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Note: This application note is also
available as an on-line
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File:

APPNOTE.TXT - .ZIP File Format Specification

Version:

4.5

Revised:

11/01/2001

Copyright © 1989 - 2001 PKWARE Inc., All Rights
Reserved.

Disclaimer

General
Format of a .ZIP file

Local
file header

File
data

Data
descriptor

Central
directory structure

Zip64
end of central directory record

Zip64
end of central directory locator

End
of central directory record

Explanation
of fields

General
notes

UnShrinking
- Method 1

Expanding
- Methods 2-5

Imploding
- Method 6

Tokenizing
- Method 7

Deflating
- Method 8

Decryption

Change
Process

Acknowledgements

References

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 zipfiles can span
multiple diskette media or be split into user-defined segment
sizes.

Overall zipfile format:

[local file header1]
[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]

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)

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.

Data descriptor:

crc-32

4 bytes

compressed size

4 bytes

uncompressed size

4 bytes

This descriptor exists only if bit 3 of the general purpose
bit flag is set (see below). It is byte aligned and
immediately follows the last byte of compressed data. This
descriptor is used only when it was not possible to seek in
the output zip file, e.g., when the output zip file was
standard output or a non seekable device. For Zip64 format
archives, the compressed and uncompressed sizes are 8 bytes
each.

Central directory structure:

[file header 1]
.
.
.
[file header
n]
[digital signature]
[end of central directory
record]

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)

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)

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

End of central dir 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)

Explanation of fields:

version made by (2 bytes)

The upper byte indicates the compatibility of the file
attribute information. If the external file attributes are
compatible with MS-DOS and can be read by PKZIP for DOS
version 2.04g then this value will be zero. If these
attributes are not compatible, then this value will identify
the host system on which the attributes are compatible.
Software can use this information to determine the line record
format for text files etc. The current mappings are:

0 -

MS-DOS and OS/2 (FAT / VFAT / FAT32 file
systems)

1 -

Amiga

2 -

OpenVMS

3 -

UNIX

4 -

VM/CMS

5 -

Atari ST

6 -

OS/2 H.P.F.S.

7 -

Macintosh

8 -

Z-System

9 -

CP/M

10 -

Windows NTFS

11 -

MVS

12 -

VSE

13 -

Acorn Risc

14 -

VFAT

15 -

Alternate MVS

16 -

BeOS

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
an 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.)

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 Date 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.

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.

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.

extra field: (Variable)

This is for future expansion. If additional information
needs to be stored in the future, 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:

0x0007

AV Info

0x0009

OS/2

0x000A

NTFS

0x000c

OpenVMS

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

Third party mappings commonly used are:

0x0065

IBM S/390 attributes - uncompressed

0x0066

IBM S/390 attributes - compressed

0x4b46

FWKCS MD5 (see below)

0x07c8

Macintosh

0x4341

Acorn/SparkFS

0x4453

Windows NT security descriptor (binary ACL)

0x4704

VM/CMS

0x470f

MVS

0x4c41

OS/2 access control list (text ACL)

0x4d49

Info-ZIP OpenVMS

0x5455

extended timestamp

0x5855

Info-ZIP UNIX (original, also OS/2, NT, etc)

0x6542

BeOS/BeBox

0x756e

ASi UNIX

0x7855

Info-ZIP UNIX (new)

0xfd4a

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.

-OS/2 Extra Field:

The following is the layout of the OS/2 attributes "extra"
block. (Last Revision 09/05/95)

Note: all fields stored in Intel
low-byte/high-byte order.

Value

Size

Description

0x0009

2 bytes

Tag for this "extra" block type

TSize

2 bytes

Size for the following data block

BSize

Long

Uncompressed Block Size

CType

2 bytes

Compression type

EACRC

Long

CRC value for uncompress block

(var)

variable

Compressed block

The OS/2 extended attribute structure (FEA2LIST) is
compressed and then stored in it's entirety within this
structure. There will only ever be one "block" of data in
VarFields[].

-UNIX Extra Field:

The following is the layout of the UNIX "extra" block.

Note: all fields are stored in Intel
low-byte/high-byte order.

Value

Size

Description

0x000d

2 bytes

Tag for this "extra" block type

TSize

2 bytes

Size for the following data block

Atime

4 bytes

File last access time

Mtime

4 bytes

File last modification time

Uid

2 bytes

File user ID

Gid

2 bytes

File group ID

(var)

variable

Variable length data field

The variable length data field will contain file type
specific data. Currently the only values allowed are the
original "linked to" file names for hard or symbolic
links.

-OpenVMS Extra Field:

The following is the layout of the OpenVMS attributes
"extra" block.

Note: all fields stored in Intel
low-byte/high-byte order.

