cture, called an onion, which implicitly defines the route path through the network. An onion is recursively encrypted using public key cryptography. The number of encryptions is equal to the number of the onion routes through which this structure should be delivered and processed towards the destination. The outer cryptographic control information refers to the first onion router in the path, while the inner cryptographic control information refers to the last onion router in the path (i.e., the predecessor of the destination). Each onion router along the route uses its public key to decrypt the entire onion that it receives. This operation exposes the embedded onion, and as a result, the C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security identity of the next onion router. After decrypting a cortex each onion router then pads the embedded onion to maintain a fixed size, and sends it to the next onion router. Once the onion reaches the destination, all the inner control data are displayed as plaintext. This establishes the anonymous end-to-end connection, and then data can be sent in both directions. As data are routed through the anonymous end-to-end connection, each onion router removes one layer of encryption, SO the data arrives as plaintext at the recipient. This lay- ering occurs in the reverse order (using different algorithms and keys) for data moving backward. Connection tear-down can be initiated by either end, or in the middle if needed. All the messages (onions and real data) that transferred through the Onion Routing network illustrate identical sizes. The messages arrive at an onion router at fixed time intervals. They are mixed to avoid cor- relation by potential attackers. Additional cover traffics the semi-permanent connections between onion-routers and thus deludes external eavesdroppers. Onion Routing effectively resists traffic analysis. Hordes [8] is an anonymity infrastructure that combines elements from Onion Routing and Crowds. It is the first protocol