NAF Authentication Filter

The UE needs to retrieve an XCAP resource, for example to discover configuration settings for an MMTEL service.

Here we assume the UE knows its XCAP server’s address, xcap.home1.net. This address may be preconfigured in the UE, or the UE may derive it from the subscriber’s private identity as per 3GPP TS 23.003 §13.9.

The UE constructs the XCAP selector for the resource and sends an HTTP GET request to the XCAP server. In this example, the UE wants to retrieve the communication diversion (CDIV) configuration for their public identity sip:user@home1.net.

The XCAP server will need to confirm that the request is being made by a device associated with the public identity sip:user@home1.net.

The web container (such as Apache Tomcat) sees that the URL is for a resource protected by the filter, so passes the request to the filter.

The filter sees that the request has not been authenticated, because there is no Authorization header. The filter generates a 401 unauthorized response, containing a challenge.

The realm value 3GPP-bootstrapping@xcap.home1.net tells the UE that bootstrapping is required. At this point the UE must perform the GBA bootstrapping procedure with the BSF. This process is covered separately in the BSF Server section.

Assuming bootstrapping is successful, the authentication process carries on.

After completing bootstrapping, the UE can retry the request using its bootstrapped security association.

The UE should now know the Bootstrapping Transaction Identifier (B-TID), which is used as the username for the HTTP Digest authentication scheme.

The password used is derived from shared secrets generated in the bootstrapping process, and information about the connection to the NAF. This password is known as the NAF-specific shared key, or Ks_NAF.

The UE generates a challenge response using the standard HTTP Digest (RFC 2617) scheme. The username, password, nonce, opaque, and other values from the challenge are combined to calculate the response value:

The filter receives the challenge response over HTTP, and must now verify that it is correct.

To do this, the filter queries the bootstrap_info Cassandra table to find an entry for the username (B-TID). If an entry exists, it will contain the user’s private identity (IMPI) and also the shared secrets that will be used to calculate Ks_NAF, the password.

The filter also looks up the user’s GBA User Security Settings (GUSS) in the GUSS cache, populated by the BSF. If no bootstrap information or GUSS is found, the filter must reject the request as it does not have enough information to continue.

Now the filter calculates Ks_NAF, and uses this value as the password in its HTTP Digest calculation. If everything is correct, the filter will calculate the same value that the UE sent in the challenge response. This means the request has been successfully authenticated — the filter can now pass the request to the XCAP servlet.

Before doing so, the filter must also determine which IMS public identities (IMPUs) are associated with the request. This is determined by the ussList element from the user’s GUSS, and also the filter’s settings for service ID, service type and NAF group. See GUSS usage in the NAF Authentication Filter for details.

The user is now authenticated, and the filter knows the public identities associated with the request. The filter passes the request to the XCAP servlet, and also adds the X-3GPP-Asserted-Identity header to the request, with the known public identities. The request header seen by the XCAP servlet looks like this:

The XCAP servlet sees that one of the asserted identities ("sip:user@home.net") matches the XCAP User Identity (XUI) in the Request-URI, so allows the request to proceed. If the XUI did not match an asserted identity, the request would be rejected immediately.

The XCAP servlet uses its Sh-Cache to retrieve the supplementary services transparent data for this identity from the HSS. Assuming the data is found, it applies the XCAP selector to extract the requested portion of the XML document, in this case the communication-diversion element.

The XCAP servlet generates a 200 OK response with the communication-diversion element, and returns from its service() method. At this point control returns to the filter.

If the requested qop (Quality of Protection) parameter was auth-int, the filter must calculate a response digest so that the UE can verify the integrity of the response. The filter calculates the digest over the response body, and adds the Authentication-Info header to the response.

Finally the HTTP response is transmitted back to the UE. The response seen at the UE will be something like this: