File Headers
All multi-byte values in the header are stored in little-endian byte order. All length fields count the length in bytes.
| Offset | Bytes | Description |
|---|---|---|
| 0 | 4 | Local file header signature = 0x04034b50 (read as a little-endian number) |
| 4 | 2 | Version needed to extract (minimum) |
| 6 | 2 | General purpose bit flag |
| 8 | 2 | Compression method |
| 10 | 2 | File last modification time |
| 12 | 2 | File last modification date |
| 14 | 4 | CRC-32 |
| 18 | 4 | Compressed size |
| 22 | 4 | Uncompressed size |
| 26 | 2 | File name length (n) |
| 28 | 2 | Extra field length (m) |
| 30 | n | File name |
| 30+n | m | Extra field |
The extra field contains a variety of optional data such as OS-specific attributes. It is divided into chunks, each with a 16-bit ID code and a 16-bit length.
This is immediately followed by the compressed data.
If bit 3 (0x08) of the general-purpose flags field is set, then the CRC-32 and file sizes are not known when the header is written. The fields in the local header are filled with zero, and the CRC-32 and size are appended in a 12-byte structure (optionally preceded by a 4-byte signature) immediately after the compressed data:
| Offset | Bytes | Description |
|---|---|---|
| 0 | 0/4 | Optional data descriptor signature = 0x08074b50 |
| 0/4 | 4 | CRC-32 |
| 4/8 | 4 | Compressed size |
| 8/12 | 4 | Uncompressed size |
The central directory entry is an expanded form of the local header:
| Offset | Bytes | Description |
|---|---|---|
| 0 | 4 | Central directory file header signature = 0x02014b50 |
| 4 | 2 | Version made by |
| 6 | 2 | Version needed to extract (minimum) |
| 8 | 2 | General purpose bit flag |
| 10 | 2 | Compression method |
| 12 | 2 | File last modification time |
| 14 | 2 | File last modification date |
| 16 | 4 | CRC-32 |
| 20 | 4 | Compressed size |
| 24 | 4 | Uncompressed size |
| 28 | 2 | File name length (n) |
| 30 | 2 | Extra field length (m) |
| 32 | 2 | File comment length (k) |
| 34 | 2 | Disk number where file starts |
| 36 | 2 | Internal file attributes |
| 38 | 4 | External file attributes |
| 42 | 4 | Relative offset of local file header. This is the number of bytes between the start of the first disk on which the file occurs, and the start of the local file header. This allows software reading the central directory to locate the position of the file inside the ZIP file. |
| 46 | n | File name |
| 46+n | m | Extra field |
| 46+n+m | k | File comment |
After all the central directory entries comes the end of central directory record, which marks the end of the ZIP file:
| Offset | Bytes | Description |
|---|---|---|
| 0 | 4 | End of central directory signature = 0x06054b50 |
| 4 | 2 | Number of this disk |
| 6 | 2 | Disk where central directory starts |
| 8 | 2 | Number of central directory records on this disk |
| 10 | 2 | Total number of central directory records |
| 12 | 4 | Size of central directory (bytes) |
| 16 | 4 | Offset of start of central directory, relative to start of archive |
| 20 | 2 | Comment length (n) |
| 22 | n | Comment |
This ordering allows a zip file to be created in one pass, but it is usually decompressed by first reading the central directory at the end.
Read more about this topic: Zip (file Format), Design
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