual values. See the section describing
+ the Strong Encryption Specification for details. Refer
+ to the section in this document entitled "Incorporating
+ PKWARE Proprietary Technology into Your Product" for
+ more information.
+
+ Bit 14: Reserved by PKWARE for alternate streams.
+
+ Bit 15: Reserved by PKWARE.
+
+ 4.4.5 compression method: (2 bytes)
+
+ 0 - The file is stored (no compression)
+ 1 - The file is Shrunk
+ 2 - The file is Reduced with compression factor 1
+ 3 - The file is Reduced with compression factor 2
+ 4 - The file is Reduced with compression factor 3
+ 5 - The file is Reduced with compression factor 4
+ 6 - The file is Imploded
+ 7 - Reserved for Tokenizing compression algorithm
+ 8 - The file is Deflated
+ 9 - Enhanced Deflating using Deflate64(tm)
+ 10 - PKWARE Data Compression Library Imploding (old IBM TERSE)
+ 11 - Reserved by PKWARE
+ 12 - File is compressed using BZIP2 algorithm
+ 13 - Reserved by PKWARE
+ 14 - LZMA
+ 15 - Reserved by PKWARE
+ 16 - IBM z/OS CMPSC Compression
+ 17 - Reserved by PKWARE
+ 18 - File is compressed using IBM TERSE (new)
+ 19 - IBM LZ77 z Architecture
+ 20 - deprecated (use method 93 for zstd)
+ 93 - Zstandard (zstd) Compression
+ 94 - MP3 Compression
+ 95 - XZ Compression
+ 96 - JPEG variant
+ 97 - WavPack compressed data
+ 98 - PPMd version I, Rev 1
+ 99 - AE-x encryption marker (see APPENDIX E)
+
+ 4.4.5.1 Methods 1-6 are legacy algorithms and are no longer
+ recommended for use when compressing files.
+
+ 4.4.6 date and time fields: (2 bytes each)
+
+ The date and time are encoded in standard MS-DOS format.
+ If input came from standard input, the date and time are
+ those at which compression was started for this data.
+ If encrypting the central directory and general purpose bit
+ flag 13 is set indicating masking, the value stored in the
+ Local Header will be zero. MS-DOS time format is different
+ from more commonly used computer time formats such as
+ UTC. For example, MS-DOS uses year values relative to 1980
+ and 2 second precision.
+
+ 4.4.7 CRC-32: (4 bytes)
+
+ The CRC-32 algorithm was generously contributed by
+ David Schwaderer and can be found in his excellent
+ book "C Programmers Guide to NetBIOS" published by
+ Howard W. Sams & Co. Inc. The 'magic number' for
+ the CRC is 0xdebb20e3. The proper CRC pre and post
+ conditioning is used, meaning that the CRC register
+ is pre-conditioned with all ones (a starting value
+ of 0xffffffff) and the value is post-conditioned by
+ taking the one's complement of the CRC residual.
+ If bit 3 of the general purpose flag is set, this
+ field is set to zero in the local header and the correct
+ value is put in the data descriptor and in the central
+ directory. When encrypting the central directory, if the
+ local header is not in ZIP64 format and general purpose
+ bit flag 13 is set indicating masking, the value stored
+ in the Local Header will be zero.
+
+ 4.4.8 compressed size: (4 bytes)
+ 4.4.9 uncompressed size: (4 bytes)
+
+ The size of the file compressed (4.4.8) and uncompressed,
+ (4.4.9) respectively. When a decryption header is present it
+ will be placed in front of the file data and the value of the
+ compressed file size will include the bytes of the decryption
+ header. If bit 3 of the general purpose bit flag is set,
+ these fields are set to zero in the local header and the
+ correct values are put in the data descriptor and
+ in the central directory. If an archive is in ZIP64 format
+ and the value in this field is 0xFFFFFFFF, the size will be
+ in the corresponding 8 byte ZIP64 extended information
+ extra field. When encrypting the central directory, if the
+ local header is not in ZIP64 format and general purpose bit
+ flag 13 is set indicating masking, the value stored for the
+ uncompressed size in the Local Header will be zero.
+
+ 4.4.10 file name length: (2 bytes)
+ 4.4.11 extra field length: (2 bytes)
+ 4.4.12 file comment length: (2 bytes)
+
+ The length of the file name, extra field, and comment
+ fields respectively. The combined length of any
+ directory record and these three fields SHOULD NOT
+ generally exceed 65,535 bytes. If input came from standard
+ input, the file name length is set to zero.
+
+
+ 4.4.13 disk number start: (2 bytes)
+
+ The number of the disk on which this file begins. If an
+ archive is in ZIP64 format and the value in this field is
+ 0xFFFF, the size will be in the corresponding 4 byte zip64
+ extended information extra field.
+
+ 4.4.14 internal file attributes: (2 bytes)
+
+ Bits 1 and 2 are reserved for use by PKWARE.
+
+ 4.4.14.1 The lowest bit of this field indicates, if set,
+ that the file is apparently an ASCII or text file. If not
+ set, that the file apparently contains binary data.
+ The remaining bits are unused in version 1.0.
+
+ 4.4.14.2 The 0x0002 bit of this field indicates, if set, that
+ a 4 byte variable record length control field precedes each
+ logical record indicating the length of the record. The
+ record length control field is stored in little-endian byte
+ order. This flag is independent of text control characters,
+ and if used in conjunction with text data, includes any
+ control characters in the total length of the record. This
+ value is provided for mainframe data transfer support.
+
+ 4.4.15 external file attributes: (4 bytes)
+
+ The mapping of the external attributes is
+ host-system dependent (see 'version made by'). For
+ MS-DOS, the low order byte is the MS-DOS directory
+ attribute byte. If input came from standard input, this
+ field is set to zero.
+
+ 4.4.16 relative offset of local header: (4 bytes)
+
+ This is the offset from the start of the first disk on
+ which this file appears, to where the local header SHOULD
+ be found. If an archive is in ZIP64 format and the value
+ in this field is 0xFFFFFFFF, the size will be in the
+ corresponding 8 byte zip64 extended information extra field.
+
+ 4.4.17 file name: (Variable)
+
+ 4.4.17.1 The name of the file, with optional relative path.
+ The path stored MUST NOT contain a drive or
+ device letter, or a leading slash. All slashes
+ MUST be forward slashes '/' as opposed to
+ backwards slashes '\' for compatibility with Amiga
+ and UNIX file systems etc. If input came from standard
+ input, there is no file name field.
+
+ 4.4.17.2 If using the Central Directory Encryption Feature and
+ general purpose bit flag 13 is set indicating masking, the file
+ name stored in the Local Header will not be the actual file name.
+ A masking value consisting of a unique hexadecimal value will
+ be stored. This value will be sequentially incremented for each
+ file in the archive. See the section on the Strong Encryption
+ Specification for details on retrieving the encrypted file name.
+ Refer to the section in this document entitled "Incorporating PKWARE
+ Proprietary Technology into Your Product" for more information.
+
+
+ 4.4.18 file comment: (Variable)
+
+ The comment for this file.
+
+ 4.4.19 number of this disk: (2 bytes)
+
+ The number of this disk, which contains central
+ directory end record. If an archive is in ZIP64 format
+ and the value in this field is 0xFFFF, the size will
+ be in the corresponding 4 byte zip64 end of central
+ directory field.
+
+
+ 4.4.20 number of the disk with the start of the central
+ directory: (2 bytes)
+
+ The number of the disk on which the central
+ directory starts. If an archive is in ZIP64 format
+ and the value in this field is 0xFFFF, the size will
+ be in the corresponding 4 byte zip64 end of central
+ directory field.
+
+ 4.4.21 total number of entries in the central dir on
+ this disk: (2 bytes)
+
+ The number of central directory entries on this disk.
+ If an archive is in ZIP64 format and the value in
+ this field is 0xFFFF, the size will be in the
+ corresponding 8 byte zip64 end of central
+ directory field.
+
+ 4.4.22 total number of entries in the central dir: (2 bytes)
+
+ The total number of files in the .ZIP file. If an
+ archive is in ZIP64 format and the value in this field
+ is 0xFFFF, the size will be in the corresponding 8 byte
+ zip64 end of central directory field.
+
+ 4.4.23 size of the central directory: (4 bytes)
+
+ The size (in bytes) of the entire central directory.
+ If an archive is in ZIP64 format and the value in
+ this field is 0xFFFFFFFF, the size will be in the
+ corresponding 8 byte zip64 end of central
+ directory field.
+
+ 4.4.24 offset of start of central directory with respect to
+ the starting disk number: (4 bytes)
+
+ Offset of the start of the central directory on the
+ disk on which the central directory starts. If an
+ archive is in ZIP64 format and the value in this
+ field is 0xFFFFFFFF, the size will be in the
+ corresponding 8 byte zip64 end of central
+ directory field.
+
+ 4.4.25 .ZIP file comment length: (2 bytes)
+
+ The length of the comment for this .ZIP file.
+
+ 4.4.26 .ZIP file comment: (Variable)
+
+ The comment for this .ZIP file. ZIP file comment data
+ is stored unsecured. No encryption or data authentication
+ is applied to this area at this time. Confidential information
+ SHOULD NOT be stored in this section.
+
+ 4.4.27 zip64 extensible data sector (variable size)
+
+ (currently reserved for use by PKWARE)
+
+
+ 4.4.28 extra field: (Variable)
+
+ This SHOULD be used for storage expansion. If additional
+ information needs to be stored within a ZIP file for special
+ application or platform needs, it SHOULD be stored here.
+ Programs supporting earlier versions of this specification can
+ then safely skip the file, and find the next file or header.
+ This field will be 0 length in version 1.0.
+
+ Existing extra fields are defined in the section
+ Extensible data fields that follows.
+
+4.5 Extensible data fields
+--------------------------
+
+ 4.5.1 In order to allow different programs and different types
+ of information to be stored in the 'extra' field in .ZIP
+ files, the following structure MUST be used for all
+ programs storing data in this field:
+
+ header1+data1 + header2+data2 . . .
+
+ Each header MUST consist of:
+
+ Header ID - 2 bytes
+ Data Size - 2 bytes
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ The Header ID field indicates the type of data that is in
+ the following data block.
+
+ Header IDs of 0 thru 31 are reserved for use by PKWARE.
+ The remaining IDs can be used by third party vendors for
+ proprietary usage.
+
+ 4.5.2 The current Header ID mappings defined by PKWARE are:
+
+ 0x0001 Zip64 extended information extra field
+ 0x0007 AV Info
+ 0x0008 Reserved for extended language encoding data (PFS)
+ (see APPENDIX D)
+ 0x0009 OS/2
+ 0x000a NTFS
+ 0x000c OpenVMS
+ 0x000d UNIX
+ 0x000e Reserved for file stream and fork descriptors
+ 0x000f Patch Descriptor
+ 0x0014 PKCS#7 Store for X.509 Certificates
+ 0x0015 X.509 Certificate ID and Signature for
+ individual file
+ 0x0016 X.509 Certificate ID for Central Directory
+ 0x0017 Strong Encryption Header
+ 0x0018 Record Management Controls
+ 0x0019 PKCS#7 Encryption Recipient Certificate List
+ 0x0020 Reserved for Timestamp record
+ 0x0021 Policy Decryption Key Record
+ 0x0022 Smartcrypt Key Provider Record
+ 0x0023 Smartcrypt Policy Key Data Record
+ 0x0065 IBM S/390 (Z390), AS/400 (I400) attributes
+ - uncompressed
+ 0x0066 Reserved for IBM S/390 (Z390), AS/400 (I400)
+ attributes - compressed
+ 0x4690 POSZIP 4690 (reserved)
+
+
+ 4.5.3 -Zip64 Extended Information Extra Field (0x0001):
+
+ The following is the layout of the zip64 extended
+ information "extra" block. If one of the size or
+ offset fields in the Local or Central directory
+ record is too small to hold the required data,
+ a Zip64 extended information record is created.
+ The order of the fields in the zip64 extended
+ information record is fixed, but the fields MUST
+ only appear if the corresponding Local or Central
+ directory record field is set to 0xFFFF or 0xFFFFFFFF.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(ZIP64) 0x0001 2 bytes Tag for this "extra" block type
+ Size 2 bytes Size of this "extra" block
+ Original
+ Size 8 bytes Original uncompressed file size
+ Compressed
+ Size 8 bytes Size of compressed data
+ Relative Header
+ Offset 8 bytes Offset of local header record
+ Disk Start
+ Number 4 bytes Number of the disk on which
+ this file starts
+
+ This entry in the Local header MUST include BOTH original
+ and compressed file size fields. If encrypting the
+ central directory and bit 13 of the general purpose bit
+ flag is set indicating masking, the value stored in the
+ Local Header for the original file size will be zero.
+
+
+ 4.5.4 -OS/2 Extra Field (0x0009):
+
+ The following is the layout of the OS/2 attributes "extra"
+ block. (Last Revision 09/05/95)
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(OS/2) 0x0009 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ BSize 4 bytes Uncompressed Block Size
+ CType 2 bytes Compression type
+ EACRC 4 bytes CRC value for uncompress block
+ (var) variable Compressed block
+
+ The OS/2 extended attribute structure (FEA2LIST) is
+ compressed and then stored in its entirety within this
+ structure. There will only ever be one "block" of data in
+ VarFields[].
+
+ 4.5.5 -NTFS Extra Field (0x000a):
+
+ The following is the layout of the NTFS attributes
+ "extra" block. (Note: At this time the Mtime, Atime
+ and Ctime values MAY be used on any WIN32 system.)
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(NTFS) 0x000a 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of the total "extra" block
+ Reserved 4 bytes Reserved for future use
+ Tag1 2 bytes NTFS attribute tag value #1
+ Size1 2 bytes Size of attribute #1, in bytes
+ (var) Size1 Attribute #1 data
+ .
+ .
+ .
+ TagN 2 bytes NTFS attribute tag value #N
+ SizeN 2 bytes Size of attribute #N, in bytes
+ (var) SizeN Attribute #N data
+
+ For NTFS, values for Tag1 through TagN are as follows:
+ (currently only one set of attributes is defined for NTFS)
+
+ Tag Size Description
+ ----- ---- -----------
+ 0x0001 2 bytes Tag for attribute #1
+ Size1 2 bytes Size of attribute #1, in bytes
+ Mtime 8 bytes File last modification time
+ Atime 8 bytes File last access time
+ Ctime 8 bytes File creation time
+
+ 4.5.6 -OpenVMS Extra Field (0x000c):
+
+ The following is the layout of the OpenVMS attributes
+ "extra" block.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+ (VMS) 0x000c 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of the total "extra" block
+ CRC 4 bytes 32-bit CRC for remainder of the block
+ Tag1 2 bytes OpenVMS attribute tag value #1
+ Size1 2 bytes Size of attribute #1, in bytes
+ (var) Size1 Attribute #1 data
+ .
