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: lower byte: |
| 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: |
| 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" |