Custom stream cipher CTF challenge

my_stream_cipher.py.sanitized

import hashlib

KEY = #NAH
FLAG = #NAH

message = 'Hello LOLCTF! Can you beat my super-leet stream cipher? If you can, the flag is: ' + FLAG

def sxor(s1, s2):
   return ''.join([chr(ord(c1) ^ ord(c2)) for c1, c2 in zip(s1, s2)])

def sha1prng(key, length):
   num_hashes = (length / 20) + 1
   state = key
   output = ''
   for i in range(num_hashes):
      hasher = hashlib.sha1()
      hasher.update(state)
      state = hasher.digest()
      output += state
   return output  

def my_stream_cipher(key, data):
   ''' Encrypt / decrypt function for my custom stream cipher '''
   return sxor(data, sha1prng(key, len(data)))

output = open('output.txt', 'w')
output.write(my_stream_cipher(KEY, message).encode('hex'))
output.close()

output.txt

13add529091b5a5df1a8a10766f711a3f8e7d1613e6d1d1b1b383039acd9ded2748e80a957a12fa8b5ecd5f741dcf6c436de3245c0e0e956771850ccee3bc2002c6bd3bc924c49991285614a874c28d0ba36e9e0d0fb5b9b5bb48cec11724030315eea7ab8d4670e3e3f95f9d6f054a5800f0d4fef3e5cb182c02364ae352ca6d9