protected Http signHttpUserPreAuthRequest(final Http request) {
        return FunctionUtils.doUnchecked(() -> {
            val resolver = SpringExpressionLanguageValueResolver.getInstance();
            request.signRequest(
                resolver.resolve(getDuoClient().getDuoIntegrationKey()),
                resolver.resolve(getDuoClient().getDuoSecretKey()));
            return request;
        });
    }
This is {@link BaseDuoSecurityAuthenticationService}.

@author Misagh Moayyed
@since 5.1.0
support/cas-server-support-duo-core/src/main/java/org/apereo/cas/adaptors/duo/authn/BaseDuoSecurityAuthenticationService.java