Overview

This document describes the on-disk structure of a PKZip (Zip) file. The documentation currently only describes the file layout format and meta information but does not address the actual compression or encryption of the file data itself. This documentation also does not discuss Zip archives that span multiple files in great detail. This documentation was created using the official documentation provided by PKWare Inc.

General structure

Each Zip file is structured in the following manner:

The archive consists of a series of local file descriptors, each containing a local file header, the actual compressed and/or encrypted data, as well as an optional data descriptor. Whether a data descriptor exists or not depends on a flag in the local file header.

Following the file descriptors is the archive decryption header, which only exists in PKZip file version 6.2 or greater. This header is only present if the central directory is encrypted and contains information about the encryption specification. The archive extra data record is also only for file of version 6.2 or greater and is not present in all Zip files. It is used in to support the encryption or compression of the central directory.

The central directory summarizes the local file descriptors and carries additional information regarding file attributes, file comments, location of the local headers, and multi-file archive information.

Local file headers

Each local file header has the following structure:

Signature The signature of the local file header. This is always '\x50\x4b\x03\x04'.
Version PKZip version needed to extract
Flags General purpose bit flag:
Bit 00: encrypted file
Bit 01: compression option 
Bit 02: compression option 
Bit 03: data descriptor
Bit 04: enhanced deflation
Bit 05: compressed patched data
Bit 06: strong encryption
Bit 07-10: unused
Bit 11: language encoding
Bit 12: reserved
Bit 13: mask header values
Bit 14-15: reserved
Compression method 00: no compression
01: shrunk
02: reduced with compression factor 1
03: reduced with compression factor 2 
04: reduced with compression factor 3 
05: reduced with compression factor 4 
06: imploded
07: reserved
08: deflated
09: enhanced deflated
10: PKWare DCL imploded
11: reserved
12: compressed using BZIP2
13: reserved
14: LZMA
15-17: reserved
18: compressed using IBM TERSE
19: IBM LZ77 z
98: PPMd version I, Rev 1 
File modification time stored in standard MS-DOS format:
Bits 00-04: seconds divided by 2 
Bits 05-10: minute
Bits 11-15: hour
File modification date stored in standard MS-DOS format:
Bits 00-04: day
Bits 05-08: month
Bits 09-15: years from 1980
Crc-32 checksum value computed over file data by CRC-32 algorithm with 'magic number' 0xdebb20e3 (little endian)
Compressed size if archive is in ZIP64 format, this filed is 0xffffffff and the length is stored in the extra field
Uncompressed size if archive is in ZIP64 format, this filed is 0xffffffff and the length is stored in the extra field
File name length the length of the file name field below
Extra field length the length of the extra field below
File name the name of the file including an optional relative path. All slashes in the path should be forward slashes '/'.
Extra field Used to store additional information. The field consistes of a sequence of header and data pairs, where the header has a 2 byte identifier and a 2 byte data size field.

Example

Our sample zip file starts with a local file header:

00000000  50 4b 03 04 14 00 00 00  08 00 1c 7d 4b 35 a6 e1  |PK.........}K5..|
00000010  90 7d 45 00 00 00 4a 00  00 00 05 00 15 00 66 69  |.}E...J.......fi|
00000020  6c 65 31 55 54 09 00 03  c7 48 2d 45 c7 48 2d 45  |le1UT....H-E.H-E|
00000030  55 78 04 00 f5 01 f5 01  0b c9 c8 2c 56 00 a2 92  |Ux.........,V...|

This results in the following fields and field values:

Signature '\x50\x4b\x03\x04'.
Version 0x14 = 20 -> 2.0
Flags no flags
Compression method 08: deflated
File modification time 0x7d1c = 0111110100011100 
hour = (01111)10100011100 = 15 
minute = 01111(101000)11100 = 40
second = 01111101000(11100) = 28 = 56 seconds 
15:40:56
File modification date 0x354b = 0011010101001011
year = (0011010)101001011 = 26
month = 0011010(1010)01011 = 10
day = 00110101010(01011) = 11 
10/11/2006
Crc-32 checksum 0x7d90e1a6
Compressed size 0x45 = 69 bytes
Uncompressed size 0x4a = 74 bytes
File name length 5 bytes
Extra field length 21 bytes
File name "file1"
Extra field id 0x5455: extended timestamp, size: 9 bytes 
Id 0x7855: Info-ZIP UNIX, size: 4 bytes

