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Rework the way IDF's SCM interface provides downloadable snapshots. Instead of returning a command which gets executed and which should pass through / stream its output data to the client, we're just returning an instance of Pluf_HTTP_Response. This is needed, because some SCMs, most noticable monotone, have no locally executable command to provide a snapshot archive (and probably never will for our kind of setup). We therefor added a little BSD-licensed class "ZipArchive" which allows the creation of pkzip-compatible archives on the fly by letting it eat the file contents directly feed from the (remote) stdio instance. Download performance is ok and lies between 15K/s and 110K/s, but at least we do no longer block the browser while we pre-generate the zip file server-side. Thanks to Patrick Georgi for all his work!
2010-10-30 21:52:40 +00:00
source: http://www.pkware.com/documents/casestudies/APPNOTE.TXT
File: APPNOTE.TXT - .ZIP File Format Specification
Version: 6.3.1
Revised: April 11, 2007
Copyright (c) 1989 - 2007 PKWARE Inc., All Rights Reserved.
The use of certain technological aspects disclosed in the current
APPNOTE is available pursuant to the below section entitled
"Incorporating PKWARE Proprietary Technology into Your Product".
I. Purpose
----------
This specification is intended to define a cross-platform,
interoperable file storage and transfer format. Since its
first publication in 1989, PKWARE has remained committed to
ensuring the interoperability of the .ZIP file format through
publication and maintenance of this specification. We trust that
all .ZIP compatible vendors and application developers that have
adopted and benefited from this format will share and support
this commitment to interoperability.
II. Contacting PKWARE
---------------------
PKWARE, Inc.
648 N. Plankinton Avenue, Suite 220
Milwaukee, WI 53203
+1-414-289-9788
+1-414-289-9789 FAX
zipformat@pkware.com
III. Disclaimer
---------------
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.
If the version of this file is marked as a NOTIFICATION OF CHANGE,
the content defines an Early Feature Specification (EFS) change
to the .ZIP file format that may be subject to modification prior
to publication of the Final Feature Specification (FFS). This
document may also contain information on Planned Feature
Specifications (PFS) defining recognized future extensions.
IV. Change Log
--------------
Version Change Description Date
------- ------------------ ----------
5.2 -Single Password Symmetric Encryption 06/02/2003
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
V. General Format of a .ZIP file
--------------------------------
Files stored in arbitrary order. Large .ZIP files can span multiple
volumes or be split into user-defined segment sizes. All values
are stored in little-endian byte order unless otherwise specified.
Overall .ZIP file format:
[local file header 1]
[file data 1]
[data descriptor 1]
.
.
.
[local file header n]
[file data n]
[data descriptor n]
[archive decryption header]
[archive extra data record]
[central directory]
[zip64 end of central directory record]
[zip64 end of central directory locator]
[end of central directory record]
A. 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)
B. File data
Immediately following the local header for a file
is the compressed or stored data for the file.
The series of [local file header][file data][data
descriptor] repeats for each file in the .ZIP archive.
C. Data descriptor:
crc-32 4 bytes
compressed size 4 bytes
uncompressed size 4 bytes
This descriptor exists only 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 is 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.
When compressing files, compressed and uncompressed sizes
should be stored in ZIP64 format (as 8 byte values) when a
files 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.
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.
When writing ZIP files, it is recommended to 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.
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.
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 should be set to binary zeros.
D. Archive decryption header:
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 will precede the encrypted data segment. The encrypted
data segment will 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. Refer to the
section on the Strong Encryption Specification for information
on the fields used in the Archive Decryption Header record.
E. Archive extra data record:
archive extra data signature 4 bytes (0x08064b50)
extra field length 4 bytes
extra field data (variable size)
The Archive Extra Data Record 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 present, this record immediately precedes the central
directory data structure. The size of this data record will 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.
F. Central directory structure:
[file header 1]
.
.
.
[file 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)
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.
G. 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)
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.
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.
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.
Multiple special purpose data blocks may be present, but 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.
H. 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
I. 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)
J. Explanation of fields:
version made by (2 bytes)
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. 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
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.
version needed to extract (2 bytes)
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 should 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.
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
* 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 should 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 should 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 should also be set.
This field should be set appropriately to indicate whether
Version 1 or Version 2 format is in use.
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 should
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.
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: Used 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.
Bit 14: Reserved by PKWARE.
