docs: def mask(key, original):
      """
      Mask an octet string using the given masking key.
      The following masking algorithm is used, as defined in RFC 6455:
  
      for each octet:
          j = i MOD 4
          transformed-octet-i = original-octet-i XOR masking-key-octet-j
      """
      if len(key) != 4:
          raise ValueError('invalid masking key "%s"' % key)
  
      key = map(ord, key)
      masked = bytearray(original)
  
      for i in xrange(len(masked)):
          masked[i] ^= key[i % 4]
  
      return masked