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[Info-ZIP note, 20040528: this file is based on PKWARE's appnote.txt of
15 February 1996, taking into account PKWARE's revised appnote.txt
version 6.2.0 of 26 April 2004. It has been unofficially corrected
and extended by Info-ZIP without explicit permission by PKWARE.
Although Info-ZIP believes the information to be accurate and complete,
it is provided under a disclaimer similar to the PKWARE disclaimer below,
differing only in the substitution of "Info-ZIP" for "PKWARE". In other
words, use this information at your own risk, but we think it's correct.
Specification info from PKWARE that was obviously wrong has been corrected
silently (e.g. missing structure fields, wrong numbers).
As of PKZIPW 2.50, two new incompatibilities have been introduced by PKWARE;
they are noted below. Note that the "NTFS tag" conflict is currently not
real; PKZIPW 2.50 actually tags NTFS files as having come from a FAT
file system, too.]
File: APPNOTE.TXT - .ZIP File Format Specification
Version: 6.2.0 - NOTIFICATION OF CHANGE
Revised: 04/26/2004 [2004-05-28 Info-ZIP]
Copyright (c) 1989 - 2004 PKWARE Inc., All Rights Reserved.
I. Purpose
----------
This specification is intended to define a cross-platform,
interoperable file format. Since its first publication
in 1989, PKWARE has remained committed to ensuring the
interoperability of the .ZIP file format through this
specification. We trust that all .ZIP compatible vendors
and application developers that have adopted this format
will share and support this commitment.
II. 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 can not
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.
III. Change Log
---------------
Version Change Description Date
------- ------------------ ----------
5.2 -Single Password Symmetric Encryption 06/02/2003
storage
6.1.0 -Smart Card 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
IV. General Format of a .ZIP file
---------------------------------
Files stored in arbitrary order. Large .ZIP files can span multiple
diskette media or be split into user-defined segment sizes. [The
minimum user-defined segment size for a split .ZIP file is 64K.
(removed by PKWare 2003-06-01)]
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] (EFS)
[archive extra data record] (EFS)
[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:
[Info-ZIP discrepancy:
The Info-ZIP zip program starts the data descriptor with a 4-byte
PK-style signature. Despite the specification, none of the PKWARE
programs supports the data descriptor. PKZIP 4.0 -fix function
(and PKZIPFIX 2.04) ignores the data descriptor info even when bit 3
of the general purpose bit flag is set.
data descriptor signature 4 bytes (0x08074b50)
]
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 format
archives, the compressed and uncompressed sizes are 8 bytes each.
D. Archive decryption header: (EFS)
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: (EFS)
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 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(tm) 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.
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)
[PKWARE describes "OS made by" now (since 1998) as follows:
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.]
The upper byte indicates the host system (OS) for the
file. Software can use this information to determine
the line record format for text files etc. The current
mappings are:
0 - FAT file system (DOS, OS/2, NT) + PKWARE 2.50+ VFAT, NTFS
1 - Amiga
2 - OpenVMS
3 - Unix
4 - VM/CMS
5 - Atari ST
6 - HPFS file system (OS/2, NT 3.x)
7 - Macintosh
8 - Z-System
9 - CP/M
---------------------------------------------------------------------
PKWARE assignment | Info-ZIP assignment
-----------------------------------|---------------------------------
10 - Windows NTFS | TOPS-20
(since PKZIPW 2.50, but | (assigned Oct-1992,
not used by any PKWARE prog) | no longer used)
11 - MVS | NTFS file system (WinNT)
| (actively used by Info-ZIP's
| Zip for NT since Sep-1993)
12 - VSE | SMS/QDOS
---------------------------------------------------------------------
13 - Acorn RISC OS
14 - VFAT file system (Win95, NT) [Info-ZIP reservation, unused]
15 - MVS [PKWARE describes this assignment as "alternate MVS"]
16 - BeOS (BeBox or PowerMac)
17 - Tandem
18 - OS/400 (IBM) | THEOS
19 - OS/X (Darwin)
20 thru 29 - unused
30 - AtheOS/Syllable
31 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
* 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).
When using ZIP64 extensions, the corresponding value in the
Zip64 end of central directory record should also be set.
This field currently supports only the value 45 to indicate
ZIP64 extensions are present.
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.
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.)
[Info-ZIP note: This bit was introduced by PKZIP 2.04 for
DOS. In general, this feature can only be reliably used
together with compression methods that allow intrinsic
detection of the "end-of-compressed-data" condition. From
the set of compression methods described in this Zip archive
specification, only "deflate" and "bzip2" fulfill this
requirement.
Especially, the method STORED does not work!
The Info-ZIP tools recognize this bit regardless of the
compression method; but, they rely on correctly set
"compressed size" information in the central directory entry.]
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: Currently unused.
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
11 - Reserved by PKWARE
12 - File is compressed using BZIP2 algorithm
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. 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.
compressed size: (4 bytes)
uncompressed size: (4 bytes)
The size of the file compressed and uncompressed,
respectively. 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. If encrypting the central directory 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.
[Info-ZIP note:
This feature is not yet supported by any PKWARE version of ZIP
(at least not in PKZIP for DOS and PKZIP for Windows/WinNT).
The Info-ZIP programs handle standard input differently:
If input came from standard input, the filename is set to "-"
(length one).]
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. 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.
[Info-ZIP discrepancy:
If input came from standard input, the file name is set
to "-" (without the quotes).
As far as we know, the PKWARE specification for "input from
stdin" is not supported by PKZIP/PKUNZIP for DOS, OS/2, Windows
Windows NT.]
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 future Unicode file name data (PFS)
0x0009 OS/2 extended attributes (also Info-ZIP)
0x000a NTFS (Win9x/WinNT FileTimes)
0x000c OpenVMS (also Info-ZIP)
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
The Header ID mappings defined by Info-ZIP and third parties are:
0x07c8 Info-ZIP Macintosh (old, J. Lee)
0x2605 ZipIt Macintosh (first version)
0x2705 ZipIt Macintosh v 1.3.5 and newer (w/o full filename)
0x2805 ZipIt Macintosh 1.3.5+
0x334d Info-ZIP Macintosh (new, D. Haase's 'Mac3' field)
0x4154 Tandem NSK
0x4341 Acorn/SparkFS (David Pilling)
0x4453 Windows NT security descriptor (binary ACL)
0x4704 VM/CMS
0x470f MVS
0x4854 Theos, old inofficial port
0x4b46 FWKCS MD5 (see below)
0x4c41 OS/2 access control list (text ACL)
0x4d49 Info-ZIP OpenVMS (obsolete)
0x4d63 Macintosh SmartZIP, by Macro Bambini
0x4f4c Xceed original location extra field
0x5356 AOS/VS (binary ACL)
0x5455 extended timestamp
0x554e Xceed unicode extra field
0x5855 Info-ZIP Unix (original; also OS/2, NT, etc.)
