protected String getThrottledRequestKeyFor(final Authentication authentication,
                                               final RequestContext requestContext) {
        val principal = resolvePrincipal(authentication.getPrincipal(), requestContext);
        return principal.getId();
    }
The resolved provider for this flow.
core/cas-server-core-webflow-mfa-api/src/main/java/org/apereo/cas/web/flow/actions/AbstractMultifactorAuthenticationAction.java