<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validation(response):
 """ Get the validation value for a challenge response. """

    h = hashlib.sha256(response.key_authorization.encode("utf-8"))
    return b64encode(h.digest()).decode()