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.