0x6542 BeOS (BeBox, PowerMac, etc.)
0x6854 Theos
0x7441 AtheOS (AtheOS/Syllable attributes)
0x756e ASi Unix
0x7855 Info-ZIP Unix (new)
0xfb4a 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.
In the following descriptions, note that "Short" means two bytes,
"Long" means four bytes, and "Long-Long" means eight bytes,
regardless of their native sizes. Unless specifically noted, all
integer fields should be interpreted as unsigned (non-negative)
numbers.
-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 sizes.
-OS/2 Extended Attributes Extra Field (0x0009):
=============================================
The following is the layout of the OS/2 extended attributes "extra"
block. (Last Revision 19960922)
Note: all fields stored in Intel low-byte/high-byte order.
Local-header version:
Value Size Description
----- ---- -----------
(OS/2) 0x0009 Short tag for this extra block type
TSize Short total data size for this block
BSize Long uncompressed EA data size
CType Short compression type
EACRC Long CRC value for uncompressed EA data
(var.) variable compressed EA data
Central-header version:
Value Size Description
----- ---- -----------
(OS/2) 0x0009 Short tag for this extra block type
TSize Short total data size for this block (4)
BSize Long size of uncompressed local EA data
The value of CType is interpreted according to the "compression
method" section above; i.e., 0 for stored, 8 for deflated, etc.
The OS/2 extended attribute structure (FEA2LIST) is
compressed and then stored in its entirety within this
structure. There will only ever be one "block" of data in
the variable-length field.
-OS/2 Access Control List Extra Field:
====================================
The following is the layout of the OS/2 ACL extra block.
(Last Revision 19960922)
Local-header version:
Value Size Description
----- ---- -----------
(ACL) 0x4c41 Short tag for this extra block type ("AL")
TSize Short total data size for this block
BSize Long uncompressed ACL data size
CType Short compression type
EACRC Long CRC value for uncompressed ACL data
(var.) variable compressed ACL data
Central-header version:
Value Size Description
----- ---- -----------
(ACL) 0x4c41 Short tag for this extra block type ("AL")
TSize Short total data size for this block (4)
BSize Long size of uncompressed local ACL data
The value of CType is interpreted according to the "compression
method" section above; i.e., 0 for stored, 8 for deflated, etc.
The uncompressed ACL data consist of a text header of the form
"ACL1:%hX,%hd\n", where the first field is the OS/2 ACCINFO acc_attr
member and the second is acc_count, followed by acc_count strings
of the form "%s,%hx\n", where the first field is acl_ugname (user
group name) and the second acl_access. This block type will be
extended for other operating systems as needed.
-Windows NT Security Descriptor Extra Field (0x4453):
===================================================
The following is the layout of the NT Security Descriptor (another
type of ACL) extra block. (Last Revision 19960922)
Local-header version:
Value Size Description
----- ---- -----------
(SD) 0x4453 Short tag for this extra block type ("SD")
TSize Short total data size for this block
BSize Long uncompressed SD data size
Version Byte version of uncompressed SD data format
CType Short compression type
EACRC Long CRC value for uncompressed SD data
(var.) variable compressed SD data
Central-header version:
Value Size Description
----- ---- -----------
(SD) 0x4453 Short tag for this extra block type ("SD")
TSize Short total data size for this block (4)
BSize Long size of uncompressed local SD data
The value of CType is interpreted according to the "compression
method" section above; i.e., 0 for stored, 8 for deflated, etc.
Version specifies how the compressed data are to be interpreted
and allows for future expansion of this extra field type. Currently
only version 0 is defined.
For version 0, the compressed data are to be interpreted as a single
valid Windows NT SECURITY_DESCRIPTOR data structure, in self-relative
format.
-PKWARE Win95/WinNT Extra Field (0x000a):
=======================================
The following description covers PKWARE's "NTFS" attributes
"extra" block, introduced with the release of PKZIP 2.50 for
Windows. (Last Revision 20001118)
(Note: At this time the Mtime, Atime and Ctime values may
be used on any WIN32 system.)
[Info-ZIP note: In the current implementations, this field has
a fixed total data size of 32 bytes and is only stored as local
extra field.]
Value Size Description
----- ---- -----------
(NTFS) 0x000a Short Tag for this "extra" block type
TSize Short Total Data Size for this block
Reserved Long for future use
Tag1 Short NTFS attribute tag value #1
Size1 Short Size of attribute #1, in bytes
(var.) SubSize1 Attribute #1 data
.
.
.
TagN Short NTFS attribute tag value #N
SizeN Short Size of attribute #N, in bytes
(var.) SubSizeN 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 (24)
Mtime 8 bytes 64-bit NTFS file last modification time
Atime 8 bytes 64-bit NTFS file last access time
Ctime 8 bytes 64-bit NTFS file creation time
The total length for this block is 28 bytes, resulting in a
fixed size value of 32 for the TSize field of the NTFS block.
The NTFS filetimes are 64-bit unsigned integers, stored in Intel
(least significant byte first) byte order. They determine the
number of 1.0E-07 seconds (1/10th microseconds!) past WinNT "epoch",
which is "01-Jan-1601 00:00:00 UTC".
-PKWARE OpenVMS Extra Field (0x000c):
===================================
The following is the layout of PKWARE's OpenVMS attributes
"extra" block. (Last Revision 12/17/91)
Note: all fields stored in Intel low-byte/high-byte order.
Value Size Description
----- ---- -----------
(VMS) 0x000c Short Tag for this "extra" block type
TSize Short Total Data Size for this block
CRC Long 32-bit CRC for remainder of the block
Tag1 Short OpenVMS attribute tag value #1
Size1 Short Size of attribute #1, in bytes
(var.) Size1 Attribute #1 data
.
.
.
TagN Short OpenVMS attribute tage value #N
SizeN Short 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.
