Break stream cipher

stream_cipher.py

import array
import base64
import copy

# Found X0R cipher on an app assessment?  Got the ciphertext and know the plaintext?  Use this to get the key.  

cipher_text = array.array('B', base64.b64decode("Some Blob of base64 encoded ciphertext remove decoder if not base64"))

plain_text = array.array('B', "some known plaintext value")

for i in range(len(plain_text)):
     plain_text[i] ^= cipher_text[i]

key_stream = copy.deepcopy(plain_text.tostring())

def findkey(key_stream):
     
     new_string = key_stream
     for i in range(len(key_stream)):
           new_string = new_string[-1] + new_string[:-1]
           if new_string == key_stream:
                return i + 1

key = key_stream[:findkey(key_stream)]
print key