Bit 15: Reserved by PKWARE.
compression method: (2 bytes)
(see accompanying documentation for algorithm
descriptions)
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 (EFS)
15 - Reserved by PKWARE
16 - Reserved by PKWARE
17 - Reserved by PKWARE
18 - File is compressed using IBM TERSE (new)
19 - IBM LZ77 z Architecture (PFS)
98 - PPMd version I, Rev 1
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.
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.
compressed size: (4 bytes)
uncompressed size: (4 bytes)
The size of the file compressed and uncompressed,
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.
file name length: (2 bytes)
extra field length: (2 bytes)
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.
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.
internal file attributes: (2 bytes)
Bits 1 and 2 are reserved for use by PKWARE.
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.
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.
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.
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.
file name: (Variable)
The name of the file, with optional relative path.
The path stored should not contain a drive or
device letter, or a leading slash. All slashes
should 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. If encrypting
the central directory 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.
extra field: (Variable)
This is for expansion. If additional information
needs to be stored for special needs or for specific
platforms, it should be stored here. Earlier versions
of the software can then safely skip this file, and
find the next file or header. This field will be 0
length in version 1.0.
In order to allow different programs and different types
of information to be stored in the 'extra' field in .ZIP
files, the following structure should be used for all
programs storing data in this field:
header1+data1 + header2+data2 . . .
Each header should 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 ID's of 0 thru 31 are reserved for use by PKWARE.
The remaining ID's can be used by third party vendors for
proprietary usage.
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
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)
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)
0x6542 BeOS/BeBox
0x756e ASi UNIX
0x7855 Info-ZIP UNIX (new)
0xa220 Microsoft Open Packaging Growth Hint
0xfd4a SMS/QDOS
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.
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.
In case two different programs should appropriate the same
Header ID value, it is strongly recommended that each
program 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.
-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 will
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.
-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 it's entirety within this
structure. There will only ever be one "block" of data in
VarFields[].
-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
-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
Rules:
1. There will be one or more of 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.
2. No word alignment or padding is performed.
3. A well-behaved PKZIP/OpenVMS program should never produce
more than one sub-block with the same TagX value. Also,
there will never be more than one "extra" block of type
0x000c in a particular directory record.
-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.
-PATCH Descriptor Extra Field (0x000f):
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
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
Actions
Action Value
------ -----
none 0
add 1
delete 2
patch 3
Reactions
Reaction Value
-------- -----
ask 0
skip 1
ignore 2
fail 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, patching, or extended tape operations requires
a license from PKWARE. Please contact PKWARE with regard to
acquiring a license.
-PKCS#7 Store for X.509 Certificates (0x0014):
This field contains 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
-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
-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
-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)
-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
-PKCS#7 Encryption Recipient Certificate List (0x0019):
This field contains 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 aide 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 0x0001 at this time
CStore (var) PKCS#7 data blob
-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)
-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)
Third-party Mappings:
-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
-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
-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;
-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."
-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
file comment: (Variable)
The comment for this file.
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.
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.
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.
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.
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.
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.
.ZIP file comment length: (2 bytes)
The length of the comment for this .ZIP file.
.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.
zip64 extensible data sector (variable size)
(currently reserved for use by PKWARE)
K. Splitting and Spanning ZIP files
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.
File splitting is a newer derivative 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...
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.
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.
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
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.
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.
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
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 never 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.
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.
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.
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.
L. General notes:
1) All fields unless otherwise noted are unsigned and stored
in Intel low-byte:high-byte, low-word:high-word order.
2) String fields are not null terminated, since the
length is given explicitly.
3) The entries in the central directory may not necessarily
be in the same order that files appear in the .ZIP file.
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.
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.
VI. UnShrinking - Method 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:
1) 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.
2) 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.
VII. Expanding - Methods 2-5
----------------------------
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.
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.
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.
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.
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.
VIII. Imploding - Method 6
--------------------------
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.
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.
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.
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.
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.
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.
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)
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.
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
IX. Tokenizing - Method 7
-------------------------
This method is not used by PKZIP.
X. Deflating - Method 8
-----------------------
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.
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.
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
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
XI. Enhanced Deflating - Method 9
---------------------------------
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.
XII. BZIP2 - Method 12
----------------------
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.
XIII. LZMA - Method 14 (EFS)
----------------------------
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:
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.
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.
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]
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]
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.
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.
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"
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.
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 should it arise in the future due
to changes in this compression algorithm.
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.
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 4.32
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.
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 must be set. If
general purpose bit 1 is not set, the EOS marker is not present.
