png_chunks.py

import struct, sys, zlib

PNG_HEADER = b"\x89PNG\r\n\x1a\n"

for path in sys.argv[1:]:
    with open(path, "rb") as png:
        png = png.read()
    print(path + ":")
    if not png.startswith(PNG_HEADER):
        print("not a png")
        continue
    png = memoryview(png)[len(PNG_HEADER):]
    while png:
        length, chunk_type = struct.unpack_from("!I4s", png)
        print(chunk_type.decode() + ":", length, "bytes")
        expected, actual = png[8 + length:12 + length], zlib.crc32(png[4:8 + length]).to_bytes(4)
        if expected != actual:
            print(f"expected {expected.hex()}, got {actual.hex()}")
        png = png[12 + length:]