-Info-ZIP VMS Extra Field:
========================
The following is the layout of Info-ZIP's VMS attributes extra
block for VAX or Alpha AXP. The local-header and central-header
versions are identical. (Last Revision 19960922)
Value Size Description
----- ---- -----------
(VMS2) 0x4d49 Short tag for this extra block type ("JM")
TSize Short total data size for this block
ID Long block ID
Flags Short info bytes
BSize Short uncompressed block size
Reserved Long (reserved)
(var.) variable compressed VMS file-attributes block
The block ID is one of the following unterminated strings:
"VFAB" struct FAB
"VALL" struct XABALL
"VFHC" struct XABFHC
"VDAT" struct XABDAT
"VRDT" struct XABRDT
"VPRO" struct XABPRO
"VKEY" struct XABKEY
"VMSV" version (e.g., "V6.1"; truncated at hyphen)
"VNAM" reserved
The lower three bits of Flags indicate the compression method. The
currently defined methods are:
0 stored (not compressed)
1 simple "RLE"
2 deflated
The "RLE" method simply replaces zero-valued bytes with zero-valued
bits and non-zero-valued bytes with a "1" bit followed by the byte
value.
The variable-length compressed data contains only the data corre-
sponding to the indicated structure or string. Typically multiple
VMS2 extra fields are present (each with a unique block type).
-Info-ZIP Macintosh Extra Field:
==============================
The following is the layout of the (old) Info-ZIP resource-fork extra
block for Macintosh. The local-header and central-header versions
are identical. (Last Revision 19960922)
Value Size Description
----- ---- -----------
(Mac) 0x07c8 Short tag for this extra block type
TSize Short total data size for this block
"JLEE" beLong extra-field signature
FInfo 16 bytes Macintosh FInfo structure
CrDat beLong HParamBlockRec fileParam.ioFlCrDat
MdDat beLong HParamBlockRec fileParam.ioFlMdDat
Flags beLong info bits
DirID beLong HParamBlockRec fileParam.ioDirID
VolName 28 bytes volume name (optional)
All fields but the first two are in native Macintosh format
(big-endian Motorola order, not little-endian Intel). The least
significant bit of Flags is 1 if the file is a data fork, 0 other-
wise. In addition, if this extra field is present, the filename
has an extra 'd' or 'r' appended to indicate data fork or resource
fork. The 28-byte VolName field may be omitted.
-ZipIt Macintosh Extra Field (long):
==================================
The following is the layout of the ZipIt extra block for Macintosh.
The local-header and central-header versions are identical.
(Last Revision 19970130)
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):
==============================================
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.
(Last Revision 20030602)
Value Size Description
----- ---- -----------
(Mac2b) 0x2705 Short tag for this extra block type
TSize Short total data size for this block (min. 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):
====================================================
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;
-Info-ZIP Macintosh Extra Field (new):
====================================
The following is the layout of the (new) Info-ZIP extra
block for Macintosh, designed by Dirk Haase.
All values are in little-endian.
(Last Revision 19981005)
Local-header version:
Value Size Description
----- ---- -----------
(Mac3) 0x334d Short tag for this extra block type ("M3")
TSize Short total data size for this block
BSize Long uncompressed finder attribute data size
Flags Short info bits
fdType Byte[4] Type of the File (4-byte string)
fdCreator Byte[4] Creator of the File (4-byte string)
(CType) Short compression type
(CRC) Long CRC value for uncompressed MacOS data
Attribs variable finder attribute data (see below)
Central-header version:
Value Size Description
----- ---- -----------
(Mac3) 0x334d Short tag for this extra block type ("M3")
TSize Short total data size for this block
BSize Long uncompressed finder attribute data size
Flags Short info bits
fdType Byte[4] Type of the File (4-byte string)
fdCreator Byte[4] Creator of the File (4-byte string)
The third bit of Flags in both headers indicates whether
the LOCAL extra field is uncompressed (and therefore whether CType
and CRC are omitted):
Bits of the Flags:
bit 0 if set, file is a data fork; otherwise unset
bit 1 if set, filename will be not changed
bit 2 if set, Attribs is uncompressed (no CType, CRC)
bit 3 if set, date and times are in 64 bit
if zero date and times are in 32 bit.
bit 4 if set, timezone offsets fields for the native
Mac times are omitted (UTC support deactivated)
bits 5-15 reserved;
Attributes:
Attribs is a Mac-specific block of data in little-endian format with
the following structure (if compressed, uncompress it first):
Value Size Description
----- ---- -----------
fdFlags Short Finder Flags
fdLocation.v Short Finder Icon Location
fdLocation.h Short Finder Icon Location
fdFldr Short Folder containing file
FXInfo 16 bytes Macintosh FXInfo structure
FXInfo-Structure:
fdIconID Short
fdUnused[3] Short unused but reserved 6 bytes
fdScript Byte Script flag and number
fdXFlags Byte More flag bits
fdComment Short Comment ID
fdPutAway Long Home Dir ID
FVersNum Byte file version number
may be not used by MacOS
ACUser Byte directory access rights
FlCrDat ULong date and time of creation
FlMdDat ULong date and time of last modification
FlBkDat ULong date and time of last backup
These time numbers are original Mac FileTime values (local time!).
Currently, date-time width is 32-bit, but future version may
support be 64-bit times (see flags)
CrGMTOffs Long(signed!) difference "local Creat. time - UTC"
MdGMTOffs Long(signed!) difference "local Modif. time - UTC"
BkGMTOffs Long(signed!) difference "local Backup time - UTC"
These "local time - UTC" differences (stored in seconds) may be
used to support timestamp adjustment after inter-timezone transfer.
These fields are optional; bit 4 of the flags word controls their
presence.
Charset Short TextEncodingBase (Charset)
valid for the following two fields
FullPath variable Path of the current file.
Zero terminated string (C-String)
Currently coded in the native Charset.
Comment variable Finder Comment of the current file.
Zero terminated string (C-String)
Currently coded in the native Charset.
-SmartZIP Macintosh Extra Field:
====================================
The following is the layout of the SmartZIP extra
block for Macintosh, designed by Marco Bambini.
