, thus rendering the handle globally unique. A possible realization of the handle 2118/r_user inside the Handle System is depicted in Figure 10.3. The handle has several fields. The HS ADMIN and HS_VLIST fields determine the administrators of the handle who are the naming authority (0.NA/2118): the handle itself (2118/r_user) and the two proxy servers in the HS_VLIST field. Any of these administra- tors has the privilege to modify the fields inside the handle provided the administrator succeeds to authenticate with the Handle System using his private key. 10.3.2 Authentication and Registration Currently, the most common authentication mechanism employed by SIP is the digest authentication [35] used by HTTP. When a user associates with a domain proxy server, he obtains an account on that server with a username and password, which he uses to authenticate himself to the server if asked. The digest authentication depicted here is domain-dependant, i.e., the user's credentials are valid for a particular domain. Briefly, digest authentication proceeds as follows: 1. The user sends a REGISTER request to a SIP proxy/registrar server. 2. The server replies with a 401 unauthorized response message challeng- ing the user to authenticate himself for the requested service (realm) through a user and password prompt. 3. The user sends back a message digest of his credentials, which include his username, password, etc. 4. The same message digest is computed internally using the server's inter- nal user information and compared to the one sent by the user. C 2009 by Taylor & Francis Group, LLC SIP Mobility Technologies and the Use of Persistent Identifiers 5. Authentication is granted if the two digests match. 6. The user registers with the SIP proxy/registrar server. In our approach, we still use digest authentication for the SIP users due to its wide support by current SIP servers and user agents, although a better authentication mechanism can be designed that would leverage the inherent security that handles expose. Access to 