d conditions [4], whereas an attacker can prove the identity of the sender to others. For instance consider a user who decided to use anonymous SIP features. The UAC uses a meaningless URI, such as sip:thisis@anonymous.invalid [5]. If this meaningless URI is always used for this particular user, then it is possible to intercept SIP traffic, and connect this URI with different "Addresses-of-Record" (AoR). Then, using commercial or open source tools, the attacker will link these AoRs with phys- ical locations, and then with end-users' identities. 20.3 Generalized Anonymity Architectures To enhance or provide privacy in the internet services several privacy enhance- ment technologies (PET) have been proposed. Chaum's Mixes [6], Stop-and- Go Mixes and MixeNets [7], Crowds [4], Hordes [8], Onion Routing [9], and Mist [10] are some of the privacy preserving techniques. Mixes [6] introduced the notion of anonymous digital communication. The Mix system provides unlinkability of sender and receiver. This ensures that while an attacker is able to determine that the sender and receiver actually sends or receives messages, they cannot determine with whom they are com- municating. The system consists of special mix nodes which store, mix, and © 2009 by Taylor & Francis Group, LLC Anonymity in SIP then forward the messages in transit. The sender predetermines the route of the message through one or more mix nodes using a well-defined protocol. A public key cryptography protocol is also used to ensure that any message cannot be tracked by an attacker as it passes through the mix network. In its simplest form (called a threshold mix) a mix node waits until it collects a num- ber of messages as input. It uses its private key to reveal the address of the next mix node (or final destination) and reorders the received and buffered mes- sages by some metric before forwarding them. In that sense, an omnipresent attacker cannot trace a message from its source to its destination without the collusion of the mix nodes. To provide a mix-n