XIV. PPMd - Method 98
---------------------
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.
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:
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
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)
XV. Traditional PKWARE Encryption
---------------------------------
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.
Decryption
----------
PKWARE is grateful to Mr. Roger Schlafly for his expert contribution
towards the development of PKWARE's traditional encryption.
PKZIP encrypts the compressed data stream. Encrypted files must
be decrypted before they can be extracted.
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.
The following is 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.
Step 1 - 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.
Step 2 - 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.
Step 3 - 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
XVI. Strong Encryption Specification
------------------------------------
The Strong Encryption technology defined in this specification is
covered under a pending patent application. The use or implementation
in a product of certain technological aspects set forth in the current
APPNOTE, including those with regard to strong encryption, patching,
or extended tape operations requires a license from PKWARE. Portions
of this Strong Encryption technology are available for use at no charge.
Contact PKWARE for licensing terms and conditions. Refer to section II
of this APPNOTE (Contacting PKWARE) for information on how to
contact PKWARE.
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.
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.
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.
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
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.
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 files name, its original
size, timestamp and CRC32 value.
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.
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.
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.
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.
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.
Encryption provides data confidentiality and privacy. It is
recommended that you combine X.509 digital signing with encryption
to add authentication and non-repudiation.
Single Password Symmetric Encryption Method:
-------------------------------------------
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:
1. 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.
2. Extra Field 0x0017 in central header only.
Fields to consider in this record are:
Format - the data format identifier for this record. The only
value allowed at this time is the integer value 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
Bitlen - Explicit bit length of key
32 - 448 bits
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
3. 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
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).
Format - the data format identifier for this record. The only
value allowed at this time is the integer value 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
Bitlen - Explicit bit length of key
32 - 448 bits
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
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.
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.
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.
VSize - This size value will always include the 4 bytes of the
VCRC32 data and will be greater than 4 bytes.
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.
4. Useful Tips
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 ID's 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.
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
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.
Single Password - Central Directory Encryption:
-----------------------------------------------
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.
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 never 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.
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.
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
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.
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.
Decrypting and decompressing the Central Directory is accomplished
in the same manner as decrypting and decompressing a file.
Certificate Processing Method:
-----------------------------
The Certificate Processing Method of for ZIP file encryption
defines the following additional data fields:
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
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.
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.
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.
Certificate Processing - Central Directory Encryption:
------------------------------------------------------
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.
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.
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.
Certificate Processing Differences:
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.
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.
OAEP Processing with Certificate-based Encryption:
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.
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.
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.
XVII. Change Process
--------------------
In order for the .ZIP file format to remain a viable definition, 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. If your application requires new definitions to the
extensible sections in this format, or if you would like to
submit new data structures, 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. Periodic revisions
to this specification will be published to ensure interoperability.
We encourage comments and feedback that may help improve clarity
or content.
XVIII. Incorporating PKWARE Proprietary Technology into Your Product
--------------------------------------------------------------------
PKWARE is committed to the interoperability and advancement of the
.ZIP format. PKWARE offers a free license for certain technological
aspects described above under certain restrictions and conditions.
However, the use or implementation in a product of certain technological
aspects set forth in the current APPNOTE, including those with regard to
strong encryption, patching, or extended tape operations requires a
license from PKWARE. Please contact PKWARE with regard to acquiring
a license.
XIX. Acknowledgements
----------------------
In addition to the above mentioned contributors to PKZIP and PKUNZIP,
I would like to extend special thanks to Robert Mahoney for suggesting
the extension .ZIP for this software.
XX. 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
--------------------------------------------------------------
Field Definition Structure:
a. field length including length 2 bytes
b. field code 2 bytes
c. data x bytes
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 Creation Time 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
------------------------------------------------------------
Field Definition Structure:
a. field length including length 2 bytes
b. field code 2 bytes
c. data x bytes
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
APPENDIX C - Zip64 Extensible Data Sector Mappings (EFS)
--------------------------------------------------------
-Z390 Extra Field:
The following is the general layout of the attributes for the
ZIP 64 "extra" block for extended tape operations. Portions of
this extended tape processing technology is covered under a
pending patent application. The use or implementation in a
product of certain technological aspects set forth in the
current APPNOTE, including those with regard to strong encryption,
patching or extended tape operations, requires a license from
PKWARE. Please contact PKWARE with regard to acquiring a license.
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)
------------------------------------
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.
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).
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.
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.
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.
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