.ZIP File Format – Shared fields

Version made by

Two bytes in the central header and the ZIP64 end record. The low byte is the highest ZIP specification version the writer supports, as the version times ten: 20 means 2.0. The high byte is the host system, which says how to read the external attributes.

Version needed to extract

Two bytes. The low byte is the lowest ZIP version a reader needs for this member, times ten. Each value is the minimum for the feature named, and a member takes the highest value of any feature it uses. A reader may treat an unknown value as a reason to refuse the member.

The high byte is unused here. Real writers do not always follow the table. For LZMA, XZ and Zstandard, libzip writes 20, with a comment that WinZip refuses other values.

General purpose bit flag

Two bytes. Bits are numbered from 0, the least significant.

MS-DOS date and time

Two 16-bit fields, time then date, in local time with no time zone. A zone can only come from an extra field, such as the extended timestamp.

Seconds can only be even. A date and time of zero means no time was recorded. In a local header with bit 13 set they are masked to zero.

Example: the date 0x5D69 is binary 0101110 1011 01001: year 46 (2026), month 11, day 9. The time 0x6000 is 01100 000000 00000: 12:00:00.

CRC-32

A 32-bit CRC of the uncompressed data, stored as a little-endian number. The polynomial is 0x04C11DB7, used in reflected form as 0xEDB88320. The register starts at 0xFFFFFFFF and the result is inverted at the end.

crc = 0xFFFFFFFF
for each byte b:
    crc = table[(crc ^ b) & 0xFF] ^ (crc >> 8)
result = crc ^ 0xFFFFFFFF

Each table entry for index i starts with the value i, and is shifted right eight times, with the polynomial XORed in whenever the bit shifted out is 1.

This is the common CRC-32. The check value for the nine ASCII bytes 123456789 is 0xCBF43926. APPNOTE gives the magic number 0xDEBB20E3, the register value after processing a complete message and its own CRC. In the sample archive the 12 bytes Hello, ZIP! and a newline have the CRC-32 0x8F865D0C.

The field is zero in the local header when bit 3 is set, and is masked in some cases when the directory is encrypted. In an AE-2 member it is zero.

Internal file attributes

Two bytes in the central header.

External file attributes

Four bytes in the central header. Their meaning depends on the host system in version made by. For an entry read from standard input, the field is zero.