Editing ZIP (file format) (section)

==== Data descriptor ====

If the bit at offset 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. If the archive is in Zip64 format, the compressed and uncompressed size fields are 8 bytes long instead of 4 bytes long (see section 4.3.9.2<ref>{{cite web |title=File: APPNOTE.TXT - .ZIP File Format Specification |url=https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT |publisher=PKWARE Inc. |access-date=21 February 2022}}</ref>). The equivalent fields in the local header (or in the Zip64 extended information extra field in the case of archives in Zip64 format) 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:

{|class="wikitable"
|+ Data descriptor
|-
! Offset !! Bytes !! Description<ref name="appnote"/>
|-
|  0 || 0 or 4 || ''Optional'' data descriptor signature = 0x08074b50
|-
|  0 or 4 || 4 || CRC-32 of uncompressed data
|-
|  4 or 8 || 4 or 8 || Compressed size
|-
|  8 or 12 or 16|| 4 or 8 || Uncompressed size
|}