Zip文件格式

官方文档

https://pkware.cachefly.net/webdocs/APPNOTE/APPNOTE-6.2.0.txt

格式说明

在官方文档中给出的ZIP格式如下：

Overall .ZIP file format:

    [local file header 1]
    [file data 1]
    [data descriptor 1]
    . 
    .
    .
    [local file header n]
    [file data n]
    [data descriptor n]
    [archive decryption header] (EFS)
    [archive extra data record] (EFS)
    [central directory]
    [zip64 end of central directory record]
    [zip64 end of central directory locator] 
    [end of central directory record]

通常情况下，我们用到的ZIP文件格式：

[local file header + file data + data descriptor]{1,n} + central directory + end of central directory record
即
[文件头+文件数据+数据描述符]{此处可重复n次}+核心目录+目录结束标识

当压缩包中有多个文件时，就会有多个[文件头+文件数据+数据描述符]

本片文章讨论的就是这种通常用到的ZIP文件格式，若想了解完整的ZIP文件格式，请看官方文档。

压缩源文件数据区

[local file header + file data + data descriptor]

记录着压缩的所有文件的内容信息，每个压缩文件都由local file header 、file data、data descriptor三部分组成，在这个数据区中每一个压缩的源文件/目录都是一条记录。

local file header 文件头

用于标识该文件的开始，记录了该压缩文件的信息。

Offset

Bytes

Description

译

0

4

Local file header signature = 0x04034b50 (read as a little-endian number)

文件头标识，值固定(0x04034b50)

4

2

Version needed to extract (minimum)

解压文件所需 pkware最低版本

6

2

General purpose bit flag

通用比特标志位(置比特0位=加密，详情见后)

8

2

Compression method

压缩方式（详情见后）

10

2

File last modification time

文件最后修改时间

12

2

File last modification date

文件最后修改日期

14

2

CRC-32

CRC-32校验码

18

4

Compressed size

压缩后的大小

22

4

Uncompressed size

未压缩的大小

26

4

File name length (n)

文件名长度

28

2

Extra field length (m)

扩展区长度

30

n

File name

文件名

30+n

m

Extra field

扩展区

general purpose bit flag: (2 bytes) 通用位标记

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.)

      Bit 4: Reserved for use with method 8, for enhanced
             deflating. 

      Bit 5: If this bit is set, this indicates that the file is 
             compressed patched data.  (Note: Requires PKZIP 
             version 2.70 or greater)

      Bit 6: Strong encryption.  If this bit is set, you should
             set the version needed to extract value to at least
             50 and you must also set bit 0.  If AES encryption
             is used, the version needed to extract value must 
             be at least 51.

      Bit 7: Currently unused.

      Bit 8: Currently unused.

      Bit 9: Currently unused.

      Bit 10: Currently unused.

      Bit 11: Currently unused.

      Bit 12: Reserved by PKWARE for enhanced compression.

      Bit 13: Used when encrypting the Central Directory to indicate 
              selected data values in the Local Header are masked to
              hide their actual values.  See the section describing 
              the Strong Encryption Specification for details.

      Bit 14: Reserved by PKWARE.

      Bit 15: Reserved by PKWARE.

compression method: (2 bytes) 压缩方式

(see accompanying documentation for algorithm
      descriptions)

      0 - The file is stored (no compression)
      1 - The file is Shrunk
      2 - The file is Reduced with compression factor 1
      3 - The file is Reduced with compression factor 2
      4 - The file is Reduced with compression factor 3
      5 - The file is Reduced with compression factor 4
      6 - The file is Imploded
      7 - Reserved for Tokenizing compression algorithm
      8 - The file is Deflated
      9 - Enhanced Deflating using Deflate64(tm)
     10 - PKWARE Data Compression Library Imploding
     11 - Reserved by PKWARE
     12 - File is compressed using BZIP2 algorithm

file data 文件数据

记录了相应压缩文件的数据

data descriptor 数据描述符

用于标识该文件压缩结束，该结构只有在相应的local file header中通用标记字段的第３bit设为１时才会出现，紧接在压缩文件源数据后。这个数据描述符只用在不能对输出的 ZIP 文件进行检索时使用。例如：在一个不能检索的驱动器（如：磁带机上）上的 ZIP 文件中。如果是磁盘上的ZIP文件一般没有这个数据描述符。

Offset

Bytes

Description

译

0

4

crc-32

CRC-32校验码

4

4

compressed size

压缩后的大小

8

4

uncompressed size

未压缩的大小

Central directory 核心目录

记录了压缩文件的目录信息，在这个数据区中每一条纪录对应在压缩源文件数据区中的一条数据。

核心目录结构如下：

Offset

Bytes

Description

译

0

4

Central directory file header signature = 0x02014b50

核心目录文件header标识=（0x02014b50）

4

2

Version made by

压缩所用的pkware版本

6

2

Version needed to extract (minimum)

解压所需pkware的最低版本

8

2

General purpose bit flag

通用位标记

10

2

Compression method

压缩方法

12

2

File last modification time

文件最后修改时间

14

2

File last modification date

文件最后修改日期

16

4

CRC-32

CRC-32校验码

20

4

Compressed size

压缩后的大小

24

4

Uncompressed size

未压缩的大小

28

2

File name length (n)

文件名长度

30

2

Extra field length (m)

扩展域长度

32

2

File comment length (k)

文件注释长度

34

2

Disk number where file starts

文件开始位置的磁盘编号

36

2

Internal file attributes

内部文件属性

38

4

External file attributes

外部文件属性

42

4

relative offset of local header

本地文件头的相对位移

46

n

File name

目录文件名

46+n

m

Extra field

扩展域

46+n+m

k

File comment

文件注释内容

End of central directory record(EOCD) 目录结束标识

目录结束标识存在于整个归档包的结尾，用于标记压缩的目录数据的结束。每个压缩文件必须有且只有一个EOCD记录。

Offset

Bytes

Description

译

0

4

End of central directory signature = 0x06054b50

核心目录结束标记（0x06054b50）

4

2

Number of this disk

当前磁盘编号

6

2

number of the disk with the start of the central directory

核心目录开始位置的磁盘编号

8

2

total number of entries in the central directory on this disk

该磁盘上所记录的核心目录数量

10

2

total number of entries in the central directory

核心目录结构总数

12

2

Size of central directory (bytes)

核心目录的大小

16

4

offset of start of central directory with respect to the starting disk number

核心目录开始位置相对于archive开始的位移

20

2

.ZIP file comment length(n)

注释长度

22

n

.ZIP Comment

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