[Info-ZIP note, 20011203: this file is based on PKWARE's appnote.txt of
15 February 1996, taking into account PKWARE's revised appnote.txt
version 4.5 of 01 November 2001. 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.]
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 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.
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.
Overall .ZIP file format:
[local file header 1]
[file data 1]
[data descriptor 1]
.
.
.
[local file header n]
[file data n]
[data descriptor n]
[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. 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)
E. 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)
F. 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
G. 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)
H. 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 thru 255 - unused
The lower byte indicates the version number of 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 software version needed to extract the
file, mapped as above. For Zip64 format archives,
this value should not be less than 45.
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" meets 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: Currently unused.
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: Reserved by PKWARE.
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
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.
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.
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.
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)
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.
Bits 1 and 2 are reserved for use by PKWARE.
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.
[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
0x0009 OS/2 extended attributes (also Info-ZIP)
0x000a NTFS (Win9x/WinNT FileTimes)
0x000c OpenVMS (also Info-ZIP)
0x000d Unix
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
The Header ID mappings defined by Info-ZIP and third parties are:
0x0065 IBM S/390 attributes - uncompressed
0x0066 IBM S/390 attributes - compressed
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)
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
0x5855 Info-ZIP Unix (original; also OS/2, NT, etc.)
0x554e Xceed unicode extra field
0x6542 BeOS (BeBox, PowerMac, etc.)
0x6854 Theos
0x756e ASi Unix
0x7855 Info-ZIP Unix (new)
0xfb4a SMS/QDOS
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.
-OS/2 Extended Attributes Extra Field:
====================================
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:
==========================================
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:
==============================
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.) SubSize1 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:
==========================
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):
===================================
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 that
do not need a "full Mac filename" record.
The local-header and central-header versions are identical.
(Last Revision 19980903)
Value Size Description
----- ---- -----------
(Mac2b) 0x2705 Short tag for this extra block type
TSize Short total data size for this block (12)
"ZPIT" beLong extra-field signature
FileType Byte[4] four-byte Mac file type string
Creator Byte[4] four-byte Mac creator string
-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:
========================
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:
============================
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 autodetection
1 Treat as selfpatch
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
-PKCS#7 Store for X.509 Certificates:
===================================
This field is contains the information about each
certificate a file is signed with. This field should only
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
SSize 2 bytes Size of the store data
SData (variable) 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:
======================================================
This field contains the information about which certificate
in the PKCS#7 Store was used to sign the 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:
========================================================
This field contains the information about which certificate
in the PKCS#7 Store was used to sign the central directory.
It should only appear with 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
CSize 2 bytes Size of Method
Method (variable)
-ZIP64 Extended Information Extra Field:
======================================
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 uncompresseed 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.
-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.
-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:
=====================
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.
zip64 extensible data sector (variable size)
(currently reserved for use by PKWARE)
I. 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.
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.
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.
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
Tokenizing - Method 7
--------------------
This method is not used by PKZIP.
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
Deflate64 - Method 9
--------------------
[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.
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
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.
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.
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.