Value

Size

Description

0x000c

2 bytes

Tag for this "extra" block type

TSize

2 bytes

Size of the total "extra" block

CRC

4 bytes

32-bit CRC for remainder of the block

Tag1

2 bytes

VMS attribute tag value #1

Size1

2 bytes

Size of attribute #1, in bytes

(var.)

Size1

Attribute #1 data

.
.
.

TagN

2 bytes

VMS attribute tage value #N

SizeN

2 bytes

Size of attribute #N, in bytes

(var.)

SizeN

Attribute #N data

Rules:

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.

No word alignment or padding is performed.

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.

-NTFS Extra Field:

The following is the layout of the NTFS attributes "extra"
block.

Note: At this time, the Mtime, Atime and
Ctime values may be used on any Win32 system.

Value

Size

Description

0x000a

2 bytes

Tag for this "extra" block type

TSize

2 bytes

Size of the total "extra" block

Reserved

4 bytes

Reserved for future use

Tag1

2 bytes

NTFS attribute tag value #1

Size1

2 bytes

Size of attribute #1, in bytes

(var.)

Size1

Attribute #1 data

.
.
.

TagN

2 bytes

NTFS attribute tag value #N

SizeN

2 bytes

Size of attribute #N, in bytes

(var.)

SizeN

Attribute #N data

For NTFS, values for Tag1 through TagN are as follows:
(currently only one set of attributes is defined for NTFS)

Tag

Size

Description

0x0001

2 bytes

Tag for attribute #1

Size1

2 bytes

Size of attribute #1, in bytes

Mtime

8 bytes

File last modification time

Atime

8 bytes

File last access time

Ctime

8 bytes

File creation time

-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

0x000f

2 bytes

Tag for this "extra" block type

TSize

2 bytes

Size of the total "extra" block

Version

2 bytes

Version of the descriptor

Flags

4 bytes

Actions and reactions (see below)

OldSize

4 bytes

Size of the file about to be patched

OldCRC

4 bytes

32-bit CRC of the file about to be patched

NewSize

4 bytes

Size of the resulting file

NewCRC

4 bytes

32-bit CRC of the resulting file

Actions and reactions

Bits

Description

0

Use for 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

Note: all fields stored in Intel
low-byte/high-byte order.

Value

Size

Description

0x0014

2 bytes

Tag for this "extra" block type

SSize

2 bytes

Size of the stored data

SData

(variable)

Data about the store

SData
Value

Size

Description

Version

2 bytes

Version number

StoreD

(variable)

Actual store data

-X.509 Certificate ID and Signature for individual
file

Note: all fields stored in Intel
low-byte/high-byte order.

Value

Size

Description

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

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

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

-X.509 Certificate ID and Signature for central
directory

Note: all fields stored in Intel
low-byte/high-byte order.

Value

Size

Description

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

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.

- FWKCS MD5 Extra Field:

The FWKCS Contents_Signature System, used in automatically
identifying files independent of file name, optionally adds
and uses an extra field to support the rapid creation of an
enhanced contents_signature:

Header ID

= 0x4b46

Data Size

= 0x0013

Preface

= 'M','D','5'

followed by 16 bytes containing the uncompressed file's
128_bit MD5 hash(1),
low byte first.

When FWKCS revises a zipfile 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 zipfile central directory. In adding this
extra field, FWKCS preserves Zipfile 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)

General notes:

All fields unless otherwise noted are unsigned and stored
in Intel low-byte:high-byte, low-word:high-word order.

String fields are not null terminated, since the length is
given explicitly.

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.

The entries in the central directory may not necessarily
be in the same order that files appear in the .ZIP file.

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 segement. 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.

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.

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:

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.

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:

Sort the Bit Lengths in ascending order, while retaining the
order of the original lengths stored in the file.

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

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).

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:

Val

Sorted

Constructed Code

Reversed
Value

Order
Restored

Original
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 - 256
All other
codes are never sent

(256 - 286)

5 Bits:

# of Dist codes - 1

(1 - 32)

4 Bits:

# of Bit Length codes - 3

(3 - 19)

The Huffman codes are sent as bit lengths and the codes are
built as described in the implode algorithm. The bit lengths
themselves are compressed with Huffman codes. There are 19 bit
length codes:

0 - 15:

Represent bit lengths of 0 - 15

16:

Copy the previous bit length 3 - 6
times.
The next 2 bits indicate repeat length (0 = 3, ...
,3 = 6)
Example: Codes 8, 16 (+2 bits 11), 16 (+2 bits
10) will expand to 12 bit lengths of 8 (1 + 6 + 5)

17:

Repeat a bit length of 0 for 3 - 10 times. (3
bits of length)

18:

Repeat a bit length of 0 for 11 - 138 times
(7 bits of length)

The lengths of the bit length codes are sent packed 3 bits per
value (0 - 7) in the following order:

16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1,
15

The Huffman codes should be built as described in the Implode
algorithm except codes are assigned starting at the shortest bit
length, i.e. the shortest code should be all 0's rather than all
1's. Also, codes with a bit length of zero do not participate in
the tree construction. The codes are then used to decode the bit
lengths for the literal and distance tables.