Data descriptor

The data descriptor is only present if bit 3 of the bit flag field is set. In this case, the CRC-32, compressed size, and uncompressed size fields in the local header are set to zero. The data descriptor field is byte aligned and immediately follows the file data. The structure is as follows:

The example file does not contain a data descriptor.

Archive decryption header

This header is used to support the Central Directory Encryption Feature. It is present when the central directory is encrypted. The format of this data record is identical to the Decryption header record preceding compressed file data.

Archive extra data record

This header is used to support the Central Directory Encryption Feature. When present, this record immediately precedes the central directory data structure. The size of this data record will be included in the Size of the Central Directory field in the End of Central Directory record. The structure is as follows:

Central directory

The central directory contains more metadata about the files in the archive and also contains encryption information and information about Zip64 (64-bit zip archives) archives. Furthermore, the central directory contains information about archives that span multiple files. The structure of the central directory is as follows:

The file headers are similar to the local file headers, but contain some extra information. The Zip64 entries handle the case of a 64-bit Zip archive, and the end of the central directory record contains information about the archive itself.

Central directory file header

The structure of the file header in the central directory is as follows:

Signature The signature of the file header. This is always '\x50\x4b\x01\x02'.
Version Version made by:

upper byte: 
0 - MS-DOS and OS/2 (FAT / VFAT / FAT32 file systems)
1 - Amiga 
2 - OpenVMS
3 - UNIX 
4 - VM/CMS
5 - Atari ST
6 - OS/2 H.P.F.S.
7 - Macintosh 
8 - Z-System
9 - CP/M 
10 - Windows NTFS
11 - MVS (OS/390 - Z/OS) 
12 - VSE
13 - Acorn Risc 
14 - VFAT
15 - alternate MVS 
16 - BeOS
17 - Tandem 
18 - OS/400
19 - OS/X (Darwin) 
20 - 255: unused

lower byte:
zip specification version
Vers. needed

PKZip version needed to extract
Flags General purpose bit flag:
Bit 00: encrypted file
Bit 01: compression option 
Bit 02: compression option 
Bit 03: data descriptor
Bit 04: enhanced deflation
Bit 05: compressed patched data
Bit 06: strong encryption
Bit 07-10: unused
Bit 11: language encoding
Bit 12: reserved
Bit 13: mask header values
Bit 14-15: reserved
Compression method 00: no compression
01: shrunk
02: reduced with compression factor 1
03: reduced with compression factor 2 
04: reduced with compression factor 3 
05: reduced with compression factor 4 
06: imploded
07: reserved
08: deflated
09: enhanced deflated
10: PKWare DCL imploded
11: reserved
12: compressed using BZIP2
13: reserved
14: LZMA
15-17: reserved
18: compressed using IBM TERSE
19: IBM LZ77 z
98: PPMd version I, Rev 1 
File modification time stored in standard MS-DOS format:
Bits 00-04: seconds divided by 2 
Bits 05-10: minute
Bits 11-15: hour
File modification date stored in standard MS-DOS format:
Bits 00-04: day
Bits 05-08: month
Bits 09-15: years from 1980
Crc-32 checksum value computed over file data by CRC-32 algorithm with 'magic number' 0xdebb20e3 (little endian)
Compressed size if archive is in ZIP64 format, this filed is 0xffffffff and the length is stored in the extra field
Uncompressed size if archive is in ZIP64 format, this filed is 0xffffffff and the length is stored in the extra field
File name length the length of the file name field below
Extra field length the length of the extra field below
File comm. len the length of the file comment
Disk # start the number of the disk on which this file exists
Internal attr.