+ .
+ .
+ TagN 2 bytes OpenVMS attribute tag value #N
+ SizeN 2 bytes Size of attribute #N, in bytes
+ (var) SizeN Attribute #N data
+
+ OpenVMS Extra Field Rules:
+
+ 4.5.6.1. There will be one or more attributes present, which
+ will each be preceded by the above TagX & SizeX values.
+ These values are identical to the ATR$C_XXXX and ATR$S_XXXX
+ constants which are defined in ATR.H under OpenVMS C. Neither
+ of these values will ever be zero.
+
+ 4.5.6.2. No word alignment or padding is performed.
+
+ 4.5.6.3. A well-behaved PKZIP/OpenVMS program SHOULD NOT produce
+ more than one sub-block with the same TagX value. Also, there MUST
+ NOT be more than one "extra" block of type 0x000c in a particular
+ directory record.
+
+ 4.5.7 -UNIX Extra Field (0x000d):
+
+ The following is the layout of the UNIX "extra" block.
+ Note: all fields are stored in Intel low-byte/high-byte
+ order.
+
+ Value Size Description
+ ----- ---- -----------
+(UNIX) 0x000d 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ Atime 4 bytes File last access time
+ Mtime 4 bytes File last modification time
+ Uid 2 bytes File user ID
+ Gid 2 bytes File group ID
+ (var) variable Variable length data field
+
+ The variable length data field will contain file type
+ specific data. Currently the only values allowed are
+ the original "linked to" file names for hard or symbolic
+ links, and the major and minor device node numbers for
+ character and block device nodes. Since device nodes
+ cannot be either symbolic or hard links, only one set of
+ variable length data is stored. Link files will have the
+ name of the original file stored. This name is NOT NULL
+ terminated. Its size can be determined by checking TSize -
+ 12. Device entries will have eight bytes stored as two 4
+ byte entries (in little endian format). The first entry
+ will be the major device number, and the second the minor
+ device number.
+
+ 4.5.8 -PATCH Descriptor Extra Field (0x000f):
+
+ 4.5.8.1 The following is the layout of the Patch Descriptor
+ "extra" block.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(Patch) 0x000f 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of the total "extra" block
+ Version 2 bytes Version of the descriptor
+ Flags 4 bytes Actions and reactions (see below)
+ OldSize 4 bytes Size of the file about to be patched
+ OldCRC 4 bytes 32-bit CRC of the file to be patched
+ NewSize 4 bytes Size of the resulting file
+ NewCRC 4 bytes 32-bit CRC of the resulting file
+
+ 4.5.8.2 Actions and reactions
+
+ Bits Description
+ ---- ----------------
+ 0 Use for auto detection
+ 1 Treat as a self-patch
+ 2-3 RESERVED
+ 4-5 Action (see below)
+ 6-7 RESERVED
+ 8-9 Reaction (see below) to absent file
+ 10-11 Reaction (see below) to newer file
+ 12-13 Reaction (see below) to unknown file
+ 14-15 RESERVED
+ 16-31 RESERVED
+
+ 4.5.8.2.1 Actions
+
+ Action Value
+ ------ -----
+ none 0
+ add 1
+ delete 2
+ patch 3
+
+ 4.5.8.2.2 Reactions
+
+ Reaction Value
+ -------- -----
+ ask 0
+ skip 1
+ ignore 2
+ fail 3
+
+ 4.5.8.3 Patch support is provided by PKPatchMaker(tm) technology
+ and is covered under U.S. Patents and Patents Pending. The use or
+ implementation in a product of certain technological aspects set
+ forth in the current APPNOTE, including those with regard to
+ strong encryption or patching requires a license from PKWARE.
+ Refer to the section in this document entitled "Incorporating
+ PKWARE Proprietary Technology into Your Product" for more
+ information.
+
+ 4.5.9 -PKCS#7 Store for X.509 Certificates (0x0014):
+
+ This field MUST contain information about each of the certificates
+ files MAY be signed with. When the Central Directory Encryption
+ feature is enabled for a ZIP file, this record will appear in
+ the Archive Extra Data Record, otherwise it will appear in the
+ first central directory record and will be ignored in any
+ other record.
+
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(Store) 0x0014 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of the store data
+ TData TSize Data about the store
+
+
+ 4.5.10 -X.509 Certificate ID and Signature for individual file (0x0015):
+
+ This field contains the information about which certificate in
+ the PKCS#7 store was used to sign a particular file. It also
+ contains the signature data. This field can appear multiple
+ times, but can only appear once per certificate.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(CID) 0x0015 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of data that follows
+ TData TSize Signature Data
+
+ 4.5.11 -X.509 Certificate ID and Signature for central directory (0x0016):
+
+ This field contains the information about which certificate in
+ the PKCS#7 store was used to sign the central directory structure.
+ When the Central Directory Encryption feature is enabled for a
+ ZIP file, this record will appear in the Archive Extra Data Record,
+ otherwise it will appear in the first central directory record.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(CDID) 0x0016 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of data that follows
+ TData TSize Data
+
+ 4.5.12 -Strong Encryption Header (0x0017):
+
+ Value Size Description
+ ----- ---- -----------
+ 0x0017 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of data that follows
+ Format 2 bytes Format definition for this record
+ AlgID 2 bytes Encryption algorithm identifier
+ Bitlen 2 bytes Bit length of encryption key
+ Flags 2 bytes Processing flags
+ CertData TSize-8 Certificate decryption extra field data
+ (refer to the explanation for CertData
+ in the section describing the
+ Certificate Processing Method under
+ the Strong Encryption Specification)
+
+ See the section describing the Strong Encryption Specification
+ for details. Refer to the section in this document entitled
+ "Incorporating PKWARE Proprietary Technology into Your Product"
+ for more information.
+
+ 4.5.13 -Record Management Controls (0x0018):
+
+ Value Size Description
+ ----- ---- -----------
+(Rec-CTL) 0x0018 2 bytes Tag for this "extra" block type
+ CSize 2 bytes Size of total extra block data
+ Tag1 2 bytes Record control attribute 1
+ Size1 2 bytes Size of attribute 1, in bytes
+ Data1 Size1 Attribute 1 data
+ .
+ .
+ .
+ TagN 2 bytes Record control attribute N
+ SizeN 2 bytes Size of attribute N, in bytes
+ DataN SizeN Attribute N data
+
+
+ 4.5.14 -PKCS#7 Encryption Recipient Certificate List (0x0019):
+
+ This field MAY contain information about each of the certificates
+ used in encryption processing and it can be used to identify who is
+ allowed to decrypt encrypted files. This field SHOULD only appear
+ in the archive extra data record. This field is not required and
+ serves only to aid archive modifications by preserving public
+ encryption key data. Individual security requirements may dictate
+ that this data be omitted to deter information exposure.
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+(CStore) 0x0019 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size of the store data
+ TData TSize Data about the store
+
+ TData:
+
+ Value Size Description
+ ----- ---- -----------
+ Version 2 bytes Format version number - MUST be 0x0001 at this time
+ CStore (var) PKCS#7 data blob
+
+ See the section describing the Strong Encryption Specification
+ for details. Refer to the section in this document entitled
+ "Incorporating PKWARE Proprietary Technology into Your Product"
+ for more information.
+
+ 4.5.15 -MVS Extra Field (0x0065):
+
+ The following is the layout of the MVS "extra" block.
+ Note: Some fields are stored in Big Endian format.
+ All text is in EBCDIC format unless otherwise specified.
+Value Size Description
+ ----- ---- -----------
+(MVS) 0x0065 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ ID 4 bytes EBCDIC "Z390" 0xE9F3F9F0 or
+ "T4MV" for TargetFour
+ (var) TSize-4 Attribute data (see APPENDIX B)
+
+
+ 4.5.16 -OS/400 Extra Field (0x0065):
+
+ The following is the layout of the OS/400 "extra" block.
+ Note: Some fields are stored in Big Endian format.
+ All text is in EBCDIC format unless otherwise specified.
+
+ Value Size Description
+ ----- ---- -----------
+(OS400) 0x0065 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ ID 4 bytes EBCDIC "I400" 0xC9F4F0F0 or
+ "T4MV" for TargetFour
+ (var) TSize-4 Attribute data (see APPENDIX A)
+
+ 4.5.17 -Policy Decryption Key Record Extra Field (0x0021):
+
+ The following is the layout of the Policy Decryption Key "extra" block.
+ TData is a variable length, variable content field. It holds
+ information about encryptions and/or encryption key sources.
+ Contact PKWARE for information on current TData structures.
+ Information in this "extra" block may aternatively be placed
+ within comment fields. Refer to the section in this document
+ entitled "Incorporating PKWARE Proprietary Technology into Your
+ Product" for more information.
+
+ Value Size Description
+ ----- ---- -----------
+ 0x0021 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ TData TSize Data about the key
+
+ 4.5.18 -Key Provider Record Extra Field (0x0022):
+
+ The following is the layout of the Key Provider "extra" block.
+ TData is a variable length, variable content field. It holds
+ information about encryptions and/or encryption key sources.
+ Contact PKWARE for information on current TData structures.
+ Information in this "extra" block may aternatively be placed
+ within comment fields. Refer to the section in this document
+ entitled "Incorporating PKWARE Proprietary Technology into Your
+ Product" for more information.
+
+ Value Size Description
+ ----- ---- -----------
+ 0x0022 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ TData TSize Data about the key
+
+ 4.5.19 -Policy Key Data Record Record Extra Field (0x0023):
+
+ The following is the layout of the Policy Key Data "extra" block.
+ TData is a variable length, variable content field. It holds
+ information about encryptions and/or encryption key sources.
+ Contact PKWARE for information on current TData structures.
+ Information in this "extra" block may aternatively be placed
+ within comment fields. Refer to the section in this document
+ entitled "Incorporating PKWARE Proprietary Technology into Your
+ Product" for more information.
+
+ Value Size Description
+ ----- ---- -----------
+ 0x0023 2 bytes Tag for this "extra" block type
+ TSize 2 bytes Size for the following data block
+ TData TSize Data about the key
+
+4.6 Third Party Mappings
+------------------------
+
+ 4.6.1 Third party mappings commonly used are:
+
+ 0x07c8 Macintosh
+ 0x1986 Pixar USD header ID
+ 0x2605 ZipIt Macintosh
+ 0x2705 ZipIt Macintosh 1.3.5+
+ 0x2805 ZipIt Macintosh 1.3.5+
+ 0x334d Info-ZIP Macintosh
+ 0x4154 Tandem
+ 0x4341 Acorn/SparkFS
+ 0x4453 Windows NT security descriptor (binary ACL)
+ 0x4704 VM/CMS
+ 0x470f MVS
+ 0x4854 THEOS (old?)
+ 0x4b46 FWKCS MD5 (see below)
+ 0x4c41 OS/2 access control list (text ACL)
+ 0x4d49 Info-ZIP OpenVMS
+ 0x4d63 Macintosh Smartzip (??)
+ 0x4f4c Xceed original location extra field
+ 0x5356 AOS/VS (ACL)
+ 0x5455 extended timestamp
+ 0x554e Xceed unicode extra field
+ 0x5855 Info-ZIP UNIX (original, also OS/2, NT, etc)
+ 0x6375 Info-ZIP Unicode Comment Extra Field
+ 0x6542 BeOS/BeBox
+ 0x6854 THEOS
+ 0x7075 Info-ZIP Unicode Path Extra Field
+ 0x7441 AtheOS/Syllable
+ 0x756e ASi UNIX
+ 0x7855 Info-ZIP UNIX (new)
+ 0x7875 Info-ZIP UNIX (newer UID/GID)
+ 0xa11e Data Stream Alignment (Apache Commons-Compress)
+ 0xa220 Microsoft Open Packaging Growth Hint
+ 0xcafe Java JAR file Extra Field Header ID
+ 0xd935 Android ZIP Alignment Extra Field
+ 0xe57a Korean ZIP code page info
+ 0xfd4a SMS/QDOS
+ 0x9901 AE-x encryption structure (see APPENDIX E)
+ 0x9902 unknown
+
+
+ Detailed descriptions of Extra Fields defined by third
+ party mappings will be documented as information on
+ these data structures is made available to PKWARE.
+ PKWARE does not guarantee the accuracy of any published
+ third party data.
+
+ 4.6.2 Third-party Extra Fields MUST include a Header ID using
+ the format defined in the section of this document
+ titled Extensible Data Fields (section 4.5).
+
+ The Data Size field indicates the size of the following
+ data block. Programs can use this value to skip to the
+ next header block, passing over any data blocks that are
+ not of interest.
+
+ Note: As stated above, the size of the entire .ZIP file
+ header, including the file name, comment, and extra
+ field SHOULD NOT exceed 64K in size.
+
+ 4.6.3 In case two different programs appropriate the same
+ Header ID value, it is strongly recommended that each
+ program SHOULD place a unique signature of at least two bytes in
+ size (and preferably 4 bytes or bigger) at the start of
+ each data area. Every program SHOULD verify that its
+ unique signature is present, in addition to the Header ID
+ value being correct, before assuming that it is a block of
+ known type.
+
+ Third-party Mappings:
+ Not all third-party extra field mappings are documented here.
+
+ 4.6.4 -ZipIt Macintosh Extra Field (long) (0x2605):
+
+ The following is the layout of the ZipIt extra block
+ for Macintosh. The local-header and central-header versions
+ are identical. This block MUST be present if the file is
+ stored MacBinary-encoded and it SHOULD NOT be used if the file
+ is not stored MacBinary-encoded.
+
+ Value Size Description
+ ----- ---- -----------
+ (Mac2) 0x2605 Short tag for this extra block type
+ TSize Short total data size for this block
+ "ZPIT" beLong extra-field signature
+ FnLen Byte length of FileName
+ FileName variable full Macintosh filename
+ FileType Byte[4] four-byte Mac file type string
+ Creator Byte[4] four-byte Mac creator string
+
+
+ 4.6.5 -ZipIt Macintosh Extra Field (short, for files) (0x2705):
+
+ The following is the layout of a shortened variant of the
+ ZipIt extra block for Macintosh (without "full name" entry).
+ This variant is used by ZipIt 1.3.5 and newer for entries of
+ files (not directories) that do not have a MacBinary encoded
+ file. The local-header and central-header versions are identical.