The bit lengths for the literal tables are sent first with the
number of entries sent described by the 5 bits sent earlier. There
are up to 286 literal characters; the first 256 represent the
respective 8 bit character, code 256 represents the End-Of-Block
code, the remaining 29 codes represent copy lengths of 3 thru 258.
There are up to 30 distance codes representing distances from 1
thru 32k as described below.

Length
Codes

Code

Extra
Bits

Length

Code

Extra
Bits

Lengths

Code

Extra
Bits

Lengths

Code

Extra
Bits

Length(s)

257

0

3

265

1

11,12

273

3

35-42

281

5

131-162

258

0

4

266

1

13,14

274

3

43-50

282

5

163-194

259

0

5

267

1

15,16

275

3

51-58

283

5

195-226

260

0

6

268

1

17,18

276

3

59-66

284

5

227-257

261

0

7

269

2

19-22

277

4

67-82

285

0

258

262

0

8

270

2

23-26

278

4

83-98

263

0

9

271

2

27-30

279

4

99-114

264

0

10

272

2

31-34

280

4

115-130

Distance
Codes

Code

Extra
Bits

Distance

Code

Extra
Bits

Distance

Code

Extra
Bits

Distance

Code

Extra
Bits

Distance

0

0

1

8

3

17-24

16

7

257-384

24

11

4097-6144

1

0

2

9

3

25-32

17

7

385-512

25

11

6145-8192

2

0

3

10

4

33-48

18

8

513-768

26

12

8193-12288

3

0

4

11

4

49-64

19

8

769-1024

27

12

12289-16384

4

1

5,6

12

5

65-96

20

9

1025-1536

28

13

16385-24576

5

1

7,8

13

5

97-128

21

9

1537-2048

29

13

24577-32768

6

2

9-12

14

6

129-192

22

10

2049-3072

7

2

13-16

15

6

193-256

23

10

3073-4096

The compressed data stream begins immediately after the
compressed header data. The compressed data stream can be
interpreted as follows:

do
read header from input stream.

if stored block
skip bits until byte aligned
read count and 1's compliment of count
copy count bytes data block
otherwise
loop until end of block code sent
decode literal character from input stream
if literal < 256
copy character to the output stream
otherwise
if literal = end of block
break from loop
otherwise
decode distance from input stream

move backwards distance bytes in the output stream, and
copy length characters from this position to the
output stream.
end loop
while not last block

if data descriptor exists
skip bits until byte aligned
read crc and sizes
endif

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:

Initialize the three 32-bit keys with the password.

Read and decrypt the 12-byte encryption header, further
initializing the encryption keys.

Read and decrypt the compressed data stream using the
encryption keys.

Step 1 - Initializing the encryption keys

Key(0) <- 305419896
Key(1) <- 591751049
Key(2) <- 878082192

loop for i <- 0 to length(password)-1
update_keys(password(i))
end loop

Where update_keys() is defined as:

update_keys(char):
Key(0) <- crc32(key(0),char)
Key(1) <- Key(1) + (Key(0) & 000000ffH)
Key(1) <- Key(1) * 134775813 + 1
Key(2) <- crc32(key(2),key(1) >> 24)
end update_keys

Where crc32(old_crc,char) is a routine that given
a CRC value and a character, returns an updated CRC value after
applying the CRC-32 algorithm described elsewhere in this
document.

Step 2 - Decrypting the encryption header

The purpose of this step is to further initialize the
encryption keys, based on random data, to render a plaintext
attack on the data ineffective.

Read the 12-byte encryption header into Buffer, in locations
Buffer(0) thru Buffer(11).

loop for i <- 0 to 11
C <- buffer(i) ^ decrypt_byte()
update_keys(C)
buffer(i) <- C
end loop

Where decrypt_byte() is defined as:

unsigned char decrypt_byte()
local unsigned short temp
temp <- Key(2) | 2
decrypt_byte <- (temp * (temp ^ 1)) >> 8
end decrypt_byte

After the header is decrypted, the last 1 or 2 bytes in Buffer
should be the high-order word/byte of the CRC for the file being
decrypted, stored in Intel low-byte/high-byte order. Versions of
PKZIP prior to 2.0 used a 2 byte CRC check; a 1 byte CRC check is
used on versions after 2.0. This can be used to test if the
password supplied is correct or not.

Step 3 - Decrypting the compressed data stream

The compressed data stream can be decrypted as follows:

loop until done
read a character into C
Temp <- C ^ decrypt_byte()
update_keys(temp)
output Temp
end loop

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 [email-masked]. 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.

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