Internal file attributes:
Bit 0: apparent ASCII/text file
Bit 1: reserved
Bit 2: control field records precede logical records
Bits 3-16: unused
External attr. External file attributes:
host-system dependent
Offset of local header Relative offset of local header. This is the offset of where to find the corresponding local file header from the start of the first disk.
File name the name of the file including an optional relative path. All slashes in the path should be forward slashes '/'.
Extra field Used to store additional information. The field consistes of a sequence of header and data pairs, where the header has a 2 byte identifier and a 2 byte data size field.
File comment An optional comment for the file.

Example:

The corresponding file header from our local file header example above starts at byte 0x9a2 in the example file:

000009a0  28 f0 50 4b 01 02 17 03  14 00 00 00 08 00 1c 7d  |(.PK...........}|
000009b0  4b 35 a6 e1 90 7d 45 00  00 00 4a 00 00 00 05 00  |K5...}E...J.....|
000009c0  0d 00 1c 00 00 00 01 00  00 00 a4 81 00 00 00 00  |................|
000009d0  66 69 6c 65 31 55 54 05  00 03 c7 48 2d 45 55 78  |file1UT....H-EUx|
000009e0  00 00 74 68 69 73 20 69  73 20 61 20 63 6f 6d 6d  |..this is a comm|
000009f0  65 6e 74 20 66 6f 72 20  66 69 6c 65 20 31 50 4b  |ent for file 1PK|

Signature '\x50\x4b\x01\x02'.
Version 0x0317
upper byte: 03 -> UNIX
lower byte: 23 -> 2.3
Version needed 0x14 = 20 -> 2.0
Flags no flags
Compression method 08: deflated
File modification time 0x7d1c = 0111110100011100 
hour = (01111)10100011100 = 15 
minute = 01111(101000)11100 = 40
second = 01111101000(11100) = 28 = 56 seconds 
15:40:56
File modification date 0x354b = 0011010101001011
year = (0011010)101001011 = 26
month = 0011010(1010)01011 = 10
day = 00110101010(01011) = 11 
10/11/2006
Crc-32 checksum 0x7d90e1a6
Compressed size 0x45 = 69 bytes
Uncompressed size 0x4a = 74 bytes
File name length 5 bytes
Extra field length 13 bytes
File comment length 28 bytes
Disk # start 0
Internal attributes Bit 0 set: ASCII/text file
External attributes 0x81a40000
Offset of local header 0
File name "file1"
Extra field id 0x5455: extended timestamp, size: 5 bytes 
Id 0x7855: Info-ZIP UNIX, size: 0 bytes
File comment "this is a comment for file 1"

End of central directory record

The structure of the end of central directory record is as follows:

Signature The signature of end of central directory record. This is always '\x50\x4b\x05\x06'.
Disk Number The number of this disk (containing the end of central directory record)
Disk # w/cd Number of the disk on which the central directory starts
Disk entries The number of central directory entries on this disk
Total entries Total number of entries in the central directory.
Central directory size Size of the central directory in bytes
Offset of cd wrt to starting disk Offset of the start of the central directory on the disk on which the central directory starts
Comment len The length of the following comment field
ZIP file comment Optional comment for the Zip file
 

Example:

The end of central directory in out example file starts at byte 0xb36:

00000b30  6f 6d 6d 65 6e 74 50 4b  05 06 00 00 00 00 04 00  |ommentPK........|
00000b40  04 00 94 01 00 00 a2 09  00 00 33 00 74 68 69 73  |..........3.this|
00000b50  20 69 73 20 61 0d 0a 6d  75 6c 74 69 6c 69 6e 65  | is a..multiline|
00000b60  20 63 6f 6d 6d 65 6e 74  20 66 6f 72 20 74 68 65  | comment for the|
00000b70  20 65 6e 74 69 72 65 20  61 72 63 68 69 76 65     | entire archive|

Signature '\x50\x4b\x05\x06'.
Disk Number 0
Disk # w/cd 0
Disk entries 4
Total entries 4
Central directory size 0x194 = 404 bytes
Offset of cd wrt to starting disk byte 0x9a2 = byte 2466
Comment len 0x33 = 51 bytes
ZIP file comment "this is a
multiline comment for the entire archive"

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