Local-header version:
Value Size Description
----- ---- -----------
0x4d63 Short tag for this extra block type ("cM")
TSize Short total data size for this block (64)
"dZip" beLong extra-field signature
fdType Byte[4] Type of the File (4-byte string)
fdCreator Byte[4] Creator of the File (4-byte string)
fdFlags beShort Finder Flags
fdLocation.v beShort Finder Icon Location
fdLocation.h beShort Finder Icon Location
fdFldr beShort Folder containing file
CrDat beLong HParamBlockRec fileParam.ioFlCrDat
MdDat beLong HParamBlockRec fileParam.ioFlMdDat
frScroll.v Byte vertical pos. of folder's scroll bar
fdScript Byte Script flag and number
frScroll.h Byte horizontal pos. of folder's scroll bar
fdXFlags Byte More flag bits
FileName Byte[32] full Macintosh filename (pascal string)
All fields but the first two are in native Macintosh format
(big-endian Motorola order, not little-endian Intel).
The extra field size is fixed to 64 bytes.
The local-header and central-header versions are identical.
-Acorn SparkFS Extra Field:
=========================
The following is the layout of David Pilling's SparkFS extra block
for Acorn RISC OS. The local-header and central-header versions are
identical. (Last Revision 19960922)
Value Size Description
----- ---- -----------
(Acorn) 0x4341 Short tag for this extra block type ("AC")
TSize Short total data size for this block (20)
"ARC0" Long extra-field signature
LoadAddr Long load address or file type
ExecAddr Long exec address
Attr Long file permissions
Zero Long reserved; always zero
The following bits of Attr are associated with the given file
permissions:
bit 0 user-writable ('W')
bit 1 user-readable ('R')
bit 2 reserved
bit 3 locked ('L')
bit 4 publicly writable ('w')
bit 5 publicly readable ('r')
bit 6 reserved
bit 7 reserved
-VM/CMS Extra Field:
==================
The following is the layout of the file-attributes extra block for
VM/CMS. The local-header and central-header versions are
identical. (Last Revision 19960922)
Value Size Description
----- ---- -----------
(VM/CMS) 0x4704 Short tag for this extra block type
TSize Short total data size for this block
flData variable file attributes data
flData is an uncompressed fldata_t struct.
-MVS Extra Field:
===============
The following is the layout of the file-attributes extra block for
MVS. The local-header and central-header versions are identical.
(Last Revision 19960922)
Value Size Description
----- ---- -----------
(MVS) 0x470f Short tag for this extra block type
TSize Short total data size for this block
flData variable file attributes data
flData is an uncompressed fldata_t struct.
-PKWARE Unix Extra Field (0x000d):
================================
The following is the layout of PKWARE's Unix "extra" block.
It was introduced with the release of PKZIP for Unix 2.50.
Note: all fields are stored in Intel low-byte/high-byte order.
(Last Revision 19980901)
This field has a minimum data size of 12 bytes and is only stored
as local extra field.
Value Size Description
----- ---- -----------
(Unix0) 0x000d Short Tag for this "extra" block type
TSize Short Total Data Size for this block
AcTime Long time of last access (UTC/GMT)
ModTime Long time of last modification (UTC/GMT)
UID Short Unix user ID
GID Short Unix 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.
[Info-ZIP note: The fixed part of this field has the same layout as
Info-ZIP's abandoned "Unix1 timestamps & owner ID info" extra field;
only the two tag bytes are different.]
-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 Short Tag for this "extra" block type
TSize Short Size of the total "extra" block
Version Short Version of the descriptor
Flags Long Actions and reactions (see below)
OldSize Long Size of the file about to be patched
OldCRC Long 32-bit CRC of the file about to be patched
NewSize Long Size of the resulting file
NewCRC Long 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.
-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
SData TSize Data about the store
SData
Value Size Description
----- ---- -----------
Version 2 bytes Version number, 0x0001 for now
StoreD (variable) Actual store data
The StoreD member is suitable for passing as the pbData
member of a CRYPT_DATA_BLOB to the CertOpenStore() function
in Microsoft's CryptoAPI. The SSize member above will be
cbData + 6, where cbData is the cbData member of the same
CRYPT_DATA_BLOB. The encoding type to pass to
CertOpenStore() should be
PKCS_7_ANS_ENCODING | X509_ASN_ENCODING.
-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
CSize 2 bytes Size of Method
Method (variable)
Method
Value Size Description
----- ---- -----------
Version 2 bytes Version number, for now 0x0001
AlgID 2 bytes Algorithm ID used for signing
IDSize 2 bytes Size of Certificate ID data
CertID (variable) Certificate ID data
SigSize 2 bytes Size of Signature data
Sig (variable) Signature data
CertID
Value Size Description
----- ---- -----------
Size1 4 bytes Size of CertID, should be (IDSize - 4)
Size1 4 bytes A bug in version one causes this value
to appear twice.
IssSize 4 bytes Issuer data size
Issuer (variable) Issuer data
SerSize 4 bytes Serial Number size
Serial (variable) Serial Number data
The Issuer and IssSize members are suitable for creating a
CRYPT_DATA_BLOB to be the Issuer member of a CERT_INFO
struct. The Serial and SerSize members would be the
SerialNumber member of the same CERT_INFO struct. This
struct would be used to find the certificate in the store
the file was signed with. Those structures are from the MS
CryptoAPI.
Sig and SigSize are the actual signature data and size
generated by signing the file with the MS CryptoAPI using a
hash created with the given AlgID.
-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,
along with the store. The data structure is the
same as the CID, except that SigSize will be 0, and there
will be no Sig member.
This field is also kept after the last central directory
record, as the signature data (ID 0x05054b50, it looks like
a central directory record of a different type). This
second copy of the data is the Signature Data member of the
record, and will have a SigSize that is non-zero, and will
have Sig data.
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) (EFS):
===============================
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): (EFS)
=====================================================
This field contains the information about each of the certificates
that files may be encrypted with. 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 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 (PKWARE, 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
-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
-Extended Timestamp Extra Field:
==============================
The following is the layout of the extended-timestamp extra block.
(Last Revision 19970118)
Local-header version:
Value Size Description
----- ---- -----------
(time) 0x5455 Short tag for this extra block type ("UT")
TSize Short total data size for this block
Flags Byte info bits
(ModTime) Long time of last modification (UTC/GMT)
(AcTime) Long time of last access (UTC/GMT)
(CrTime) Long time of original creation (UTC/GMT)
Central-header version:
Value Size Description
----- ---- -----------
(time) 0x5455 Short tag for this extra block type ("UT")
TSize Short total data size for this block
Flags Byte info bits (refers to local header!)
