National Trust Collections image downloader-decrypter

#! /usr/local/bin/python3

# National Trust Collections zoomable image downloader-decrypter

# -input parameters-
# URL of the image page with zoomable content
url = "http://www.nationaltrustcollections.org.uk/object/593156"
# Name/path of the file that will be saved to disk
output_filename = "decrypted.jpg"

########################

from urllib.request import urlopen
import re

#       RC4, ARC4, ARCFOUR algorithm
#
#       Copyright (c) 2009 joonis new media
#       Author: Thimo Kraemer <thimo.kraemer@joonis.de>
#       License: GPL
def rc4crypt(data, key):
    x = 0
    box = list(range(256))
    for i in range(256):
        x = (x + box[i] + ord(key[i % len(key)])) % 256
        box[i], box[x] = box[x], box[i]
    x = 0
    y = 0
    out = bytearray()
    for char in data:
        x = (x + 1) % 256
        y = (y + box[x]) % 256
        box[x], box[y] = box[y], box[x]
        out.append(char ^ box[(box[x] + box[y]) % 256])
    return out

fileurl = "http://www.nationaltrustcollections.org.uk"
with urlopen(url) as imgpage:
    fileurl += re.search(r"/EncryptedZoomImage\.ashx\?id_fichier=\d+", imgpage.read().decode()).group(0)

key = "asdas98as9dasdhas9yd9a8sd8uasu0asdu0uas0asdasd979asyyua98sdyhd08a0889daysdt6cf8dagbcsdahc9gs8ydgb"
with urlopen(fileurl) as inf, open(output_filename, 'wb') as outf:
    outf.write(rc4crypt(inf.read(), key))