+
+ Value Size Description
+ ----- ---- -----------
+ (Mac2b) 0x2705 Short tag for this extra block type
+ TSize Short total data size for this block (12)
+ "ZPIT" beLong extra-field signature
+ FileType Byte[4] four-byte Mac file type string
+ Creator Byte[4] four-byte Mac creator string
+ fdFlags beShort attributes from FInfo.frFlags,
+ MAY be omitted
+ 0x0000 beShort reserved, MAY be omitted
+
+
+ 4.6.6 -ZipIt Macintosh Extra Field (short, for directories) (0x2805):
+
+ The following is the layout of a shortened variant of the
+ ZipIt extra block for Macintosh used only for directory
+ entries. This variant is used by ZipIt 1.3.5 and newer to
+ save some optional Mac-specific information about directories.
+ The local-header and central-header versions are identical.
+
+ Value Size Description
+ ----- ---- -----------
+ (Mac2c) 0x2805 Short tag for this extra block type
+ TSize Short total data size for this block (12)
+ "ZPIT" beLong extra-field signature
+ frFlags beShort attributes from DInfo.frFlags, MAY
+ be omitted
+ View beShort ZipIt view flag, MAY be omitted
+
+
+ The View field specifies ZipIt-internal settings as follows:
+
+ Bits of the Flags:
+ bit 0 if set, the folder is shown expanded (open)
+ when the archive contents are viewed in ZipIt.
+ bits 1-15 reserved, zero;
+
+
+ 4.6.7 -FWKCS MD5 Extra Field (0x4b46):
+
+ The FWKCS Contents_Signature System, used in
+ automatically identifying files independent of file name,
+ optionally adds and uses an extra field to support the
+ rapid creation of an enhanced contents_signature:
+
+ Header ID = 0x4b46
+ Data Size = 0x0013
+ Preface = 'M','D','5'
+ followed by 16 bytes containing the uncompressed file's
+ 128_bit MD5 hash(1), low byte first.
+
+ When FWKCS revises a .ZIP file central directory to add
+ this extra field for a file, it also replaces the
+ central directory entry for that file's uncompressed
+ file length with a measured value.
+
+ FWKCS provides an option to strip this extra field, if
+ present, from a .ZIP file central directory. In adding
+ this extra field, FWKCS preserves .ZIP file Authenticity
+ Verification; if stripping this extra field, FWKCS
+ preserves all versions of AV through PKZIP version 2.04g.
+
+ FWKCS, and FWKCS Contents_Signature System, are
+ trademarks of Frederick W. Kantor.
+
+ (1) R. Rivest, RFC1321.TXT, MIT Laboratory for Computer
+ Science and RSA Data Security, Inc., April 1992.
+ ll.76-77: "The MD5 algorithm is being placed in the
+ public domain for review and possible adoption as a
+ standard."
+
+
+ 4.6.8 -Info-ZIP Unicode Comment Extra Field (0x6375):
+
+ Stores the UTF-8 version of the file comment as stored in the
+ central directory header. (Last Revision 20070912)
+
+ Value Size Description
+ ----- ---- -----------
+ (UCom) 0x6375 Short tag for this extra block type ("uc")
+ TSize Short total data size for this block
+ Version 1 byte version of this extra field, currently 1
+ ComCRC32 4 bytes Comment Field CRC32 Checksum
+ UnicodeCom Variable UTF-8 version of the entry comment
+
+ Currently Version is set to the number 1. If there is a need
+ to change this field, the version will be incremented. Changes
+ MAY NOT be backward compatible so this extra field SHOULD NOT be
+ used if the version is not recognized.
+
+ The ComCRC32 is the standard zip CRC32 checksum of the File Comment
+ field in the central directory header. This is used to verify that
+ the comment field has not changed since the Unicode Comment extra field
+ was created. This can happen if a utility changes the File Comment
+ field but does not update the UTF-8 Comment extra field. If the CRC
+ check fails, this Unicode Comment extra field SHOULD be ignored and
+ the File Comment field in the header SHOULD be used instead.
+
+ The UnicodeCom field is the UTF-8 version of the File Comment field
+ in the header. As UnicodeCom is defined to be UTF-8, no UTF-8 byte
+ order mark (BOM) is used. The length of this field is determined by
+ subtracting the size of the previous fields from TSize. If both the
+ File Name and Comment fields are UTF-8, the new General Purpose Bit
+ Flag, bit 11 (Language encoding flag (EFS)), can be used to indicate
+ both the header File Name and Comment fields are UTF-8 and, in this
+ case, the Unicode Path and Unicode Comment extra fields are not
+ needed and SHOULD NOT be created. Note that, for backward
+ compatibility, bit 11 SHOULD only be used if the native character set
+ of the paths and comments being zipped up are already in UTF-8. It is
+ expected that the same file comment storage method, either general
+ purpose bit 11 or extra fields, be used in both the Local and Central
+ Directory Header for a file.
+
+
+ 4.6.9 -Info-ZIP Unicode Path Extra Field (0x7075):
+
+ Stores the UTF-8 version of the file name field as stored in the
+ local header and central directory header. (Last Revision 20070912)
+
+ Value Size Description
+ ----- ---- -----------
+ (UPath) 0x7075 Short tag for this extra block type ("up")
+ TSize Short total data size for this block
+ Version 1 byte version of this extra field, currently 1
+ NameCRC32 4 bytes File Name Field CRC32 Checksum
+ UnicodeName Variable UTF-8 version of the entry File Name
+
+ Currently Version is set to the number 1. If there is a need
+ to change this field, the version will be incremented. Changes
+ MAY NOT be backward compatible so this extra field SHOULD NOT be
+ used if the version is not recognized.
+
+ The NameCRC32 is the standard zip CRC32 checksum of the File Name
+ field in the header. This is used to verify that the header
+ File Name field has not changed since the Unicode Path extra field
+ was created. This can happen if a utility renames the File Name but
+ does not update the UTF-8 path extra field. If the CRC check fails,
+ this UTF-8 Path Extra Field SHOULD be ignored and the File Name field
+ in the header SHOULD be used instead.
+
+ The UnicodeName is the UTF-8 version of the contents of the File Name
+ field in the header. As UnicodeName is defined to be UTF-8, no UTF-8
+ byte order mark (BOM) is used. The length of this field is determined
+ by subtracting the size of the previous fields from TSize. If both
+ the File Name and Comment fields are UTF-8, the new General Purpose
+ Bit Flag, bit 11 (Language encoding flag (EFS)), can be used to
+ indicate that both the header File Name and Comment fields are UTF-8
+ and, in this case, the Unicode Path and Unicode Comment extra fields
+ are not needed and SHOULD NOT be created. Note that, for backward
+ compatibility, bit 11 SHOULD only be used if the native character set
+ of the paths and comments being zipped up are already in UTF-8. It is
+ expected that the same file name storage method, either general
+ purpose bit 11 or extra fields, be used in both the Local and Central
+ Directory Header for a file.
+
+
+ 4.6.10 -Microsoft Open Packaging Growth Hint (0xa220):
+
+ Value Size Description
+ ----- ---- -----------
+ 0xa220 Short tag for this extra block type
+ TSize Short size of Sig + PadVal + Padding
+ Sig Short verification signature (A028)
+ PadVal Short Initial padding value
+ Padding variable filled with NULL characters
+
+ 4.6.11 -Data Stream Alignment (Apache Commons-Compress) (0xa11e):
+
+ (per Zbynek Vyskovsky) Defines alignment of data stream of this
+ entry within the zip archive. Additionally, indicates whether the
+ compression method should be kept when re-compressing the zip file.
+
+ The purpose of this extra field is to align specific resources to
+ word or page boundaries so they can be easily mapped into memory.
+
+ Value Size Description
+ ----- ---- -----------
+ 0xa11e Short tag for this extra block type
+ TSize Short total data size for this block (2+padding)
+ alignment Short required alignment and indicator
+ 0x00 Variable padding
+
+ The alignment field (lower 15 bits) defines the minimal alignment
+ required by the data stream. Bit 15 of alignment field indicates
+ whether the compression method of this entry can be changed when
+ recompressing the zip file. The value 0 means the compression method
+ should not be changed. The value 1 indicates the compression method
+ may be changed. The padding field contains padding to ensure the correct
+ alignment. It can be changed at any time when the offset or required
+ alignment changes. (see https://issues.apache.org/jira/browse/COMPRESS-391)
+
+
+4.7 Manifest Files
+------------------
+
+ 4.7.1 Applications using ZIP files MAY have a need for additional
+ information that MUST be included with the files placed into
+ a ZIP file. Application specific information that cannot be
+ stored using the defined ZIP storage records SHOULD be stored
+ using the extensible Extra Field convention defined in this
+ document. However, some applications MAY use a manifest
+ file as a means for storing additional information. One
+ example is the META-INF/MANIFEST.MF file used in ZIP formatted
+ files having the .JAR extension (JAR files).
+
+ 4.7.2 A manifest file is a file created for the application process
+ that requires this information. A manifest file MAY be of any
+ file type required by the defining application process. It is
+ placed within the same ZIP file as files to which this information
+ applies. By convention, this file is typically the first file placed
+ into the ZIP file and it MAY include a defined directory path.
+
+ 4.7.3 Manifest files MAY be compressed or encrypted as needed for
+ application processing of the files inside the ZIP files.
+
+ Manifest files are outside of the scope of this specification.
+
+
+5.0 Explanation of compression methods
+--------------------------------------
+
+
+5.1 UnShrinking - Method 1
+--------------------------
+
+ 5.1.1 Shrinking is a Dynamic Ziv-Lempel-Welch compression algorithm
+ with partial clearing. The initial code size is 9 bits, and the
+ maximum code size is 13 bits. Shrinking differs from conventional
+ Dynamic Ziv-Lempel-Welch implementations in several respects:
+
+ 5.1.2 The code size is controlled by the compressor, and is
+ not automatically increased when codes larger than the current
+ code size are created (but not necessarily used). When
+ the decompressor encounters the code sequence 256
+ (decimal) followed by 1, it SHOULD increase the code size
+ read from the input stream to the next bit size. No
+ blocking of the codes is performed, so the next code at
+ the increased size SHOULD be read from the input stream
+ immediately after where the previous code at the smaller
+ bit size was read. Again, the decompressor SHOULD NOT
+ increase the code size used until the sequence 256,1 is
+ encountered.
+
+ 5.1.3 When the table becomes full, total clearing is not
+ performed. Rather, when the compressor emits the code
+ sequence 256,2 (decimal), the decompressor SHOULD clear
+ all leaf nodes from the Ziv-Lempel tree, and continue to
+ use the current code size. The nodes that are cleared
+ from the Ziv-Lempel tree are then re-used, with the lowest
+ code value re-used first, and the highest code value
+ re-used last. The compressor can emit the sequence 256,2
+ at any time.
+
+5.2 Expanding - Methods 2-5
+---------------------------
+
+ 5.2.1 The Reducing algorithm is actually a combination of two
+ distinct algorithms. The first algorithm compresses repeated
+ byte sequences, and the second algorithm takes the compressed
+ stream from the first algorithm and applies a probabilistic
+ compression method.
+
+ 5.2.2 The probabilistic compression stores an array of 'follower
+ sets' S(j), for j=0 to 255, corresponding to each possible
+ ASCII character. Each set contains between 0 and 32
+ characters, to be denoted as S(j)[0],...,S(j)[m], where m<32.
+ The sets are stored at the beginning of the data area for a
+ Reduced file, in reverse order, with S(255) first, and S(0)
+ last.
+
+ 5.2.3 The sets are encoded as { N(j), S(j)[0],...,S(j)[N(j)-1] },
+ where N(j) is the size of set S(j). N(j) can be 0, in which
+ case the follower set for S(j) is empty. Each N(j) value is
+ encoded in 6 bits, followed by N(j) eight bit character values
+ corresponding to S(j)[0] to S(j)[N(j)-1] respectively. If
+ N(j) is 0, then no values for S(j) are stored, and the value
+ for N(j-1) immediately follows.
+
+ 5.2.4 Immediately after the follower sets, is the compressed data
+ stream. The compressed data stream can be interpreted for the
+ probabilistic decompression as follows:
+
+ let Last-Character <- 0.
+ loop until done
+ if the follower set S(Last-Character) is empty then
+ read 8 bits from the input stream, and copy this
+ value to the output stream.
+ otherwise if the follower set S(Last-Character) is non-empty then
+ read 1 bit from the input stream.
+ if this bit is not zero then
+ read 8 bits from the input stream, and copy this
+ value to the output stream.
+ otherwise if this bit is zero then
+ read B(N(Last-Character)) bits from the input
+ stream, and assign this value to I.
+ Copy the value of S(Last-Character)[I] to the
+ output stream.
+
+ assign the last value placed on the output stream to
+ Last-Character.
+ end loop
+
+ B(N(j)) is defined as the minimal number of bits required to
+ encode the value N(j)-1.
+
+ 5.2.5 The decompressed stream from above can then be expanded to
+ re-create the original file as follows:
+
+ let State <- 0.
+
+ loop until done
+ read 8 bits from the input stream into C.
+ case State of
+ 0: if C is not equal to DLE (144 decimal) then
+ copy C to the output stream.
+ otherwise if C is equal to DLE then
+ let State <- 1.
+
+ 1: if C is non-zero then
+ let V <- C.
+ let Len <- L(V)
+ let State <- F(Len).
+ otherwise if C is zero then
+ copy the value 144 (decimal) to the output stream.
+ let State <- 0
+
+ 2: let Len <- Len + C
+ let State <- 3.
+
+ 3: move backwards D(V,C) bytes in the output stream
+ (if this position is before the start of the output
+ stream, then assume that all the data before the
+ start of the output stream is filled with zeros).
+ copy Len+3 bytes from this position to the output stream.
+ let State <- 0.
+ end case
+ end loop
+
+ The functions F,L, and D are dependent on the 'compression
+ factor', 1 through 4, and are defined as follows:
+
+ For compression factor 1:
+ L(X) equals the lower 7 bits of X.
+ F(X) equals 2 if X equals 127 otherwise F(X) equals 3.
+ D(X,Y) equals the (upper 1 bit of X) * 256 + Y + 1.
+ For compression factor 2:
+ L(X) equals the lower 6 bits of X.
+ F(X) equals 2 if X equals 63 otherwise F(X) equals 3.
+ D(X,Y) equals the (upper 2 bits of X) * 256 + Y + 1.
+ For compression factor 3:
+ L(X) equals the lower 5 bits of X.
+ F(X) equals 2 if X equals 31 otherwise F(X) equals 3.
+ D(X,Y) equals the (upper 3 bits of X) * 256 + Y + 1.
+ For compression factor 4:
+ L(X) equals the lower 4 bits of X.