(ModTime) Long time of last modification (UTC/GMT)
The central-header extra field contains the modification time only,
or no timestamp at all. TSize is used to flag its presence or
absence. But note:
If "Flags" indicates that Modtime is present in the local header
field, it MUST be present in the central header field, too!
This correspondence is required because the modification time
value may be used to support trans-timezone freshening and
updating operations with zip archives.
The time values are in standard Unix signed-long format, indicating
the number of seconds since 1 January 1970 00:00:00. The times
are relative to Coordinated Universal Time (UTC), also sometimes
referred to as Greenwich Mean Time (GMT). To convert to local time,
the software must know the local timezone offset from UTC/GMT.
The lower three bits of Flags in both headers indicate which time-
stamps are present in the LOCAL extra field:
bit 0 if set, modification time is present
bit 1 if set, access time is present
bit 2 if set, creation time is present
bits 3-7 reserved for additional timestamps; not set
Those times that are present will appear in the order indicated, but
any combination of times may be omitted. (Creation time may be
present without access time, for example.) TSize should equal
(1 + 4*(number of set bits in Flags)), as the block is currently
defined. Other timestamps may be added in the future.
-Info-ZIP Unix Extra Field (type 1):
==================================
The following is the layout of the old Info-ZIP extra block for
Unix. It has been replaced by the extended-timestamp extra block
(0x5455) and the Unix type 2 extra block (0x7855).
(Last Revision 19970118)
Local-header version:
Value Size Description
----- ---- -----------
(Unix1) 0x5855 Short tag for this extra block type ("UX")
TSize Short total data size for this block
AcTime Long time of last access (UTC/GMT)
ModTime Long time of last modification (UTC/GMT)
UID Short Unix user ID (optional)
GID Short Unix group ID (optional)
Central-header version:
Value Size Description
----- ---- -----------
(Unix1) 0x5855 Short tag for this extra block type ("UX")
TSize Short total data size for this block
AcTime Long time of last access (GMT/UTC)
ModTime Long time of last modification (GMT/UTC)
The file access and modification times are in standard Unix signed-
long format, indicating the number of seconds since 1 January 1970
00:00:00. The times are relative to Coordinated Universal Time
(UTC), also sometimes referred to as Greenwich Mean Time (GMT). To
convert to local time, the software must know the local timezone
offset from UTC/GMT. The modification time may be used by non-Unix
systems to support inter-timezone freshening and updating of zip
archives.
The local-header extra block may optionally contain UID and GID
info for the file. The local-header TSize value is the only
indication of this. Note that Unix UIDs and GIDs are usually
specific to a particular machine, and they generally require root
access to restore.
This extra field type is obsolete, but it has been in use since
mid-1994. Therefore future archiving software should continue to
support it. Some guidelines:
An archive member should either contain the old "Unix1"
extra field block or the new extra field types "time" and/or
"Unix2".
If both the old "Unix1" block type and one or both of the new
block types "time" and "Unix2" are found, the "Unix1" block
should be considered invalid and ignored.
Unarchiving software should recognize both old and new extra
field block types, but the info from new types overrides the
old "Unix1" field.
Archiving software should recognize "Unix1" extra fields for
timestamp comparison but never create it for updated, freshened
or new archive members. When copying existing members to a new
archive, any "Unix1" extra field blocks should be converted to
the new "time" and/or "Unix2" types.
-Info-ZIP Unix Extra Field (type 2):
==================================
The following is the layout of the new Info-ZIP extra block for
Unix. (Last Revision 19960922)
Local-header version:
Value Size Description
----- ---- -----------
(Unix2) 0x7855 Short tag for this extra block type ("Ux")
TSize Short total data size for this block (4)
UID Short Unix user ID
GID Short Unix group ID
Central-header version:
Value Size Description
----- ---- -----------
(Unix2) 0x7855 Short tag for this extra block type ("Ux")
TSize Short total data size for this block (0)
The data size of the central-header version is zero; it is used
solely as a flag that UID/GID info is present in the local-header
extra field. If additional fields are ever added to the local
version, the central version may be extended to indicate this.
Note that Unix UIDs and GIDs are usually specific to a particular
machine, and they generally require root access to restore.
-ASi Unix Extra Field:
====================
The following is the layout of the ASi extra block for Unix. The
local-header and central-header versions are identical.
(Last Revision 19960916)
Value Size Description
----- ---- -----------
(Unix3) 0x756e Short tag for this extra block type ("nu")
TSize Short total data size for this block
CRC Long CRC-32 of the remaining data
Mode Short file permissions
SizDev Long symlink'd size OR major/minor dev num
UID Short user ID
GID Short group ID
(var.) variable symbolic link filename
Mode is the standard Unix st_mode field from struct stat, containing
user/group/other permissions, setuid/setgid and symlink info, etc.
If Mode indicates that this file is a symbolic link, SizDev is the
size of the file to which the link points. Otherwise, if the file
is a device, SizDev contains the standard Unix st_rdev field from
struct stat (includes the major and minor numbers of the device).
SizDev is undefined in other cases.
If Mode indicates that the file is a symbolic link, the final field
will be the name of the file to which the link points. The file-
name length can be inferred from TSize.
[Note that TSize may incorrectly refer to the data size not counting
the CRC; i.e., it may be four bytes too small.]
-BeOS Extra Field:
================
The following is the layout of the file-attributes extra block for
BeOS. (Last Revision 19970531)
Local-header version:
Value Size Description
----- ---- -----------
(BeOS) 0x6542 Short tag for this extra block type ("Be")
TSize Short total data size for this block
BSize Long uncompressed file attribute data size
Flags Byte info bits
(CType) Short compression type
(CRC) Long CRC value for uncompressed file attribs
Attribs variable file attribute data
Central-header version:
Value Size Description
----- ---- -----------
(BeOS) 0x6542 Short tag for this extra block type ("Be")
TSize Short total data size for this block (5)
BSize Long size of uncompr. local EF block data
Flags Byte info bits
The least significant bit of Flags in both headers indicates whether
the LOCAL extra field is uncompressed (and therefore whether CType
and CRC are omitted):
bit 0 if set, Attribs is uncompressed (no CType, CRC)
bits 1-7 reserved; if set, assume error or unknown data
Currently the only supported compression types are deflated (type 8)
and stored (type 0); the latter is not used by Info-ZIP's Zip but is
supported by UnZip.
