# PicoCTF 2k13 - Black Hole

In [1]: rf = open('masked_key.png', 'r')
In [2]: mk = rf.read()
In [3]: rf.close()
In [4]: key = bytearray(mk[-64:])
In [5]: png = bytearray(mk[:-64])
In [6]: for byte in xrange(len(png)):
   ...:     png[byte] ^= key[byte % 64]
   ...:     
In [7]: wf = open('key.png', 'w')
In [8]: wf.write(png)
In [9]: wf.close()