artifactview/crates/async_zip/SPECIFICATION.md

165 KiB

File: APPNOTE.TXT - .ZIP File Format Specification Version: 6.3.9 Status: FINAL - replaces version 6.3.8 Revised: July 15, 2020 Copyright (c) 1989 - 2014, 2018, 2019, 2020 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)

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
      0x2605        ZipIt Macintosh
      0x2705        ZipIt Macintosh 1.3.5+
      0x2805        ZipIt Macintosh 1.3.5+
      0x334d        Info-ZIP Macintosh
      0x4341        Acorn/SparkFS 
      0x4453        Windows NT security descriptor (binary ACL)
      0x4704        VM/CMS
      0x470f        MVS
      0x4b46        FWKCS MD5 (see below)
      0x4c41        OS/2 access control list (text ACL)
      0x4d49        Info-ZIP OpenVMS
      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
      0x7075        Info-ZIP Unicode Path Extra Field
      0x756e        ASi UNIX
      0x7855        Info-ZIP UNIX (new)
      0xa11e        Data Stream Alignment (Apache Commons-Compress)
      0xa220        Microsoft Open Packaging Growth Hint
      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:

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 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.