+ F(X) equals 2 if X equals 15 otherwise F(X) equals 3.
+ D(X,Y) equals the (upper 4 bits of X) * 256 + Y + 1.
+
+5.3 Imploding - Method 6
+------------------------
+
+ 5.3.1 The Imploding algorithm is actually a combination of two
+ distinct algorithms. The first algorithm compresses repeated byte
+ sequences using a sliding dictionary. The second algorithm is
+ used to compress the encoding of the sliding dictionary output,
+ using multiple Shannon-Fano trees.
+
+ 5.3.2 The Imploding algorithm can use a 4K or 8K sliding dictionary
+ size. The dictionary size used can be determined by bit 1 in the
+ general purpose flag word; a 0 bit indicates a 4K dictionary
+ while a 1 bit indicates an 8K dictionary.
+
+ 5.3.3 The Shannon-Fano trees are stored at the start of the
+ compressed file. The number of trees stored is defined by bit 2 in
+ the general purpose flag word; a 0 bit indicates two trees stored,
+ a 1 bit indicates three trees are stored. If 3 trees are stored,
+ the first Shannon-Fano tree represents the encoding of the
+ Literal characters, the second tree represents the encoding of
+ the Length information, the third represents the encoding of the
+ Distance information. When 2 Shannon-Fano trees are stored, the
+ Length tree is stored first, followed by the Distance tree.
+
+ 5.3.4 The Literal Shannon-Fano tree, if present is used to represent
+ the entire ASCII character set, and contains 256 values. This
+ tree is used to compress any data not compressed by the sliding
+ dictionary algorithm. When this tree is present, the Minimum
+ Match Length for the sliding dictionary is 3. If this tree is
+ not present, the Minimum Match Length is 2.
+
+ 5.3.5 The Length Shannon-Fano tree is used to compress the Length
+ part of the (length,distance) pairs from the sliding dictionary
+ output. The Length tree contains 64 values, ranging from the
+ Minimum Match Length, to 63 plus the Minimum Match Length.
+
+ 5.3.6 The Distance Shannon-Fano tree is used to compress the Distance
+ part of the (length,distance) pairs from the sliding dictionary
+ output. The Distance tree contains 64 values, ranging from 0 to
+ 63, representing the upper 6 bits of the distance value. The
+ distance values themselves will be between 0 and the sliding
+ dictionary size, either 4K or 8K.
+
+ 5.3.7 The Shannon-Fano trees themselves are stored in a compressed
+ format. The first byte of the tree data represents the number of
+ bytes of data representing the (compressed) Shannon-Fano tree
+ minus 1. The remaining bytes represent the Shannon-Fano tree
+ data encoded as:
+
+ High 4 bits: Number of values at this bit length + 1. (1 - 16)
+ Low 4 bits: Bit Length needed to represent value + 1. (1 - 16)
+
+ 5.3.8 The Shannon-Fano codes can be constructed from the bit lengths
+ using the following algorithm:
+
+ 1) Sort the Bit Lengths in ascending order, while retaining the
+ order of the original lengths stored in the file.
+
+ 2) Generate the Shannon-Fano trees:
+
+ Code <- 0
+ CodeIncrement <- 0
+ LastBitLength <- 0
+ i <- number of Shannon-Fano codes - 1 (either 255 or 63)
+
+ loop while i >= 0
+ Code = Code + CodeIncrement
+ if BitLength(i) <> LastBitLength then
+ LastBitLength=BitLength(i)
+ CodeIncrement = 1 shifted left (16 - LastBitLength)
+ ShannonCode(i) = Code
+ i <- i - 1
+ end loop
+
+ 3) Reverse the order of all the bits in the above ShannonCode()
+ vector, so that the most significant bit becomes the least
+ significant bit. For example, the value 0x1234 (hex) would
+ become 0x2C48 (hex).
+
+ 4) Restore the order of Shannon-Fano codes as originally stored
+ within the file.
+
+ Example:
+
+ This example will show the encoding of a Shannon-Fano tree
+ of size 8. Notice that the actual Shannon-Fano trees used
+ for Imploding are either 64 or 256 entries in size.
+
+ Example: 0x02, 0x42, 0x01, 0x13
+
+ The first byte indicates 3 values in this table. Decoding the
+ bytes:
+ 0x42 = 5 codes of 3 bits long
+ 0x01 = 1 code of 2 bits long
+ 0x13 = 2 codes of 4 bits long
+
+ This would generate the original bit length array of:
+ (3, 3, 3, 3, 3, 2, 4, 4)
+
+ There are 8 codes in this table for the values 0 thru 7. Using
+ the algorithm to obtain the Shannon-Fano codes produces:
+
+ Reversed Order Original
+ Val Sorted Constructed Code Value Restored Length
+ --- ------ ----------------- -------- -------- ------
+ 0: 2 1100000000000000 11 101 3
+ 1: 3 1010000000000000 101 001 3
+ 2: 3 1000000000000000 001 110 3
+ 3: 3 0110000000000000 110 010 3
+ 4: 3 0100000000000000 010 100 3
+ 5: 3 0010000000000000 100 11 2
+ 6: 4 0001000000000000 1000 1000 4
+ 7: 4 0000000000000000 0000 0000 4
+
+ The values in the Val, Order Restored and Original Length columns
+ now represent the Shannon-Fano encoding tree that can be used for
+ decoding the Shannon-Fano encoded data. How to parse the
+ variable length Shannon-Fano values from the data stream is beyond
+ the scope of this document. (See the references listed at the end of
+ this document for more information.) However, traditional decoding
+ schemes used for Huffman variable length decoding, such as the
+ Greenlaw algorithm, can be successfully applied.
+
+ 5.3.9 The compressed data stream begins immediately after the
+ compressed Shannon-Fano data. The compressed data stream can be
+ interpreted as follows:
+
+ loop until done
+ read 1 bit from input stream.
+
+ if this bit is non-zero then (encoded data is literal data)
+ if Literal Shannon-Fano tree is present
+ read and decode character using Literal Shannon-Fano tree.
+ otherwise
+ read 8 bits from input stream.
+ copy character to the output stream.
+ otherwise (encoded data is sliding dictionary match)
+ if 8K dictionary size
+ read 7 bits for offset Distance (lower 7 bits of offset).
+ otherwise
+ read 6 bits for offset Distance (lower 6 bits of offset).
+
+ using the Distance Shannon-Fano tree, read and decode the
+ upper 6 bits of the Distance value.
+
+ using the Length Shannon-Fano tree, read and decode
+ the Length value.
+
+ Length <- Length + Minimum Match Length
+
+ if Length = 63 + Minimum Match Length
+ read 8 bits from the input stream,
+ add this value to Length.
+
+ move backwards Distance+1 bytes in the output stream, and
+ copy Length characters from this position to the output
+ stream. (if this position is before the start of the output
+ stream, then assume that all the data before the start of
+ the output stream is filled with zeros).
+ end loop
+
+5.4 Tokenizing - Method 7
+-------------------------
+
+ 5.4.1 This method is not used by PKZIP.
+
+5.5 Deflating - Method 8
+------------------------
+
+ 5.5.1 The Deflate algorithm is similar to the Implode algorithm using
+ a sliding dictionary of up to 32K with secondary compression
+ from Huffman/Shannon-Fano codes.
+
+ 5.5.2 The compressed data is stored in blocks with a header describing
+ the block and the Huffman codes used in the data block. The header
+ format is as follows:
+
+ Bit 0: Last Block bit This bit is set to 1 if this is the last
+ compressed block in the data.
+ Bits 1-2: Block type
+ 00 (0) - Block is stored - All stored data is byte aligned.
+ Skip bits until next byte, then next word = block
+ length, followed by the ones compliment of the block
+ length word. Remaining data in block is the stored
+ data.
+
+ 01 (1) - Use fixed Huffman codes for literal and distance codes.
+ Lit Code Bits Dist Code Bits
+ --------- ---- --------- ----
+ 0 - 143 8 0 - 31 5
+ 144 - 255 9
+ 256 - 279 7
+ 280 - 287 8
+
+ Literal codes 286-287 and distance codes 30-31 are
+ never used but participate in the huffman construction.
+
+ 10 (2) - Dynamic Huffman codes. (See expanding Huffman codes)
+
+ 11 (3) - Reserved - Flag a "Error in compressed data" if seen.
+
+ 5.5.3 Expanding Huffman Codes
+
+ If the data block is stored with dynamic Huffman codes, the Huffman
+ codes are sent in the following compressed format:
+
+ 5 Bits: # of Literal codes sent - 256 (256 - 286)
+ All other codes are never sent.
+ 5 Bits: # of Dist codes - 1 (1 - 32)
+ 4 Bits: # of Bit Length codes - 3 (3 - 19)
+
+ The Huffman codes are sent as bit lengths and the codes are built as
+ described in the implode algorithm. The bit lengths themselves are
+ compressed with Huffman codes. There are 19 bit length codes:
+
+ 0 - 15: Represent bit lengths of 0 - 15
+ 16: Copy the previous bit length 3 - 6 times.
+ The next 2 bits indicate repeat length (0 = 3, ... ,3 = 6)
+ Example: Codes 8, 16 (+2 bits 11), 16 (+2 bits 10) will
+ expand to 12 bit lengths of 8 (1 + 6 + 5)
+ 17: Repeat a bit length of 0 for 3 - 10 times. (3 bits of length)
+ 18: Repeat a bit length of 0 for 11 - 138 times (7 bits of length)
+
+ The lengths of the bit length codes are sent packed 3 bits per value
+ (0 - 7) in the following order:
+
+ 16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
+
+ The Huffman codes SHOULD be built as described in the Implode algorithm
+ except codes are assigned starting at the shortest bit length, i.e. the
+ shortest code SHOULD be all 0's rather than all 1's. Also, codes with
+ a bit length of zero do not participate in the tree construction. The
+ codes are then used to decode the bit lengths for the literal and
+ distance tables.
+
+ The bit lengths for the literal tables are sent first with the number
+ of entries sent described by the 5 bits sent earlier. There are up
+ to 286 literal characters; the first 256 represent the respective 8
+ bit character, code 256 represents the End-Of-Block code, the remaining
+ 29 codes represent copy lengths of 3 thru 258. There are up to 30
+ distance codes representing distances from 1 thru 32k as described
+ below.
+
+ Length Codes
+ ------------
+ Extra Extra Extra Extra
+ Code Bits Length Code Bits Lengths Code Bits Lengths Code Bits Length(s)
+ ---- ---- ------ ---- ---- ------- ---- ---- ------- ---- ---- ---------
+ 257 0 3 265 1 11,12 273 3 35-42 281 5 131-162
+ 258 0 4 266 1 13,14 274 3 43-50 282 5 163-194
+ 259 0 5 267 1 15,16 275 3 51-58 283 5 195-226
+ 260 0 6 268 1 17,18 276 3 59-66 284 5 227-257
+ 261 0 7 269 2 19-22 277 4 67-82 285 0 258
+ 262 0 8 270 2 23-26 278 4 83-98
+ 263 0 9 271 2 27-30 279 4 99-114
+ 264 0 10 272 2 31-34 280 4 115-130
+
+ Distance Codes
+ --------------
+ Extra Extra Extra Extra
+ Code Bits Dist Code Bits Dist Code Bits Distance Code Bits Distance
+ ---- ---- ---- ---- ---- ------ ---- ---- -------- ---- ---- --------
+ 0 0 1 8 3 17-24 16 7 257-384 24 11 4097-6144
+ 1 0 2 9 3 25-32 17 7 385-512 25 11 6145-8192
+ 2 0 3 10 4 33-48 18 8 513-768 26 12 8193-12288
+ 3 0 4 11 4 49-64 19 8 769-1024 27 12 12289-16384
+ 4 1 5,6 12 5 65-96 20 9 1025-1536 28 13 16385-24576
+ 5 1 7,8 13 5 97-128 21 9 1537-2048 29 13 24577-32768
+ 6 2 9-12 14 6 129-192 22 10 2049-3072
+ 7 2 13-16 15 6 193-256 23 10 3073-4096
+
+ 5.5.4 The compressed data stream begins immediately after the
+ compressed header data. The compressed data stream can be
+ interpreted as follows:
+
+ do
+ read header from input stream.
+
+ if stored block
+ skip bits until byte aligned
+ read count and 1's compliment of count
+ copy count bytes data block
+ otherwise
+ loop until end of block code sent
+ decode literal character from input stream
+ if literal < 256
+ copy character to the output stream
+ otherwise
+ if literal = end of block
+ break from loop
+ otherwise
+ decode distance from input stream
+
+ move backwards distance bytes in the output stream, and
+ copy length characters from this position to the output
+ stream.
+ end loop
+ while not last block
+
+ if data descriptor exists
+ skip bits until byte aligned
+ read crc and sizes
+ endif
+
+5.6 Enhanced Deflating - Method 9
+---------------------------------
+
+ 5.6.1 The Enhanced Deflating algorithm is similar to Deflate but uses
+ a sliding dictionary of up to 64K. Deflate64(tm) is supported
+ by the Deflate extractor.
+
+5.7 BZIP2 - Method 12
+---------------------
+
+ 5.7.1 BZIP2 is an open-source data compression algorithm developed by
+ Julian Seward. Information and source code for this algorithm
+ can be found on the internet.
+
+5.8 LZMA - Method 14
+---------------------
+
+ 5.8.1 LZMA is a block-oriented, general purpose data compression
+ algorithm developed and maintained by Igor Pavlov. It is a derivative
+ of LZ77 that utilizes Markov chains and a range coder. Information and
+ source code for this algorithm can be found on the internet. Consult
+ with the author of this algorithm for information on terms or
+ restrictions on use.
+
+ Support for LZMA within the ZIP format is defined as follows:
+
+ 5.8.2 The Compression method field within the ZIP Local and Central
+ Header records will be set to the value 14 to indicate data was
+ compressed using LZMA.
+
+ 5.8.3 The Version needed to extract field within the ZIP Local and
+ Central Header records will be set to 6.3 to indicate the minimum
+ ZIP format version supporting this feature.
+
+ 5.8.4 File data compressed using the LZMA algorithm MUST be placed
+ immediately following the Local Header for the file. If a standard
+ ZIP encryption header is required, it will follow the Local Header
+ and will precede the LZMA compressed file data segment. The location
+ of LZMA compressed data segment within the ZIP format will be as shown:
+
+ [local header file 1]
+ [encryption header file 1]
+ [LZMA compressed data segment for file 1]
+ [data descriptor 1]
+ [local header file 2]
+
+ 5.8.5 The encryption header and data descriptor records MAY
+ be conditionally present. The LZMA Compressed Data Segment
+ will consist of an LZMA Properties Header followed by the
+ LZMA Compressed Data as shown:
+
+ [LZMA properties header for file 1]
+ [LZMA compressed data for file 1]
+
+ 5.8.6 The LZMA Compressed Data will be stored as provided by the
+ LZMA compression library. Compressed size, uncompressed size and
+ other file characteristics about the file being compressed MUST be
+ stored in standard ZIP storage format.
