ied by different types of party information, which are authenticated by a trusted entity [22]. These trusted entities are delegated by end-users, and include the logic to reveal the identity of the calling or called party to peer entities. Trusted peers might receive information that identi- fies an end-user, if these entities are supposed to provide the same level of privacy, i.e., to reveal party information to other trusted peers. In addition to suppressing the delivery of party information, this draft proposal provides extensions to enable a party to obtain IP address privacy. Sipanon is another SIP anonymity proposal that introduces an architec- ture two user agents (a User Agent Client and User Agent Server) that are coupled back-to-back (B2BUA) [23]. A message from the anonymous user's UAC is received by an Anonymizer's UAS. This message is then anonymized. The "From" header is changed and sent out from the Anonymizer's UAS to the remote UAC. The Anonymizers maintain a store of mappings between "real" SIP addresses and "anonymous" ones. This approach is very close to Chaum mixes solutions, and, thus, it inherits the corresponding drawbacks and vulnerabilities, such as clocking attacks. 20.5 A New Proposal for Anonymity in SIP In this section we introduce the use of the Mist architecture in order to enable anonymity in SIP. 20.5.1 Mist at a Glance Mist provides several advantages that led us to this decision. The key point of the Mist architecture is the distribution of knowledge. The "Lighthouse" routers are aware of the user's identity without knowing his/her exact location while the "Portal" routers are aware of the user's location, without know- ing the corresponding identity. Furthermore, due to the decentralized Mist C 2009 by Taylor & Francis Group, LLC Anonymity in SIP User's A L: Lighthouse lighthouse P: Portal R: Mist router User's B lighthouse User's B User's A portal portal User B User A FIGURE 20.1 The Mist hierarchical structure. architecture, a possible collusion between the aforementioned Mi