'Build the IdP name from the given config option issuer_attribute.
The default issuer attribute SSL_CLIENT_I_DN in the environment is
built with the following formula -
base64_idp = sha1(env[\'SSL_CLIENT_I_DN\'])
:returns: base64_idp like the above example
:rtype: str'

def _build_idp_id(self):

    idp = self.env.get(CONF.tokenless_auth.issuer_attribute)
    if (idp is None):
        raise exception.TokenlessAuthConfigError(issuer_attribute=CONF.tokenless_auth.issuer_attribute)
    hashed_idp = hashlib.sha256(idp.encode('utf-8'))
    return hashed_idp.hexdigest()