+
+ 5.8.7 The LZMA Properties Header will store specific data required
+ to decompress the LZMA compressed Data. This data is set by the
+ LZMA compression engine using the function WriteCoderProperties()
+ as documented within the LZMA SDK.
+
+ 5.8.8 Storage fields for the property information within the LZMA
+ Properties Header are as follows:
+
+ LZMA Version Information 2 bytes
+ LZMA Properties Size 2 bytes
+ LZMA Properties Data variable, defined by "LZMA Properties Size"
+
+ 5.8.8.1 LZMA Version Information - this field identifies which version
+ of the LZMA SDK was used to compress a file. The first byte will
+ store the major version number of the LZMA SDK and the second
+ byte will store the minor number.
+
+ 5.8.8.2 LZMA Properties Size - this field defines the size of the
+ remaining property data. Typically this size SHOULD be determined by
+ the version of the SDK. This size field is included as a convenience
+ and to help avoid any ambiguity arising in the future due
+ to changes in this compression algorithm.
+
+ 5.8.8.3 LZMA Property Data - this variable sized field records the
+ required values for the decompressor as defined by the LZMA SDK.
+ The data stored in this field SHOULD be obtained using the
+ WriteCoderProperties() in the version of the SDK defined by
+ the "LZMA Version Information" field.
+
+ 5.8.8.4 The layout of the "LZMA Properties Data" field is a function of
+ the LZMA compression algorithm. It is possible that this layout MAY be
+ changed by the author over time. The data layout in version 4.3 of the
+ LZMA SDK defines a 5 byte array that uses 4 bytes to store the dictionary
+ size in little-endian order. This is preceded by a single packed byte as
+ the first element of the array that contains the following fields:
+
+ PosStateBits
+ LiteralPosStateBits
+ LiteralContextBits
+
+ Refer to the LZMA documentation for a more detailed explanation of
+ these fields.
+
+ 5.8.9 Data compressed with method 14, LZMA, MAY include an end-of-stream
+ (EOS) marker ending the compressed data stream. This marker is not
+ required, but its use is highly recommended to facilitate processing
+ and implementers SHOULD include the EOS marker whenever possible.
+ When the EOS marker is used, general purpose bit 1 MUSY be set. If
+ general purpose bit 1 is not set, the EOS marker is not present.
+
+5.9 WavPack - Method 97
+-----------------------
+
+ 5.9.1 Information describing the use of compression method 97 is
+ provided by WinZIP International, LLC. This method relies on the
+ open source WavPack audio compression utility developed by David Bryant.
+ Information on WavPack is available at www.wavpack.com. Please consult
+ with the author of this algorithm for information on terms and
+ restrictions on use.
+
+ 5.9.2 WavPack data for a file begins immediately after the end of the
+ local header data. This data is the output from WavPack compression
+ routines. Within the ZIP file, the use of WavPack compression is
+ indicated by setting the compression method field to a value of 97
+ in both the local header and the central directory header. The Version
+ needed to extract and version made by fields use the same values as are
+ used for data compressed using the Deflate algorithm.
+
+ 5.9.3 An implementation note for storing digital sample data when using
+ WavPack compression within ZIP files is that all of the bytes of
+ the sample data SHOULD be compressed. This includes any unused
+ bits up to the byte boundary. An example is a 2 byte sample that
+ uses only 12 bits for the sample data with 4 unused bits. If only
+ 12 bits are passed as the sample size to the WavPack routines, the 4
+ unused bits will be set to 0 on extraction regardless of their original
+ state. To avoid this, the full 16 bits of the sample data size
+ SHOULD be provided.
+
+5.10 PPMd - Method 98
+---------------------
+
+ 5.10.1 PPMd is a data compression algorithm developed by Dmitry Shkarin
+ which includes a carryless rangecoder developed by Dmitry Subbotin.
+ This algorithm is based on predictive phrase matching on multiple
+ order contexts. Information and source code for this algorithm
+ can be found on the internet. Consult with the author of this
+ algorithm for information on terms or restrictions on use.
+
+ 5.10.2 Support for PPMd within the ZIP format currently is provided only
+ for version I, revision 1 of the algorithm. Storage requirements
+ for using this algorithm are as follows:
+
+ 5.10.3 Parameters needed to control the algorithm are stored in the two
+ bytes immediately preceding the compressed data. These bytes are
+ used to store the following fields:
+
+ Model order - sets the maximum model order, default is 8, possible
+ values are from 2 to 16 inclusive
+
+ Sub-allocator size - sets the size of sub-allocator in MB, default is 50,
+ possible values are from 1MB to 256MB inclusive
+
+ Model restoration method - sets the method used to restart context
+ model at memory insufficiency, values are:
+
+ 0 - restarts model from scratch - default
+ 1 - cut off model - decreases performance by as much as 2x
+ 2 - freeze context tree - not recommended
+
+ 5.10.4 An example for packing these fields into the 2 byte storage field is
+ illustrated below. These values are stored in Intel low-byte/high-byte
+ order.
+
+ wPPMd = (Model order - 1) +
+ ((Sub-allocator size - 1) << 4) +
+ (Model restoration method << 12)
+
+
+5.11 AE-x Encryption marker - Method 99
+-------------------------------------------
+
+5.12 JPEG variant - Method 96
+-------------------------------------------
+
+5.13 PKWARE Data Compression Library Imploding - Method 10
+-----------------------------------------------------------
+
+5.14 Reserved - Method 11
+-------------------------------------------
+
+5.15 Reserved - Method 13
+-------------------------------------------
+
+5.16 Reserved - Method 15
+-------------------------------------------
+
+5.17 IBM z/OS CMPSC Compression - Method 16
+-------------------------------------------
+
+Method 16 utilizes the IBM hardware compression facility available
+on most IBM mainframes. Hardware compression can significantly
+increase the speed of data compression. This method uses a variant
+of the LZ78 algorithm. CMPSC hardware compression is performed
+using the COMPRESSION CALL instruction.
+
+ZIP archives can be created using this method only on mainframes
+supporting the CP instruction. Extraction MAY occur on any
+platform supporting this compression algorithm. Use of this
+algorithm requires creation of a compression dictionary and
+an expansion dictionary. The expansion dictionary MUST be
+placed into the ZIP archive for use on the system where
+extraction will occur.
+
+Additional information on this compression algorithm and dictionaries
+can be found in the IBM provided document titled IBM ESA/390 Data
+Compression (SA22-7208-01). Storage requirements for using CMPSC
+compression are as follows.
+
+The format for the compressed data stream placed into the ZIP
+archive following the Local Header is:
+
+ [dictionary header]
+ [expansion dictionary]
+ [CMPSC compressed data]
+
+If encryption is used to encrypt a file compressed with CMPSC, these
+sections MUST be encrypted as a single entity.
+
+The format of the dictionary header is:
+
+ Value Size Description
+ ----- ---- -----------
+ Version 1 byte 1
+ Flags/Symsize 1 byte Processing flags and
+ symbol size
+ DictionaryLen 4 bytes Length of the
+ expansion dictionary
+
+Explanation of processing flags and symbol size:
+
+The high 4 bits are used to store the processing flags. The low
+4 bits represent the size of a symbol, in bits (values range
+from 9-13). Flag values are defined below.
+
+ 0x80 - expansion dictionary
+ 0x40 - expansion dictionary is compressed using Deflate
+ 0x20 - Reserved
+ 0x10 - Reserved
+
+
+5.18 Reserved - Method 17
+-------------------------------------------
+
+5.19 IBM TERSE - Method 18
+-------------------------------------------
+
+5.20 IBM LZ77 z Architecture - Method 19
+-----------------------------------------
+
+6.0 Traditional PKWARE Encryption
+----------------------------------
+
+ 6.0.1 The following information discusses the decryption steps
+ required to support traditional PKWARE encryption. This
+ form of encryption is considered weak by today's standards
+ and its use is recommended only for situations with
+ low security needs or for compatibility with older .ZIP
+ applications.
+
+6.1 Traditional PKWARE Decryption
+---------------------------------
+
+ 6.1.1 PKWARE is grateful to Mr. Roger Schlafly for his expert
+ contribution towards the development of PKWARE's traditional
+ encryption.
+
+ 6.1.2 PKZIP encrypts the compressed data stream. Encrypted files
+ MUST be decrypted before they can be extracted to their original
+ form.
+
+ 6.1.3 Each encrypted file has an extra 12 bytes stored at the start
+ of the data area defining the encryption header for that file. The
+ encryption header is originally set to random values, and then
+ itself encrypted, using three, 32-bit keys. The key values are
+ initialized using the supplied encryption password. After each byte
+ is encrypted, the keys are then updated using pseudo-random number
+ generation techniques in combination with the same CRC-32 algorithm
+ used in PKZIP and described elsewhere in this document.
+
+ 6.1.4 The following are the basic steps required to decrypt a file:
+
+ 1) Initialize the three 32-bit keys with the password.
+ 2) Read and decrypt the 12-byte encryption header, further
+ initializing the encryption keys.
+ 3) Read and decrypt the compressed data stream using the
+ encryption keys.
+
+ 6.1.5 Initializing the encryption keys
+
+ Key(0) <- 305419896
+ Key(1) <- 591751049
+ Key(2) <- 878082192
+
+ loop for i <- 0 to length(password)-1
+ update_keys(password(i))
+ end loop
+
+ Where update_keys() is defined as:
+
+ update_keys(char):
+ Key(0) <- crc32(key(0),char)
+ Key(1) <- Key(1) + (Key(0) & 000000ffH)
+ Key(1) <- Key(1) * 134775813 + 1
+ Key(2) <- crc32(key(2),key(1) >> 24)
+ end update_keys
+
+ Where crc32(old_crc,char) is a routine that given a CRC value and a
+ character, returns an updated CRC value after applying the CRC-32
+ algorithm described elsewhere in this document.
+
+ 6.1.6 Decrypting the encryption header
+
+ The purpose of this step is to further initialize the encryption
+ keys, based on random data, to render a plaintext attack on the
+ data ineffective.
+
+ Read the 12-byte encryption header into Buffer, in locations
+ Buffer(0) thru Buffer(11).
+
+ loop for i <- 0 to 11
+ C <- buffer(i) ^ decrypt_byte()
+ update_keys(C)
+ buffer(i) <- C
+ end loop
+
+ Where decrypt_byte() is defined as:
+
+ unsigned char decrypt_byte()
+ local unsigned short temp
+ temp <- Key(2) | 2
+ decrypt_byte <- (temp * (temp ^ 1)) >> 8
+ end decrypt_byte
+
+ After the header is decrypted, the last 1 or 2 bytes in Buffer
+ SHOULD be the high-order word/byte of the CRC for the file being
+ decrypted, stored in Intel low-byte/high-byte order. Versions of
+ PKZIP prior to 2.0 used a 2 byte CRC check; a 1 byte CRC check is
+ used on versions after 2.0. This can be used to test if the password
+ supplied is correct or not.
+
+ 6.1.7 Decrypting the compressed data stream
+
+ The compressed data stream can be decrypted as follows:
+
+ loop until done
+ read a character into C
+ Temp <- C ^ decrypt_byte()
+ update_keys(temp)
+ output Temp
+ end loop
+
+
+7.0 Strong Encryption Specification
+-----------------------------------
+
+ 7.0.1 Portions of the Strong Encryption technology defined in this
+ specification are covered under patents and pending patent applications.
+ Refer to the section in this document entitled "Incorporating
+ PKWARE Proprietary Technology into Your Product" for more information.
+
+7.1 Strong Encryption Overview
+------------------------------
+
+ 7.1.1 Version 5.x of this specification introduced support for strong
+ encryption algorithms. These algorithms can be used with either
+ a password or an X.509v3 digital certificate to encrypt each file.
+ This format specification supports either password or certificate
+ based encryption to meet the security needs of today, to enable
+ interoperability between users within both PKI and non-PKI
+ environments, and to ensure interoperability between different
+ computing platforms that are running a ZIP program.
+
+ 7.1.2 Password based encryption is the most common form of encryption
+ people are familiar with. However, inherent weaknesses with
+ passwords (e.g. susceptibility to dictionary/brute force attack)
+ as well as password management and support issues make certificate
+ based encryption a more secure and scalable option. Industry
+ efforts and support are defining and moving towards more advanced
+ security solutions built around X.509v3 digital certificates and
+ Public Key Infrastructures(PKI) because of the greater scalability,
+ administrative options, and more robust security over traditional
+ password based encryption.
+
+ 7.1.3 Most standard encryption algorithms are supported with this
+ specification. Reference implementations for many of these
+ algorithms are available from either commercial or open source
+ distributors. Readily available cryptographic toolkits make
+ implementation of the encryption features straight-forward.
+ This document is not intended to provide a treatise on data
+ encryption principles or theory. Its purpose is to document the
+ data structures required for implementing interoperable data
+ encryption within the .ZIP format. It is strongly recommended that
+ you have a good understanding of data encryption before reading
+ further.
+
+ 7.1.4 The algorithms introduced in Version 5.0 of this specification
+ include:
+
+ RC2 40 bit, 64 bit, and 128 bit
+ RC4 40 bit, 64 bit, and 128 bit
+ DES
+ 3DES 112 bit and 168 bit
+
+ Version 5.1 adds support for the following:
+
+ AES 128 bit, 192 bit, and 256 bit
+
+
+ 7.1.5 Version 6.1 introduces encryption data changes to support
+ interoperability with Smartcard and USB Token certificate storage
+ methods which do not support the OAEP strengthening standard.
+
+ 7.1.6 Version 6.2 introduces support for encrypting metadata by compressing
+ and encrypting the central directory data structure to reduce information
+ leakage. Information leakage can occur in legacy ZIP applications
+ through exposure of information about a file even though that file is
+ stored encrypted. The information exposed consists of file
+ characteristics stored within the records and fields defined by this
+ specification. This includes data such as a file's name, its original
+ size, timestamp and CRC32 value.
+
+ 7.1.7 Version 6.3 introduces support for encrypting data using the Blowfish
+ and Twofish algorithms. These are symmetric block ciphers developed
+ by Bruce Schneier. Blowfish supports using a variable length key from
+ 32 to 448 bits. Block size is 64 bits. Implementations SHOULD use 16
+ rounds and the only mode supported within ZIP files is CBC. Twofish
+ supports key sizes 128, 192 and 256 bits. Block size is 128 bits.
