Alternatively, for increased security or privacy, messages may be passed "by reference". For example, an identity provider may supply a reference to a SAML assertion (called an "artifact") instead of transmitting the assertion directly through the user agent. Subsequently, the service provider requests the actual assertion via a back channel. Such a back-channel exchange is specified as a SOAP message exchange (SAML over SOAP over HTTP). In general, any SAML exchange over a secure back channel is conducted as a SOAP message exchange.