Exercise 1 solution

    import struct
    import zlib

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


    def chunk(ctype, data=b""):
        body = ctype + data
        return struct.pack(">I", len(data)) + body + struct.pack(">I", zlib.crc32(body))


    def list_chunks(raw):
        if raw[:8] != SIG:
            raise ValueError("not a PNG: bad signature")
        pos, out = 8, []
        while pos < len(raw):
            if pos + 8 > len(raw):
                raise ValueError("truncated chunk header")
            length, ctype = struct.unpack_from(">I4s", raw, pos)
            if pos + 12 + length > len(raw):
                raise ValueError(f"chunk {ctype!r} runs past end of file")
            out.append((ctype, length))
            pos += 12 + length
        return out


    ihdr = struct.pack(">IIBBBBB", 3, 1, 8, 0, 0, 0, 0)
    idat = zlib.compress(b"\x00\x0a\x14\x1e")
    png = SIG + chunk(b"IHDR", ihdr) + chunk(b"IDAT", idat) + chunk(b"IEND")

    print(list_chunks(png))
    for label, raw in (("bad signature", b"GIF89a" + bytes(30)),
                       ("cut mid-chunk", png[:45]),
                       ("cut in header", png[:-5])):
        try:
            list_chunks(raw)
        except ValueError as e:
            print(label + ":", e)

    # [(b'IHDR', 13), (b'IDAT', 12), (b'IEND', 0)]
    # bad signature: not a PNG: bad signature
    # cut mid-chunk: chunk b'IDAT' runs past end of file
    # cut in header: truncated chunk header

(Run as written; the output matched the comments.)

Explanation: Every chunk is length (4 bytes, big-endian), type (4 bytes), data, then a 4-byte CRC,
so the next chunk starts at pos + 12 + length. The walker never trusts a length
beyond the end of the data. The last two cases fail differently: one chunk header is
whole but its data is cut off, the other has too few bytes left to hold a header.