+ Implementations SHOULD use 16 rounds and the only mode supported within
+ ZIP files is CBC. Information and source code for both Blowfish and
+ Twofish algorithms can be found on the internet. Consult with the author
+ of these algorithms for information on terms or restrictions on use.
+
+ 7.1.8 Central Directory Encryption provides greater protection against
+ information leakage by encrypting the Central Directory structure and
+ by masking key values that are replicated in the unencrypted Local
+ Header. ZIP compatible programs that cannot interpret an encrypted
+ Central Directory structure cannot rely on the data in the corresponding
+ Local Header for decompression information.
+
+ 7.1.9 Extra Field records that MAY contain information about a file that SHOULD
+ not be exposed SHOULD NOT be stored in the Local Header and SHOULD only
+ be written to the Central Directory where they can be encrypted. This
+ design currently does not support streaming. Information in the End of
+ Central Directory record, the Zip64 End of Central Directory Locator,
+ and the Zip64 End of Central Directory records are not encrypted. Access
+ to view data on files within a ZIP file with an encrypted Central Directory
+ requires the appropriate password or private key for decryption prior to
+ viewing any files, or any information about the files, in the archive.
+
+ 7.1.10 Older ZIP compatible programs not familiar with the Central Directory
+ Encryption feature will no longer be able to recognize the Central
+ Directory and MAY assume the ZIP file is corrupt. Programs that
+ attempt streaming access using Local Headers will see invalid
+ information for each file. Central Directory Encryption need not be
+ used for every ZIP file. Its use is recommended for greater security.
+ ZIP files not using Central Directory Encryption SHOULD operate as
+ in the past.
+
+ 7.1.11 This strong encryption feature specification is intended to provide for
+ scalable, cross-platform encryption needs ranging from simple password
+ encryption to authenticated public/private key encryption.
+
+ 7.1.12 Encryption provides data confidentiality and privacy. It is
+ recommended that you combine X.509 digital signing with encryption
+ to add authentication and non-repudiation.
+
+
+7.2 Single Password Symmetric Encryption Method
+-----------------------------------------------
+
+ 7.2.1 The Single Password Symmetric Encryption Method using strong
+ encryption algorithms operates similarly to the traditional
+ PKWARE encryption defined in this format. Additional data
+ structures are added to support the processing needs of the
+ strong algorithms.
+
+ The Strong Encryption data structures are:
+
+ 7.2.2 General Purpose Bits - Bits 0 and 6 of the General Purpose bit
+ flag in both local and central header records. Both bits set
+ indicates strong encryption. Bit 13, when set indicates the Central
+ Directory is encrypted and that selected fields in the Local Header
+ are masked to hide their actual value.
+
+
+ 7.2.3 Extra Field 0x0017 in central header only.
+
+ Fields to consider in this record are:
+
+ 7.2.3.1 Format - the data format identifier for this record. The only
+ value allowed at this time is the integer value 2.
+
+ 7.2.3.2 AlgId - integer identifier of the encryption algorithm from the
+ following range
+
+ 0x6601 - DES
+ 0x6602 - RC2 (version needed to extract < 5.2)
+ 0x6603 - 3DES 168
+ 0x6609 - 3DES 112
+ 0x660E - AES 128
+ 0x660F - AES 192
+ 0x6610 - AES 256
+ 0x6702 - RC2 (version needed to extract >= 5.2)
+ 0x6720 - Blowfish
+ 0x6721 - Twofish
+ 0x6801 - RC4
+ 0xFFFF - Unknown algorithm
+
+ 7.2.3.3 Bitlen - Explicit bit length of key
+
+ 32 - 448 bits
+
+ 7.2.3.4 Flags - Processing flags needed for decryption
+
+ 0x0001 - Password is required to decrypt
+ 0x0002 - Certificates only
+ 0x0003 - Password or certificate required to decrypt
+
+ Values > 0x0003 reserved for certificate processing
+
+
+ 7.2.4 Decryption header record preceding compressed file data.
+
+ -Decryption Header:
+
+ Value Size Description
+ ----- ---- -----------
+ IVSize 2 bytes Size of initialization vector (IV)
+ IVData IVSize Initialization vector for this file
+ Size 4 bytes Size of remaining decryption header data
+ Format 2 bytes Format definition for this record
+ AlgID 2 bytes Encryption algorithm identifier
+ Bitlen 2 bytes Bit length of encryption key
+ Flags 2 bytes Processing flags
+ ErdSize 2 bytes Size of Encrypted Random Data
+ ErdData ErdSize Encrypted Random Data
+ Reserved1 4 bytes Reserved certificate processing data
+ Reserved2 (var) Reserved for certificate processing data
+ VSize 2 bytes Size of password validation data
+ VData VSize-4 Password validation data
+ VCRC32 4 bytes Standard ZIP CRC32 of password validation data
+
+ 7.2.4.1 IVData - The size of the IV SHOULD match the algorithm block size.
+ The IVData can be completely random data. If the size of
+ the randomly generated data does not match the block size
+ it SHOULD be complemented with zero's or truncated as
+ necessary. If IVSize is 0,then IV = CRC32 + Uncompressed
+ File Size (as a 64 bit little-endian, unsigned integer value).
+
+ 7.2.4.2 Format - the data format identifier for this record. The only
+ value allowed at this time is the integer value 3.
+
+ 7.2.4.3 AlgId - integer identifier of the encryption algorithm from the
+ following range
+
+ 0x6601 - DES
+ 0x6602 - RC2 (version needed to extract < 5.2)
+ 0x6603 - 3DES 168
+ 0x6609 - 3DES 112
+ 0x660E - AES 128
+ 0x660F - AES 192
+ 0x6610 - AES 256
+ 0x6702 - RC2 (version needed to extract >= 5.2)
+ 0x6720 - Blowfish
+ 0x6721 - Twofish
+ 0x6801 - RC4
+ 0xFFFF - Unknown algorithm
+
+ 7.2.4.4 Bitlen - Explicit bit length of key
+
+ 32 - 448 bits
+
+ 7.2.4.5 Flags - Processing flags needed for decryption
+
+ 0x0001 - Password is required to decrypt
+ 0x0002 - Certificates only
+ 0x0003 - Password or certificate required to decrypt
+
+ Values > 0x0003 reserved for certificate processing
+
+ 7.2.4.6 ErdData - Encrypted random data is used to store random data that
+ is used to generate a file session key for encrypting
+ each file. SHA1 is used to calculate hash data used to
+ derive keys. File session keys are derived from a master
+ session key generated from the user-supplied password.
+ If the Flags field in the decryption header contains
+ the value 0x4000, then the ErdData field MUST be
+ decrypted using 3DES. If the value 0x4000 is not set,
+ then the ErdData field MUST be decrypted using AlgId.
+
+
+ 7.2.4.7 Reserved1 - Reserved for certificate processing, if value is
+ zero, then Reserved2 data is absent. See the explanation
+ under the Certificate Processing Method for details on
+ this data structure.
+
+ 7.2.4.8 Reserved2 - If present, the size of the Reserved2 data structure
+ is located by skipping the first 4 bytes of this field
+ and using the next 2 bytes as the remaining size. See
+ the explanation under the Certificate Processing Method
+ for details on this data structure.
+
+ 7.2.4.9 VSize - This size value will always include the 4 bytes of the
+ VCRC32 data and will be greater than 4 bytes.
+
+ 7.2.4.10 VData - Random data for password validation. This data is VSize
+ in length and VSize MUST be a multiple of the encryption
+ block size. VCRC32 is a checksum value of VData.
+ VData and VCRC32 are stored encrypted and start the
+ stream of encrypted data for a file.
+
+
+ 7.2.5 Useful Tips
+
+ 7.2.5.1 Strong Encryption is always applied to a file after compression. The
+ block oriented algorithms all operate in Cypher Block Chaining (CBC)
+ mode. The block size used for AES encryption is 16. All other block
+ algorithms use a block size of 8. Two IDs are defined for RC2 to
+ account for a discrepancy found in the implementation of the RC2
+ algorithm in the cryptographic library on Windows XP SP1 and all
+ earlier versions of Windows. It is recommended that zero length files
+ not be encrypted, however programs SHOULD be prepared to extract them
+ if they are found within a ZIP file.
+
+ 7.2.5.2 A pseudo-code representation of the encryption process is as follows:
+
+ Password = GetUserPassword()
+ MasterSessionKey = DeriveKey(SHA1(Password))
+ RD = CryptographicStrengthRandomData()
+ For Each File
+ IV = CryptographicStrengthRandomData()
+ VData = CryptographicStrengthRandomData()
+ VCRC32 = CRC32(VData)
+ FileSessionKey = DeriveKey(SHA1(IV + RD)
+ ErdData = Encrypt(RD,MasterSessionKey,IV)
+ Encrypt(VData + VCRC32 + FileData, FileSessionKey,IV)
+ Done
+
+ 7.2.5.3 The function names and parameter requirements will depend on
+ the choice of the cryptographic toolkit selected. Almost any
+ toolkit supporting the reference implementations for each
+ algorithm can be used. The RSA BSAFE(r), OpenSSL, and Microsoft
+ CryptoAPI libraries are all known to work well.
+
+
+ 7.3 Single Password - Central Directory Encryption
+ --------------------------------------------------
+
+ 7.3.1 Central Directory Encryption is achieved within the .ZIP format by
+ encrypting the Central Directory structure. This encapsulates the metadata
+ most often used for processing .ZIP files. Additional metadata is stored for
+ redundancy in the Local Header for each file. The process of concealing
+ metadata by encrypting the Central Directory does not protect the data within
+ the Local Header. To avoid information leakage from the exposed metadata
+ in the Local Header, the fields containing information about a file are masked.
+
+ 7.3.2 Local Header
+
+ Masking replaces the true content of the fields for a file in the Local
+ Header with false information. When masked, the Local Header is not
+ suitable for streaming access and the options for data recovery of damaged
+ archives is reduced. Extra Data fields that MAY contain confidential
+ data SHOULD NOT be stored within the Local Header. The value set into
+ the Version needed to extract field SHOULD be the correct value needed to
+ extract the file without regard to Central Directory Encryption. The fields
+ within the Local Header targeted for masking when the Central Directory is
+ encrypted are:
+
+ Field Name Mask Value
+ ------------------ ---------------------------
+ compression method 0
+ last mod file time 0
+ last mod file date 0
+ crc-32 0
+ compressed size 0
+ uncompressed size 0
+ file name (variable size) Base 16 value from the
+ range 1 - 0xFFFFFFFFFFFFFFFF
+ represented as a string whose
+ size will be set into the
+ file name length field
+
+ The Base 16 value assigned as a masked file name is simply a sequentially
+ incremented value for each file starting with 1 for the first file.
+ Modifications to a ZIP file MAY cause different values to be stored for
+ each file. For compatibility, the file name field in the Local Header
+ SHOULD NOT be left blank. As of Version 6.2 of this specification,
+ the Compression Method and Compressed Size fields are not yet masked.
+ Fields having a value of 0xFFFF or 0xFFFFFFFF for the ZIP64 format
+ SHOULD NOT be masked.
+
+ 7.3.3 Encrypting the Central Directory
+
+ Encryption of the Central Directory does not include encryption of the
+ Central Directory Signature data, the Zip64 End of Central Directory
+ record, the Zip64 End of Central Directory Locator, or the End
+ of Central Directory record. The ZIP file comment data is never
+ encrypted.
+
+ Before encrypting the Central Directory, it MAY optionally be compressed.
+ Compression is not required, but for storage efficiency it is assumed
+ this structure will be compressed before encrypting. Similarly, this
+ specification supports compressing the Central Directory without
+ requiring that it also be encrypted. Early implementations of this
+ feature will assume the encryption method applied to files matches the
+ encryption applied to the Central Directory.
+
+ Encryption of the Central Directory is done in a manner similar to
+ that of file encryption. The encrypted data is preceded by a
+ decryption header. The decryption header is known as the Archive
+ Decryption Header. The fields of this record are identical to
+ the decryption header preceding each encrypted file. The location
+ of the Archive Decryption Header is determined by the value in the
+ Start of the Central Directory field in the Zip64 End of Central
+ Directory record. When the Central Directory is encrypted, the
+ Zip64 End of Central Directory record will always be present.
+
+ The layout of the Zip64 End of Central Directory record for all
+ versions starting with 6.2 of this specification will follow the
+ Version 2 format. The Version 2 format is as follows:
+
+ The leading fixed size fields within the Version 1 format for this
+ record remain unchanged. The record signature for both Version 1
+ and Version 2 will be 0x06064b50. Immediately following the last
+ byte of the field known as the Offset of Start of Central
+ Directory With Respect to the Starting Disk Number will begin the
+ new fields defining Version 2 of this record.
+
+ 7.3.4 New fields for Version 2
+
+ Note: all fields stored in Intel low-byte/high-byte order.
+
+ Value Size Description
+ ----- ---- -----------
+ Compression Method 2 bytes Method used to compress the
+ Central Directory
+ Compressed Size 8 bytes Size of the compressed data
+ Original Size 8 bytes Original uncompressed size
+ AlgId 2 bytes Encryption algorithm ID
+ BitLen 2 bytes Encryption key length
+ Flags 2 bytes Encryption flags
+ HashID 2 bytes Hash algorithm identifier
+ Hash Length 2 bytes Length of hash data
+ Hash Data (variable) Hash data
+
+ The Compression Method accepts the same range of values as the
+ corresponding field in the Central Header.
+
+ The Compressed Size and Original Size values will not include the
+ data of the Central Directory Signature which is compressed or
+ encrypted.
+
+ The AlgId, BitLen, and Flags fields accept the same range of values
+ the corresponding fields within the 0x0017 record.
+
+ Hash ID identifies the algorithm used to hash the Central Directory
+ data. This data does not have to be hashed, in which case the
+ values for both the HashID and Hash Length will be 0. Possible
+ values for HashID are:
+
+ Value Algorithm
+ ------ ---------
+ 0x0000 none
+ 0x0001 CRC32
+ 0x8003 MD5
+ 0x8004 SHA1
+ 0x8007 RIPEMD160
+ 0x800C SHA256
+ 0x800D SHA384
+ 0x800E SHA512
+
+ 7.3.5 When the Central Directory data is signed, the same hash algorithm
+ used to hash the Central Directory for signing SHOULD be used.
+ This is recommended for processing efficiency, however, it is
+ permissible for any of the above algorithms to be used independent
+ of the signing process.
+
+ The Hash Data will contain the hash data for the Central Directory.
+ The length of this data will vary depending on the algorithm used.
