def METHOD_NAME(ikey, skey, akey, sig_response):
    """Validate the signed response returned from Duo.
    Returns the username of the enrolled user, or None.
    Arguments:
    ikey          -- Duo integration key
    skey          -- Duo secret key
    akey          -- Application secret key
    sig_response  -- The signed response POST'ed to the server
    """
    return _verify_response(ikey, skey, akey, ENROLL_PREFIX, sig_response)