Attribs is a BeOS-specific block of data in big-endian format with
the following structure (if compressed, uncompress it first):
Value Size Description
----- ---- -----------
Name variable attribute name (null-terminated string)
Type Long attribute type (32-bit unsigned integer)
Size Long Long data size for this sub-block (64 bits)
Data variable attribute data
The attribute structure is repeated for every attribute. The Data
field may contain anything--text, flags, bitmaps, etc.
-AtheOS Extra Field:
==================
The following is the layout of the file-attributes extra block for
AtheOS. This field is a very close spin-off from the BeOS e.f.
The only differences are:
- a new extra field signature
- numeric field in the attributes data are stored in little-endian
format ("i386" was initial hardware for AtheOS)
(Last Revision 20040908)
Local-header version:
Value Size Description
----- ---- -----------
(AtheOS) 0x7441 Short tag for this extra block type ("At")
TSize Short total data size for this block
BSize Long uncompressed file attribute data size
Flags Byte info bits
(CType) Short compression type
(CRC) Long CRC value for uncompressed file attribs
Attribs variable file attribute data
Central-header version:
Value Size Description
----- ---- -----------
(AtheOS) 0x7441 Short tag for this extra block type ("At")
TSize Short total data size for this block (5)
BSize Long size of uncompr. local EF block data
Flags Byte info bits
The least significant bit of Flags in both headers indicates whether
the LOCAL extra field is uncompressed (and therefore whether CType
and CRC are omitted):
bit 0 if set, Attribs is uncompressed (no CType, CRC)
bits 1-7 reserved; if set, assume error or unknown data
Currently the only supported compression types are deflated (type 8)
and stored (type 0); the latter is not used by Info-ZIP's Zip but is
supported by UnZip.
Attribs is a AtheOS-specific block of data in little-endian format
with the following structure (if compressed, uncompress it first):
Value Size Description
----- ---- -----------
Name variable attribute name (null-terminated string)
Type Long attribute type (32-bit unsigned integer)
Size Long Long data size for this sub-block (64 bits)
Data variable attribute data
The attribute structure is repeated for every attribute. The Data
field may contain anything--text, flags, bitmaps, etc.
-SMS/QDOS Extra Field:
====================
The following is the layout of the file-attributes extra block for
SMS/QDOS. The local-header and central-header versions are identical.
(Last Revision 19960929)
Value Size Description
----- ---- -----------
(QDOS) 0xfb4a Short tag for this extra block type
TSize Short total data size for this block
LongID Long extra-field signature
(ExtraID) Long additional signature/flag bytes
QDirect 64 bytes qdirect structure
LongID may be "QZHD" or "QDOS". In the latter case, ExtraID will
be present. Its first three bytes are "02\0"; the last byte is
currently undefined.
QDirect contains the file's uncompressed directory info (qdirect
struct). Its elements are in native (big-endian) format:
d_length beLong file length
d_access byte file access type
d_type byte file type
d_datalen beLong data length
d_reserved beLong unused
d_szname beShort size of filename
d_name 36 bytes filename
d_update beLong time of last update
d_refdate beLong file version number
d_backup beLong time of last backup (archive date)
-AOS/VS Extra Field:
==================
The following is the layout of the extra block for Data General
AOS/VS. The local-header and central-header versions are identical.
(Last Revision 19961125)
Value Size Description
----- ---- -----------
(AOSVS) 0x5356 Short tag for this extra block type ("VS")
TSize Short total data size for this block
"FCI\0" Long extra-field signature
Version Byte version of AOS/VS extra block (10 = 1.0)
Fstat variable fstat packet
AclBuf variable raw ACL data ($MXACL bytes)
Fstat contains the file's uncompressed fstat packet, which is one of
the following:
normal fstat packet (P_FSTAT struct)
DIR/CPD fstat packet (P_FSTAT_DIR struct)
unit (device) fstat packet (P_FSTAT_UNIT struct)
IPC file fstat packet (P_FSTAT_IPC struct)
AclBuf contains the raw ACL data; its length is $MXACL.
-Tandem NSK Extra Field:
======================
The following is the layout of the file-attributes extra block for
Tandem NSK. The local-header and central-header versions are
identical. (Last Revision 19981221)
Value Size Description
----- ---- -----------
(TA) 0x4154 Short tag for this extra block type ("TA")
TSize Short total data size for this block (20)
NSKattrs 20 Bytes NSK attributes
-THEOS Extra Field:
=================
The following is the layout of the file-attributes extra block for
Theos. The local-header and central-header versions are identical.
(Last Revision 19990206)
Value Size Description
----- ---- -----------
(Theos) 0x6854 Short 'Th' signature
size Short size of extra block
flags Byte reserved for future use
filesize Long file size
fileorg Byte type of file (see below)
keylen Short key length for indexed and keyed files,
data segment size for 16 bits programs
reclen Short record length for indexed,keyed and direct,
text segment size for 16 bits programs
filegrow Byte growing factor for indexed,keyed and direct
protect Byte protections (see below)
reserved Short reserved for future use
File types
==========
0x80 library (keyed access list of files)
0x40 directory
0x10 stream file
0x08 direct file
0x04 keyed file
0x02 indexed file
0x0e reserved
0x01 16 bits real mode program (obsolete)
0x21 16 bits protected mode program
0x41 32 bits protected mode program
Protection codes
================
User protection
---------------
0x01 non readable
0x02 non writable
0x04 non executable
0x08 non erasable
Other protection
----------------
0x10 non readable
0x20 non writable
0x40 non executable Theos before 4.0
0x40 modified Theos 4.x
0x80 not hidden
-THEOS old inofficial Extra Field:
================================
The following is the layout of an inoffical former version of a
Theos file-attributes extra blocks. This layout was never published
and is no longer created. However, UnZip can optionally support it
when compiling with the option flag OLD_THEOS_EXTRA defined.
Both the local-header and central-header versions are identical.
(Last Revision 19990206)
Value Size Description
----- ---- -----------
(THS0) 0x4854 Short 'TH' signature
size Short size of extra block
flags Short reserved for future use
filesize Long file size
reclen Short record length for indexed,keyed and direct,
text segment size for 16 bits programs
keylen Short key length for indexed and keyed files,
data segment size for 16 bits programs
filegrow Byte growing factor for indexed,keyed and direct
reserved 3 Bytes reserved for future use
-FWKCS MD5 Extra Field (0x4b46):
==============================
The FWKCS Contents_Signature System, used in automatically
identifying files independent of filename, optionally adds
and uses an extra field to support the rapid creation of
an enhanced contents_signature.