+
+ The Version Needed to Extract SHOULD be set to 62.
+
+ The value for the Total Number of Entries on the Current Disk will
+ be 0. These records will no longer support random access when
+ encrypting the Central Directory.
+
+ 7.3.6 When the Central Directory is compressed and/or encrypted, the
+ End of Central Directory record will store the value 0xFFFFFFFF
+ as the value for the Total Number of Entries in the Central
+ Directory. The value stored in the Total Number of Entries in
+ the Central Directory on this Disk field will be 0. The actual
+ values will be stored in the equivalent fields of the Zip64
+ End of Central Directory record.
+
+ 7.3.7 Decrypting and decompressing the Central Directory is accomplished
+ in the same manner as decrypting and decompressing a file.
+
+ 7.4 Certificate Processing Method
+ ---------------------------------
+
+ The Certificate Processing Method for ZIP file encryption
+ defines the following additional data fields:
+
+ 7.4.1 Certificate Flag Values
+
+ Additional processing flags that can be present in the Flags field of both
+ the 0x0017 field of the central directory Extra Field and the Decryption
+ header record preceding compressed file data are:
+
+ 0x0007 - reserved for future use
+ 0x000F - reserved for future use
+ 0x0100 - Indicates non-OAEP key wrapping was used. If this
+ this field is set, the version needed to extract MUST
+ be at least 61. This means OAEP key wrapping is not
+ used when generating a Master Session Key using
+ ErdData.
+ 0x4000 - ErdData MUST be decrypted using 3DES-168, otherwise use the
+ same algorithm used for encrypting the file contents.
+ 0x8000 - reserved for future use
+
+
+ 7.4.2 CertData - Extra Field 0x0017 record certificate data structure
+
+ The data structure used to store certificate data within the section
+ of the Extra Field defined by the CertData field of the 0x0017
+ record are as shown:
+
+ Value Size Description
+ ----- ---- -----------
+ RCount 4 bytes Number of recipients.
+ HashAlg 2 bytes Hash algorithm identifier
+ HSize 2 bytes Hash size
+ SRList (var) Simple list of recipients hashed public keys
+
+
+ RCount This defines the number intended recipients whose
+ public keys were used for encryption. This identifies
+ the number of elements in the SRList.
+
+ HashAlg This defines the hash algorithm used to calculate
+ the public key hash of each public key used
+ for encryption. This field currently supports
+ only the following value for SHA-1
+
+ 0x8004 - SHA1
+
+ HSize This defines the size of a hashed public key.
+
+ SRList This is a variable length list of the hashed
+ public keys for each intended recipient. Each
+ element in this list is HSize. The total size of
+ SRList is determined using RCount * HSize.
+
+
+ 7.4.3 Reserved1 - Certificate Decryption Header Reserved1 Data
+
+ Value Size Description
+ ----- ---- -----------
+ RCount 4 bytes Number of recipients.
+
+ RCount This defines the number intended recipients whose
+ public keys were used for encryption. This defines
+ the number of elements in the REList field defined below.
+
+
+ 7.4.4 Reserved2 - Certificate Decryption Header Reserved2 Data Structures
+
+
+ Value Size Description
+ ----- ---- -----------
+ HashAlg 2 bytes Hash algorithm identifier
+ HSize 2 bytes Hash size
+ REList (var) List of recipient data elements
+
+
+ HashAlg This defines the hash algorithm used to calculate
+ the public key hash of each public key used
+ for encryption. This field currently supports
+ only the following value for SHA-1
+
+ 0x8004 - SHA1
+
+ HSize This defines the size of a hashed public key
+ defined in REHData.
+
+ REList This is a variable length of list of recipient data.
+ Each element in this list consists of a Recipient
+ Element data structure as follows:
+
+
+ Recipient Element (REList) Data Structure:
+
+ Value Size Description
+ ----- ---- -----------
+ RESize 2 bytes Size of REHData + REKData
+ REHData HSize Hash of recipients public key
+ REKData (var) Simple key blob
+
+
+ RESize This defines the size of an individual REList
+ element. This value is the combined size of the
+ REHData field + REKData field. REHData is defined by
+ HSize. REKData is variable and can be calculated
+ for each REList element using RESize and HSize.
+
+ REHData Hashed public key for this recipient.
+
+ REKData Simple Key Blob. The format of this data structure
+ is identical to that defined in the Microsoft
+ CryptoAPI and generated using the CryptExportKey()
+ function. The version of the Simple Key Blob
+ supported at this time is 0x02 as defined by
+ Microsoft.
+
+7.5 Certificate Processing - Central Directory Encryption
+---------------------------------------------------------
+
+ 7.5.1 Central Directory Encryption using Digital Certificates will
+ operate in a manner similar to that of Single Password Central
+ Directory Encryption. This record will only be present when there
+ is data to place into it. Currently, data is placed into this
+ record when digital certificates are used for either encrypting
+ or signing the files within a ZIP file. When only password
+ encryption is used with no certificate encryption or digital
+ signing, this record is not currently needed. When present, this
+ record will appear before the start of the actual Central Directory
+ data structure and will be located immediately after the Archive
+ Decryption Header if the Central Directory is encrypted.
+
+ 7.5.2 The Archive Extra Data record will be used to store the following
+ information. Additional data MAY be added in future versions.
+
+ Extra Data Fields:
+
+ 0x0014 - PKCS#7 Store for X.509 Certificates
+ 0x0016 - X.509 Certificate ID and Signature for central directory
+ 0x0019 - PKCS#7 Encryption Recipient Certificate List
+
+ The 0x0014 and 0x0016 Extra Data records that otherwise would be
+ located in the first record of the Central Directory for digital
+ certificate processing. When encrypting or compressing the Central
+ Directory, the 0x0014 and 0x0016 records MUST be located in the
+ Archive Extra Data record and they SHOULD NOT remain in the first
+ Central Directory record. The Archive Extra Data record will also
+ be used to store the 0x0019 data.
+
+ 7.5.3 When present, the size of the Archive Extra Data record will be
+ included in the size of the Central Directory. The data of the
+ Archive Extra Data record will also be compressed and encrypted
+ along with the Central Directory data structure.
+
+7.6 Certificate Processing Differences
+--------------------------------------
+
+ 7.6.1 The Certificate Processing Method of encryption differs from the
+ Single Password Symmetric Encryption Method as follows. Instead
+ of using a user-defined password to generate a master session key,
+ cryptographically random data is used. The key material is then
+ wrapped using standard key-wrapping techniques. This key material
+ is wrapped using the public key of each recipient that will need
+ to decrypt the file using their corresponding private key.
+
+ 7.6.2 This specification currently assumes digital certificates will follow
+ the X.509 V3 format for 1024 bit and higher RSA format digital
+ certificates. Implementation of this Certificate Processing Method
+ requires supporting logic for key access and management. This logic
+ is outside the scope of this specification.
+
+7.7 OAEP Processing with Certificate-based Encryption
+-----------------------------------------------------
+
+ 7.7.1 OAEP stands for Optimal Asymmetric Encryption Padding. It is a
+ strengthening technique used for small encoded items such as decryption
+ keys. This is commonly applied in cryptographic key-wrapping techniques
+ and is supported by PKCS #1. Versions 5.0 and 6.0 of this specification
+ were designed to support OAEP key-wrapping for certificate-based
+ decryption keys for additional security.
+
+ 7.7.2 Support for private keys stored on Smartcards or Tokens introduced
+ a conflict with this OAEP logic. Most card and token products do
+ not support the additional strengthening applied to OAEP key-wrapped
+ data. In order to resolve this conflict, versions 6.1 and above of this
+ specification will no longer support OAEP when encrypting using
+ digital certificates.
+
+ 7.7.3 Versions of PKZIP available during initial development of the
+ certificate processing method set a value of 61 into the
+ version needed to extract field for a file. This indicates that
+ non-OAEP key wrapping is used. This affects certificate encryption
+ only, and password encryption functions SHOULD NOT be affected by
+ this value. This means values of 61 MAY be found on files encrypted
+ with certificates only, or on files encrypted with both password
+ encryption and certificate encryption. Files encrypted with both
+ methods can safely be decrypted using the password methods documented.
+
+7.8 Additional Encryption/Decryption Data Records
+-----------------------------------------------------
+
+ 7.8.1 Additional information MAY be stored within a ZIP file in support
+ of the strong password and certificate encryption methods defined above.
+ These include, but are not limited to the following record types.
+
+ 0x0021 Policy Decryption Key Record
+ 0x0022 Smartcrypt Key Provider Record
+ 0x0023 Smartcrypt Policy Key Data Record
+
+8.0 Splitting and Spanning ZIP files
+-------------------------------------
+
+ 8.1 Spanned ZIP files
+
+ 8.1.1 Spanning is the process of segmenting a ZIP file across
+ multiple removable media. This support has typically only
+ been provided for DOS formatted floppy diskettes.
+
+ 8.2 Split ZIP files
+
+ 8.2.1 File splitting is a newer derivation of spanning.
+ Splitting follows the same segmentation process as
+ spanning, however, it does not require writing each
+ segment to a unique removable medium and instead supports
+ placing all pieces onto local or non-removable locations
+ such as file systems, local drives, folders, etc.
+
+ 8.3 File Naming Differences
+
+ 8.3.1 A key difference between spanned and split ZIP files is
+ that all pieces of a spanned ZIP file have the same name.
+ Since each piece is written to a separate volume, no name
+ collisions occur and each segment can reuse the original
+ .ZIP file name given to the archive.
+
+ 8.3.2 Sequence ordering for DOS spanned archives uses the DOS
+ volume label to determine segment numbers. Volume labels
+ for each segment are written using the form PKBACK#xxx,
+ where xxx is the segment number written as a decimal
+ value from 001 - nnn.
+
+ 8.3.3 Split ZIP files are typically written to the same location
+ and are subject to name collisions if the spanned name
+ format is used since each segment will reside on the same
+ drive. To avoid name collisions, split archives are named
+ as follows.
+
+ Segment 1 = filename.z01
+ Segment n-1 = filename.z(n-1)
+ Segment n = filename.zip
+
+ 8.3.4 The .ZIP extension is used on the last segment to support
+ quickly reading the central directory. The segment number
+ n SHOULD be a decimal value.
+
+ 8.4 Spanned Self-extracting ZIP Files
+
+ 8.4.1 Spanned ZIP files MAY be PKSFX Self-extracting ZIP files.
+ PKSFX files MAY also be split, however, in this case
+ the first segment MUST be named filename.exe. The first
+ segment of a split PKSFX archive MUST be large enough to
+ include the entire executable program.
+
+ 8.5 Capacities and Markers
+
+ 8.5.1 Capacities for split archives are as follows:
+
+ Maximum number of segments = 4,294,967,295 - 1
+ Maximum .ZIP segment size = 4,294,967,295 bytes
+ Minimum segment size = 64K
+ Maximum PKSFX segment size = 2,147,483,647 bytes
+
+ 8.5.2 Segment sizes MAY be different however by convention, all
+ segment sizes SHOULD be the same with the exception of the
+ last, which MAY be smaller. Local and central directory
+ header records MUST NOT be split across a segment boundary.
+ When writing a header record, if the number of bytes remaining
+ within a segment is less than the size of the header record,
+ end the current segment and write the header at the start
+ of the next segment. The central directory MAY span segment
+ boundaries, but no single record in the central directory
+ SHOULD be split across segments.
+
+ 8.5.3 Spanned/Split archives created using PKZIP for Windows
+ (V2.50 or greater), PKZIP Command Line (V2.50 or greater),
+ or PKZIP Explorer will include a special spanning
+ signature as the first 4 bytes of the first segment of
+ the archive. This signature (0x08074b50) will be
+ followed immediately by the local header signature for
+ the first file in the archive.
+
+ 8.5.4 A special spanning marker MAY also appear in spanned/split
+ archives if the spanning or splitting process starts but
+ only requires one segment. In this case the 0x08074b50
+ signature will be replaced with the temporary spanning
+ marker signature of 0x30304b50. Split archives can
+ only be uncompressed by other versions of PKZIP that
+ know how to create a split archive.
+
+ 8.5.5 The signature value 0x08074b50 is also used by some
+ ZIP implementations as a marker for the Data Descriptor
+ record. Conflict in this alternate assignment can be
+ avoided by ensuring the position of the signature
+ within the ZIP file to determine the use for which it
+ is intended.
+
+9.0 Change Process
+------------------
+
+ 9.1 In order for the .ZIP file format to remain a viable technology, this
+ specification SHOULD be considered as open for periodic review and
+ revision. Although this format was originally designed with a
+ certain level of extensibility, not all changes in technology
+ (present or future) were or will be necessarily considered in its
+ design.
+
+ 9.2 If your application requires new definitions to the
+ extensible sections in this format, or if you would like to
+ submit new data structures or new capabilities, please forward
+ your request to zipformat@pkware.com. All submissions will be
+ reviewed by the ZIP File Specification Committee for possible
+ inclusion into future versions of this specification.
+
+ 9.3 Periodic revisions to this specification will be published as
+ DRAFT or as FINAL status to ensure interoperability. We encourage
+ comments and feedback that MAY help improve clarity or content.
+
+
+10.0 Incorporating PKWARE Proprietary Technology into Your Product
+------------------------------------------------------------------
+
+ 10.1 The Use or Implementation in a product of APPNOTE technological
+ components pertaining to either strong encryption or patching requires
+ a separate, executed license agreement from PKWARE. Please contact
+ PKWARE at zipformat@pkware.com or +1-414-289-9788 with regard to
+ acquiring such a license.
+
+ 10.2 Additional information regarding PKWARE proprietary technology is
+ available at http://www.pkware.com/appnote.
+
+11.0 Acknowledgements
+---------------------
+
+ In addition to the above mentioned contributors to PKZIP and PKUNZIP,
+ PKWARE would like to extend special thanks to Robert Mahoney for
+ suggesting the extension .ZIP for this software.
+
+12.0 References
+---------------
+
+ Fiala, Edward R., and Greene, Daniel H., "Data compression with
+ finite windows", Communications of the ACM, Volume 32, Number 4,
+ April 1989, pages 490-505.
+
+ Held, Gilbert, "Data Compression, Techniques and Applications,
+ Hardware and Software Considerations", John Wiley & Sons, 1987.
+
+ Huffman, D.A., "A method for the construction of minimum-redundancy
+ codes", Proceedings of the IRE, Volume 40, Number 9, September 1952,
+ pages 1098-1101.
+
+ Nelson, Mark, "LZW Data Compression", Dr. Dobbs Journal, Volume 14,
+ Number 10, October 1989, pages 29-37.
+
+ Nelson, Mark, "The Data Compression Book", M&T Books, 1991.
