Editing ZIP (file format) (section)

==== Local file header ====

{|class="wikitable"
|+ Local file header
|-
! Offset !! Bytes !! Description<ref name="appnote">{{cite web|url=http://www.pkware.com/documents/casestudies/APPNOTE.TXT|format=TXT|title=File : APPNOTE.TXT - .ZIP File Format Specification : Version: 6.3.4|website=Pkware.com|access-date=2017-09-09}}</ref>
|-
|  0 || 4 || Local file header signature = 0x04034b50 (PK♥♦ or "PK\3\4")
|-
|  4 || 2 || Version needed to extract (minimum)
|-
|  6 || 2 || General purpose bit flag
|-
|  8 || 2 || Compression method; e.g. none = 0, DEFLATE = 8 (or "\0x08\0x00")
|-
| 10 || 2 || File last modification time
|-
| 12 || 2 || File last modification date
|-
| 14 || 4 || CRC-32 of uncompressed data
|-
| 18 || 4 || Compressed size (or 0xffffffff for ZIP64)
|-
| 22 || 4 || Uncompressed size (or 0xffffffff for ZIP64)
|-
| 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 records, each with at minimum a 16-bit signature and a 16-bit length. A ZIP64 local file extra field record, for example, has the signature 0x0001 and a length of 16 bytes (or more) so that two 64-bit values (the uncompressed and compressed sizes) may follow. Another common local file extension is 0x5455 (or "UT") which contains 32-bit UTC UNIX timestamps.

This is immediately followed by the compressed data.