There is no local-header version; the following applies
only to the central header. (Last Revision 19961207)
Central-header version:
Value Size Description
----- ---- -----------
(MD5) 0x4b46 Short tag for this extra block type ("FK")
TSize Short total data size for this block (19)
"MD5" 3 bytes extra-field signature
MD5hash 16 bytes 128-bit MD5 hash of uncompressed data
(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."
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. 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) Local headers should not span disk boundaries. Also, even
though the central directory can span disk boundaries, no
single record in the central directory should be split
across disks.
4) The entries in the central directory may not necessarily
be in the same order that files appear in the .ZIP file.
5) 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. Spanned/split archives
created with this special signature are compatible with
all versions of PKZIP from PKWARE. Split archives can
only be uncompressed by other versions of PKZIP that
know how to create a split archive.
6) 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.
7) 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.
V. 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.
VI. 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.
VII. 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
VIII. Tokenizing - Method 7
---------------------------
This method is not used by PKZIP.
IX. 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 - 257 (257 - 286)
All other codes are never sent.
5 Bits: # of Dist codes - 1 (1 - 32)
4 Bits: # of Bit Length codes - 4 (4 - 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 5 227-258
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
check data descriptor signature
read crc and sizes
endif
X. 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.
[This description is inofficial. It has been deduced by Info-ZIP from
close inspection of PKZIP 4.x Deflate64(tm) compressed output.]
The Deflate64 algorithm is almost identical to the normal Deflate algorithm.
Differences are:
- The sliding window size is 64k.
- The previously unused distance codes 30 and 31 are now used to describe
match distances from 32k-48k and 48k-64k.
Extra
Code Bits Distance
---- ---- -----------
.. .. ...
29 13 24577-32768
30 14 32769-49152
31 14 49153-65536
- The semantics of the "maximum match length" code #258 has been changed to
allow the specification of arbitrary large match lengths (up to 64k).
Extra
Code Bits Lengths
---- ---- ------
... .. ...
284 5 227-258
285 16 3-65538
Whereas the first two modifications fit into the framework of Deflate,
this last change breaks compatibility with Deflate method 8. Thus, a
Deflate64 decompressor cannot decode normal deflated data.
XI. 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.
XII. 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.
XIII. Decryption
----------------
The encryption used in PKZIP was generously supplied by Roger
Schlafly. PKWARE is grateful to Mr. Schlafly for his expert
help and advice in the field of data 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, or the high-order
byte of the file time if bit 3 of the general purpose bit flag is set.
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
XIV. Strong Encryption Specification (EFS)
------------------------------------------
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.
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 record 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.
The details of the strong encryption specification for certificates
remain under development as design and testing issues are worked out
for the range of algorithms, encryption methods, certificate processing
and cross-platform support necessary to meet the advanced security needs
of .ZIP file users today and in the future.
This feature specification is intended to support basic encryption needs
of today, such as password support. However this specification is also
designed to lay the foundation for future advanced security needs.
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)
0x6801 - RC4
0xFFFF - Unknown algorithm
Bitlen - Explicit bit length of key
40
56
64
112
128
168
192
256
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 preceeding 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)
0x6801 - RC4
0xFFFF - Unknown algorithm
Bitlen - Explicit bit length of key
40
56
64
112
128
168
192
256
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 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.
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. 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 - FFFFFFFFFFFFFFFF
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.
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 first 48 bytes will remain identical to that of Version 1.
The record signature for both Version 1 and Version 2 will be
0x06064b50. Immediately following the 48th byte, which identifies
the end 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
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.
5. 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.
A pseudo-code representation of the encryption process is as follows:
Password = GetUserPassword()
RD = Random()
ERD = Encrypt(RD,DeriveKey(SHA1(Password)))
For Each File
IV = Random()
VData = Random()
FileSessionKey = DeriveKey(SHA1(IV + RD))
Encrypt(VData + VCRC32 + FileData,FileSessionKey)
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.
Certificate Processing Method:
-----------------------------
The Certificate Processing Method for ZIP file encryption remains
under development. The information provided here serves as a guide
to those interested in certificate-based data decryption. This
information may be subject to change in future versions of this
specification and is subject to change without notice.
OAEP Processing with Certificate-based Encryption:
Versions of PKZIP available during this development phase of the
certificate processing method may 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.
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 Smart Cards 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.
Certificate Processing Data Fields:
The Certificate Processing Method of this specification 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.
5. 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.
6. 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.
License Agreement:
-----------------
The features set forth in this Section XIV (the "Strong Encryption
Specification") are covered by a pending patent application. Portions of
this Strong Encryption technology are available for use at no charge
under the following terms and conditions.
1. License Grant.
a. NOTICE TO USER. PLEASE READ THIS ENTIRE SECTION XIV OF THE
APPNOTE (THE "AGREEMENT") CAREFULLY. BY USING ALL OR ANY PORTION OF THE
LICENSED TECHNOLOGY, YOU ACCEPT ALL THE TERMS AND CONDITIONS OF THIS
AGREEMENT AND YOU AGREE THAT THIS AGREEMENT IS ENFORCEABLE LIKE ANY
WRITTEN NEGOTIATED AGREEMENT SIGNED BY YOU. IF YOU DO NOT AGREE, DO NOT
USE THE LICENSED TECHNOLOGY.
b. Definitions.
i. "Licensed Technology" shall mean that proprietary technology now or
hereafter owned or controlled by PKWare, Inc. ("PKWARE") or any
subsidiary or affiliate that covers or is necessary to be used to give
software the ability to a) extract and decrypt data from zip files
encrypted using any methods of data encryption and key processing which
are published in this APPNOTE or any prior APPNOTE, as supplemented by
any Additional Compatibility Information; and b) encrypt file contents
as part of .ZIP file processing using only the Single Password Symmetric
Encryption Method as published in this APPNOTE or any prior APPNOTE, as
supplemented by any Additional Compatibility Information. For purposes
of this AGREEMENT, "Additional Compatibility Information" means, with
regard to any method of data encryption and key processing published in
this or any prior APPNOTE, any corrections, additions, or clarifications
to the information in such APPNOTE that are required in order to give
software the ability to successfully extract and decrypt zip files (or,
but solely in the case of the Single Password Symmetric Encryption Method,
to successfully encrypt zip files) in a manner interoperable with the
actual implementation of such method in any PKWARE product that is
documented or publicly described by PKWARE as being able to create, or
to extract and decrypt, zip files using that method.
ii. "Licensed Products" shall mean any products you produce that
incorporate the Licensed Technology.
c. License to Licensed Technology.
