def _get_client_side_certificate_subject():
    """Return the DN from the client side certificate as a D1 subject if a client side
    cert has been configured.

    Else return None.

    """
    subject = django.core.cache.cache.get('client_side_certificate_subject')
    if subject is not None:
        return subject
    cert_pem = _get_client_side_certificate_pem()
    if cert_pem is None:
        return None
    subject = _extract_subject_from_pem(cert_pem)
    django.core.cache.cache.set('client_side_certificate_subject', subject)
    return subject
Return the DN from the client side certificate as a D1 subject if a client side
    cert has been configured.

    Else return None.