+
+ Storer, James A., "Data Compression, Methods and Theory",
+ Computer Science Press, 1988
+
+ Welch, Terry, "A Technique for High-Performance Data Compression",
+ IEEE Computer, Volume 17, Number 6, June 1984, pages 8-19.
+
+ Ziv, J. and Lempel, A., "A universal algorithm for sequential data
+ compression", Communications of the ACM, Volume 30, Number 6,
+ June 1987, pages 520-540.
+
+ Ziv, J. and Lempel, A., "Compression of individual sequences via
+ variable-rate coding", IEEE Transactions on Information Theory,
+ Volume 24, Number 5, September 1978, pages 530-536.
+
+
+APPENDIX A - AS/400 Extra Field (0x0065) Attribute Definitions
+--------------------------------------------------------------
+
+A.1 Field Definition Structure:
+
+ a. field length including length 2 bytes Big Endian
+ b. field code 2 bytes
+ c. data x bytes
+
+A.2 Field Code Description
+
+ 4001 Source type i.e. CLP etc
+ 4002 The text description of the library
+ 4003 The text description of the file
+ 4004 The text description of the member
+ 4005 x'F0' or 0 is PF-DTA, x'F1' or 1 is PF_SRC
+ 4007 Database Type Code 1 byte
+ 4008 Database file and fields definition
+ 4009 GZIP file type 2 bytes
+ 400B IFS code page 2 bytes
+ 400C IFS Time of last file status change 4 bytes
+ 400D IFS Access Time 4 bytes
+ 400E IFS Modification time 4 bytes
+ 005C Length of the records in the file 2 bytes
+ 0068 GZIP two words 8 bytes
+
+APPENDIX B - z/OS Extra Field (0x0065) Attribute Definitions
+------------------------------------------------------------
+
+B.1 Field Definition Structure:
+
+ a. field length including length 2 bytes Big Endian
+ b. field code 2 bytes
+ c. data x bytes
+
+B.2 Field Code Description
+
+ 0001 File Type 2 bytes
+ 0002 NonVSAM Record Format 1 byte
+ 0003 Reserved
+ 0004 NonVSAM Block Size 2 bytes Big Endian
+ 0005 Primary Space Allocation 3 bytes Big Endian
+ 0006 Secondary Space Allocation 3 bytes Big Endian
+ 0007 Space Allocation Type1 byte flag
+ 0008 Modification Date Retired with PKZIP 5.0 +
+ 0009 Expiration Date Retired with PKZIP 5.0 +
+ 000A PDS Directory Block Allocation 3 bytes Big Endian binary value
+ 000B NonVSAM Volume List variable
+ 000C UNIT Reference Retired with PKZIP 5.0 +
+ 000D DF/SMS Management Class 8 bytes EBCDIC Text Value
+ 000E DF/SMS Storage Class 8 bytes EBCDIC Text Value
+ 000F DF/SMS Data Class 8 bytes EBCDIC Text Value
+ 0010 PDS/PDSE Member Info. 30 bytes
+ 0011 VSAM sub-filetype 2 bytes
+ 0012 VSAM LRECL 13 bytes EBCDIC "(num_avg num_max)"
+ 0013 VSAM Cluster Name Retired with PKZIP 5.0 +
+ 0014 VSAM KSDS Key Information 13 bytes EBCDIC "(num_length num_position)"
+ 0015 VSAM Average LRECL 5 bytes EBCDIC num_value padded with blanks
+ 0016 VSAM Maximum LRECL 5 bytes EBCDIC num_value padded with blanks
+ 0017 VSAM KSDS Key Length 5 bytes EBCDIC num_value padded with blanks
+ 0018 VSAM KSDS Key Position 5 bytes EBCDIC num_value padded with blanks
+ 0019 VSAM Data Name 1-44 bytes EBCDIC text string
+ 001A VSAM KSDS Index Name 1-44 bytes EBCDIC text string
+ 001B VSAM Catalog Name 1-44 bytes EBCDIC text string
+ 001C VSAM Data Space Type 9 bytes EBCDIC text string
+ 001D VSAM Data Space Primary 9 bytes EBCDIC num_value left-justified
+ 001E VSAM Data Space Secondary 9 bytes EBCDIC num_value left-justified
+ 001F VSAM Data Volume List variable EBCDIC text list of 6-character Volume IDs
+ 0020 VSAM Data Buffer Space 8 bytes EBCDIC num_value left-justified
+ 0021 VSAM Data CISIZE 5 bytes EBCDIC num_value left-justified
+ 0022 VSAM Erase Flag 1 byte flag
+ 0023 VSAM Free CI % 3 bytes EBCDIC num_value left-justified
+ 0024 VSAM Free CA % 3 bytes EBCDIC num_value left-justified
+ 0025 VSAM Index Volume List variable EBCDIC text list of 6-character Volume IDs
+ 0026 VSAM Ordered Flag 1 byte flag
+ 0027 VSAM REUSE Flag 1 byte flag
+ 0028 VSAM SPANNED Flag 1 byte flag
+ 0029 VSAM Recovery Flag 1 byte flag
+ 002A VSAM WRITECHK Flag 1 byte flag
+ 002B VSAM Cluster/Data SHROPTS 3 bytes EBCDIC "n,y"
+ 002C VSAM Index SHROPTS 3 bytes EBCDIC "n,y"
+ 002D VSAM Index Space Type 9 bytes EBCDIC text string
+ 002E VSAM Index Space Primary 9 bytes EBCDIC num_value left-justified
+ 002F VSAM Index Space Secondary 9 bytes EBCDIC num_value left-justified
+ 0030 VSAM Index CISIZE 5 bytes EBCDIC num_value left-justified
+ 0031 VSAM Index IMBED 1 byte flag
+ 0032 VSAM Index Ordered Flag 1 byte flag
+ 0033 VSAM REPLICATE Flag 1 byte flag
+ 0034 VSAM Index REUSE Flag 1 byte flag
+ 0035 VSAM Index WRITECHK Flag 1 byte flag Retired with PKZIP 5.0 +
+ 0036 VSAM Owner 8 bytes EBCDIC text string
+ 0037 VSAM Index Owner 8 bytes EBCDIC text string
+ 0038 Reserved
+ 0039 Reserved
+ 003A Reserved
+ 003B Reserved
+ 003C Reserved
+ 003D Reserved
+ 003E Reserved
+ 003F Reserved
+ 0040 Reserved
+ 0041 Reserved
+ 0042 Reserved
+ 0043 Reserved
+ 0044 Reserved
+ 0045 Reserved
+ 0046 Reserved
+ 0047 Reserved
+ 0048 Reserved
+ 0049 Reserved
+ 004A Reserved
+ 004B Reserved
+ 004C Reserved
+ 004D Reserved
+ 004E Reserved
+ 004F Reserved
+ 0050 Reserved
+ 0051 Reserved
+ 0052 Reserved
+ 0053 Reserved
+ 0054 Reserved
+ 0055 Reserved
+ 0056 Reserved
+ 0057 Reserved
+ 0058 PDS/PDSE Member TTR Info. 6 bytes Big Endian
+ 0059 PDS 1st LMOD Text TTR 3 bytes Big Endian
+ 005A PDS LMOD EP Rec # 4 bytes Big Endian
+ 005B Reserved
+ 005C Max Length of records 2 bytes Big Endian
+ 005D PDSE Flag 1 byte flag
+ 005E Reserved
+ 005F Reserved
+ 0060 Reserved
+ 0061 Reserved
+ 0062 Reserved
+ 0063 Reserved
+ 0064 Reserved
+ 0065 Last Date Referenced 4 bytes Packed Hex "yyyymmdd"
+ 0066 Date Created 4 bytes Packed Hex "yyyymmdd"
+ 0068 GZIP two words 8 bytes
+ 0071 Extended NOTE Location 12 bytes Big Endian
+ 0072 Archive device UNIT 6 bytes EBCDIC
+ 0073 Archive 1st Volume 6 bytes EBCDIC
+ 0074 Archive 1st VOL File Seq# 2 bytes Binary
+ 0075 Native I/O Flags 2 bytes
+ 0081 Unix File Type 1 byte enumerated
+ 0082 Unix File Format 1 byte enumerated
+ 0083 Unix File Character Set Tag Info 4 bytes
+ 0090 ZIP Environmental Processing Info 4 bytes
+ 0091 EAV EATTR Flags 1 byte
+ 0092 DSNTYPE Flags 1 byte
+ 0093 Total Space Allocation (Cyls) 4 bytes Big Endian
+ 009D NONVSAM DSORG 2 bytes
+ 009E Program Virtual Object Info 3 bytes
+ 009F Encapsulated file Info 9 bytes
+ 00A2 Cluster Log 4 bytes Binary
+ 00A3 Cluster LSID Length 4 bytes Binary
+ 00A4 Cluster LSID 26 bytes EBCDIC
+ 400C Unix File Creation Time 4 bytes
+ 400D Unix File Access Time 4 bytes
+ 400E Unix File Modification time 4 bytes
+ 4101 IBMCMPSC Compression Info variable
+ 4102 IBMCMPSC Compression Size 8 bytes Big Endian
+
+APPENDIX C - Zip64 Extensible Data Sector Mappings
+---------------------------------------------------
+
+ -Z390 Extra Field:
+
+ The following is the general layout of the attributes for the
+ ZIP 64 "extra" block for extended tape operations.
+
+ Note: some fields stored in Big Endian format. All text is
+ in EBCDIC format unless otherwise specified.
+
+ Value Size Description
+ ----- ---- -----------
+ (Z390) 0x0065 2 bytes Tag for this "extra" block type
+ Size 4 bytes Size for the following data block
+ Tag 4 bytes EBCDIC "Z390"
+ Length71 2 bytes Big Endian
+ Subcode71 2 bytes Enote type code
+ FMEPos 1 byte
+ Length72 2 bytes Big Endian
+ Subcode72 2 bytes Unit type code
+ Unit 1 byte Unit
+ Length73 2 bytes Big Endian
+ Subcode73 2 bytes Volume1 type code
+ FirstVol 1 byte Volume
+ Length74 2 bytes Big Endian
+ Subcode74 2 bytes FirstVol file sequence
+ FileSeq 2 bytes Sequence
+
+APPENDIX D - Language Encoding (EFS)
+------------------------------------
+
+D.1 The ZIP format has historically supported only the original IBM PC character
+encoding set, commonly referred to as IBM Code Page 437. This limits storing
+file name characters to only those within the original MS-DOS range of values
+and does not properly support file names in other character encodings, or
+languages. To address this limitation, this specification will support the
+following change.
+
+D.2 If general purpose bit 11 is unset, the file name and comment SHOULD conform
+to the original ZIP character encoding. If general purpose bit 11 is set, the
+filename and comment MUST support The Unicode Standard, Version 4.1.0 or
+greater using the character encoding form defined by the UTF-8 storage
+specification. The Unicode Standard is published by the The Unicode
+Consortium (www.unicode.org). UTF-8 encoded data stored within ZIP files
+is expected to not include a byte order mark (BOM).
+
+D.3 Applications MAY choose to supplement this file name storage through the use
+of the 0x0008 Extra Field. Storage for this optional field is currently
+undefined, however it will be used to allow storing extended information
+on source or target encoding that MAY further assist applications with file
+name, or file content encoding tasks. Please contact PKWARE with any
+requirements on how this field SHOULD be used.
+
+D.4 The 0x0008 Extra Field storage MAY be used with either setting for general
+purpose bit 11. Examples of the intended usage for this field is to store
+whether "modified-UTF-8" (JAVA) is used, or UTF-8-MAC. Similarly, other
+commonly used character encoding (code page) designations can be indicated
+through this field. Formalized values for use of the 0x0008 record remain
+undefined at this time. The definition for the layout of the 0x0008 field
+will be published when available. Use of the 0x0008 Extra Field provides
+for storing data within a ZIP file in an encoding other than IBM Code
+Page 437 or UTF-8.
+
+D.5 General purpose bit 11 will not imply any encoding of file content or
+password. Values defining character encoding for file content or
+password MUST be stored within the 0x0008 Extended Language Encoding
+Extra Field.
+
+D.6 Ed Gordon of the Info-ZIP group has defined a pair of "extra field" records
+that can be used to store UTF-8 file name and file comment fields. These
+records can be used for cases when the general purpose bit 11 method
+for storing UTF-8 data in the standard file name and comment fields is
+not desirable. A common case for this alternate method is if backward
+compatibility with older programs is required.
+
+D.7 Definitions for the record structure of these fields are included above
+in the section on 3rd party mappings for "extra field" records. These
+records are identified by Header ID's 0x6375 (Info-ZIP Unicode Comment
+Extra Field) and 0x7075 (Info-ZIP Unicode Path Extra Field).
+
+D.8 The choice of which storage method to use when writing a ZIP file is left
+to the implementation. Developers SHOULD expect that a ZIP file MAY
+contain either method and SHOULD provide support for reading data in
+either format. Use of general purpose bit 11 reduces storage requirements
+for file name data by not requiring additional "extra field" data for
+each file, but can result in older ZIP programs not being able to extract
+files. Use of the 0x6375 and 0x7075 records will result in a ZIP file
+that SHOULD always be readable by older ZIP programs, but requires more
+storage per file to write file name and/or file comment fields.
+
+APPENDIX E - AE-x encryption marker
+-----------------------------------
+
+E.1 AE-x defines an alternate password-based encryption method used
+in ZIP files that is based on a file encryption utility developed by
+Dr. Brian Gladman. Information on Dr. Gladman's method is available at
+
+ http://www.gladman.me.uk/cryptography_technology/fileencrypt/
+
+E.2 AE-x uses AES with CTR (counter mode) and HMAC-SHA1. It defines
+encryption using key sizes of 128 bits or 256 bits. It does not
+restrict support for decrypting 192 bits.
+
+E.3 This method uses the standard ZIP encryption bit (bit 0)
+of the general purpose bit flag (section 4.4.4) to indicate a
+file is encrypted.
+
+E.4 The compression method field (section 4.4.5) is set to 99
+to indicate a file has been encrypted using this method.
+
+E.5 The actual compression method is stored in an extra field
+structure identified by a Header ID of 0x9901. Information on this
+record structure can be found at http://www.winzip.com/aes_info.htm.
+
+E.6 Two versions are defined for the 0x9901 structure.
+
+ E.6.1 Version 1 stores the file CRC value in the CRC-32 field
+ (section 4.4.7).
+
+ E.6.2 Version 2 stores a value of 0 in the CRC-32 field.
+
generated by cgit v1.2.3-ZIG (git v2.39.0) at 2024-11-22 00:42:19 +0000