PKWARE hereby grants to you a non-exclusive license to use the Licensed
Technology for the purpose of manufacturing, offering, selling and using
Licensed Products, which license shall extend to permit the practice of all
claims in any patent or patent application (collectively, "Patents") now or
hereafter owned or controlled by PKWARE in any jurisdiction in the world
that are infringed by implementation of the Licensed Technology. You have
the right to sublicense rights you receive under the terms of this AGREEMENT
for the purpose of allowing sublicensee to manufacture, offer, sell and use
products that incorporate all or a portion of any of your Licensed Products,
but if you do, you agree to i) impose the same restrictions on any such
sublicensee as these terms impose on you and ii) notify the sublicensee,
by means chosen by you in your unfettered discretion, including a notice on
your web site, of the terms of this AGREEMENT and make available to each
sublicensee the full text of this APPNOTE. Further, PKWARE hereby grants to
you a non-exclusive right to reproduce and distribute, in any form, copies of
this APPNOTE, without modification. Notwithstanding anything to the contrary
in this AGREEMENT, you have the right to sublicense the rights, without any of
the restrictions described above or elsewhere in this AGREEMENT, to use, offer
to sell and sell Licensed Technology as incorporated in executable object code
or byte code forms of your Licensed Products. Any sublicense to use the
Licensed Technology incorporated in a Licensed Product granted by you shall
survive the termination of this AGREEMENT for any reason. PKWARE warrants that
this license shall continue to encumber the Licensed Technology regardless of
changes in ownership of the Licensed Technology.
d. Proprietary Notices.
i. With respect to any Licensed Product that is distributed by you either
in source code form or in the form of an object code library of externally
callable functions that has been designed by you for incorporation into third
party products, you agree to include, in the source code, or in the case of
an object code library, in accompanying documentation, a notice using the
words "patent pending" until a patent is issued to PKWARE covering any
portion of the Licensed Technology or PKWARE provides notice, by means
chosen by PKWARE in its unfettered discretion, that it no longer has any
patent pending covering any portion of the Licensed Technology. With respect
to any Licensed Product, upon your becoming aware that at least one patent has
been granted covering the Licensed Technology, you agree to include in any
revisions made by you to the documentation (or any source code distributed
by you) the words "Pat. No.", or "Patent Number" and the patent number or
numbers of the applicable patent or patents. PKWARE shall, from time to time,
inform you of the patent number or numbers of the patents covering the
Licensed Technology, by means chosen by PKWARE in its unfettered discretion,
including a notice on its web site. It shall be a violation of the terms of
this AGREEMENT for you to sell Licensed Products without complying with the
foregoing marking provisions.
ii. You acknowledge that the terms of this AGREEMENT do not grant you any
license or other right to use any PKWARE trademark in connection with the sale,
offering for sale, distribution and delivery of the Licensed Products, or in
connection with the advertising, promotion and offering of the Licensed Products.
You acknowledge PKWARE's ownership of the PKZIP trademark and all other marks
owned by PKWARE.
e. Covenant of Compliance and Remedies.
To the extent that you have elected to implement portions of the Licensed
Technology, you agree to use reasonable diligence to comply with those portions
of this Section XIV, as modified or supplemented by Additional Compatibility
Information available to you, describing the portions of the Licensed Technology
that you have elected to implement. Upon reasonable request by PKWARE, you will
provide written notice to PKWARE identifying which version of this APPNOTE you
have relied upon for your implementation of any specified Licensed Product.
If any substantial non-compliance with the terms of this AGREEMENT is determined
to exist, you will make such changes as necessary to bring your Licensed Products
into substantial compliance with the terms of this AGREEMENT. If, within sixty
days of receipt of notice that a Licensed Product fails to comply with the terms
of this AGREEMENT, you fail to make such changes as necessary to bring your
Licensed Products into compliance with the terms of this AGREEMENT, PKWARE may
terminate your rights under this AGREEMENT. PKWARE does not waive and expressly
reserves the right to pursue any and all additional remedies that are or may
become available to PKWARE.
f. Warranty and Indemnification Regarding Exportation.
You realize and acknowledge that, as between yourself and PKWARE, you are fully
responsible for compliance with the import and export laws and regulations of
any country in or to which you import or export any Licensed Products, and you
agree to hold PKWARE harmless from any claim of violation of any such import
or export laws.
g. Patent Infringement.
You agree that you will not bring or threaten to bring any action against PKWARE
for infringement of the claims of any patent owned or controlled by you solely
as a result of PKWARE's own implementation of the Licensed Technology. As its
exclusive remedy for your breach of the foregoing agreement, PKWARE reserves
the right to suspend or terminate all rights granted under the terms of this
AGREEMENT if you bring or threaten to bring any such action against PKWARE,
effective immediately upon delivery of written notice of suspension or
termination to you.
h. Governing Law.
The license granted in this AGREEMENT shall be governed by and construed under
the laws of the State of Wisconsin and the United States.
i. Revisions and Notice.
The license granted in this APPNOTE is irrevocable, except as expressly set
forth above. You agree and understand that any changes which PKWARE determines
to make to this APPNOTE shall be posted at the same location as the current
APPNOTE or at a location which will be identified by means chosen by PKWARE,
including a notice on its web site, and shall be available for adoption by you
immediately upon such posting, or at such other time as PKWARE shall determine.
Any changes to the terms of the license published in a subsequent version of
this AGREEMENT shall be binding upon you only with respect to your products
that (i) incorporate any Licensed Technology (as defined in the subsequent
AGREEMENT) that is not otherwise included in the definition of Licensed
Technology under this AGREEMENT, or (ii) that you expressly identify are to
be licensed under the subsequent AGREEMENT, which identification shall be by
written notice with reference to the APPNOTE (version and release date or other
unique identifier) in which the subsequent AGREEMENT is published. PKWARE
agrees to identify each change to this APPNOTE by using a unique version and
release date identifier or other unique identifier.
j. Warranty by PKWARE
PKWare, Inc. warrants that it has the right to grant the license hereunder.
XV. 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.
XVI. 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.
XVII. 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.