Extract data after IEND from PNG (read from stdin, write to stdout)

png_catch_the_tail.py

#!/usr/bin/env python3
import sys

# read all from stdin
data = sys.stdin.buffer.read()

# check header
png_header = b"\x89PNG\x0D\x0A\x1A\x0A"
header_len = len(png_header)
if data[0:header_len] != png_header:
    print("The input is not a valid PNG", file=sys.stderr)
    sys.exit(1)

# find IEND
iend = b"\x00\x00\x00\x00IEND\xAE\x42\x60\x82"
iend_len = len(iend)
iend_start = data.find(iend)
if iend_start == -1:
    print("IEND was not found", file=sys.stderr)
    sys.exit(1)

iend_after = iend_start + iend_len

if iend_after == len(data):
    print("No extra data after IEND", file=sys.stderr)
    sys.exit(1)

# write all data after IEND to stdout
sys.stdout.buffer.write(data[iend_after:])