xor.py

#!/usr/bin/env python

def xor(data,key):
    return bytearray(((data[i]^key[i%len(key)]) for i in range(0,len(data))))

data = bytearray(open("my_magic_bytes.jpg.enc","rb").read())
# Known plaintext from wikipedia https://en.wikipedia.org/wiki/List_of_file_signatures - XOR the enc file with it first
#key = bytearray([0xFF,0xD8,0xFF,0xE0,0x00,0x10,0x4A,0x46,0x49,0x46,0x00,0x01])
# 12 bytes key extracted from the file after the first above XOR
key = bytearray([0x46,0xcc,0xf9,0xa5,0x71,0xf0,0xff,0xb1,0x7e,0x41,0xcb,0x84])
out = xor(data,key)
fout = open("my_magic_bytes.jpg","wb")
fout.write(out)
fout.close()