How to validate CRC-32 calculation of Zip file - checksum

Question

I want to validate that my ZIP file has a correct CRC-32 checksum.
I read that in a ZIP file the CRC-32 data is in bytes 14 to 17:
Offset Bytes Description[30]
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 of uncompressed data
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
I wanted to validate a CRC-32 checksum of a simple ZIP file I created:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
-----------------------------------------------
50 4B 03 04 14 00 00 00 00 00 38 81 1C 51 4C 18 | PK........8..QL.
C7 8C 02 00 00 00 02 00 00 00 07 00 00 00 31 32 | nj............12
33 2E 64 61 74 73 73 50 4B 01 02 14 00 14 00 00 | 3.datssPK.......
00 00 00 38 81 1C 51 4C 18 C7 8C 02 00 00 00 02 | ...8..QL.nj.....
00 00 00 07 00 00 00 00 00 00 00 01 00 20 00 00 | ............. ..
00 00 00 00 00 31 32 33 2E 64 61 74 50 4B 05 06 | .....123.datPK..
00 00 00 00 01 00 01 00 35 00 00 00 27 00 00 00 | ........5...'...
00 00 | ..
The CRC-32 is: 0x4C18C78C
I went to this CRC-32 online calculator and added the following un-compressed row from the file:
50 4B 03 04 14 00 00 00 00 00 38 81 1C 51
This is the result:
Algorithm Result Check Poly Init RefIn RefOut XorOut
CRC-32 0x6A858174 0xCBF43926 0x04C11DB7 0xFFFFFFFF true true 0xFFFFFFFF
CRC-32/BZIP2 0xE3FA1205 0xFC891918 0x04C11DB7 0xFFFFFFFF false false 0xFFFFFFFF
CRC-32C 0xB578110E 0xE3069283 0x1EDC6F41 0xFFFFFFFF true true 0xFFFFFFFF
CRC-32D 0xAFE2EEA4 0x87315576 0xA833982B 0xFFFFFFFF true true 0xFFFFFFFF
CRC-32/MPEG-2 0x1C05EDFA 0x0376E6E7 0x04C11DB7 0xFFFFFFFF false false 0x00000000
CRC-32/POSIX 0xFF9B3071 0x765E7680 0x04C11DB7 0x00000000 false false 0xFFFFFFFF
CRC-32Q 0x79334F11 0x3010BF7F 0x814141AB 0x00000000 false false 0x00000000
CRC-32/JAMCRC 0x957A7E8B 0x340BC6D9 0x04C11DB7 0xFFFFFFFF true true 0x00000000
CRC-32/XFER 0xA7F36A3F 0xBD0BE338 0x000000AF 0x00000000 false false 0x00000000
But none of them equal to: 0x4C18C78C.
What am I doing wrong? The CRC-32 of the ZIP is the calculation of all the bytes (0-13) before, no?

Answer 1

The byte sequence you are running against the online CRC calculator are not uncompressed bytes.
50 4B 03 04 14 00 00 00 00 00 38 81 1C 51
Those bytes are the first few bytes of the zip file. The CRC32 value in a zip is calculated by running the CRC32 algorithm against the complete uncompressed payload. In your case the payload is the two byte sequence "ss".
To work that out, I converted your hex dump back into a zip file, tmp.zip. It contains a single member 123.dat
$ unzip -lv tmp.zip
Archive: tmp.zip
Length Method Size Cmpr Date Time CRC-32 Name
-------- ------ ------- ---- ---------- ----- -------- ----
2 Stored 2 0% 2020-08-28 16:09 8cc7184c 123.dat
-------- ------- --- -------
2 2 0% 1 file
When I extract that member to stdout & pipe though hexdump, we find it contains the two bytes string "ss" (hex 73 73)
$ unzip -p tmp.zip | hexdump -C
00000000 73 73 |ss|
Finally, as already mentioned in another comment, you can check that the CRC value is correct by running unzip -t
$ unzip -t tmp.zip
Archive: tmp.zip
testing: 123.dat OK
No errors detected in compressed data of tmp.zip.

Answer 2

I was able to create a zip file that matches the one in the question. The header shows that the compression type == 0, which means no compression, the uncompressed size == 2, the data == {73 73}. CRC32 uses reflected input and output, and the CRC is stored in little endian format, so the CRC == 0x8CC7184C.
I get a match using CRC32 on data of {73 73} using this online CRC calculator:
http://www.sunshine2k.de/coding/javascript/crc/crc_js.html