haracteristic. For example, this type of attack can be employed for diverted flow of signaling by forcing a particular itinerary or for changed recording of user or modifing a profile of service [11]. The attack type depends on the type of security (or lack of security) employed (the type of authentication, etc.). These attacks can also be employed for the denial of service. The main two mechanisms of security employed with SIP are authentication and data encryption. The authentication of data is employed to authenticate the sender of message and insure that certain critical message information was not modified in transit. It has to prevent an assailant from modifying and/or listening to SIP requests and reply. SIP employs Proxy-Authentication, Proxy- Authorization, authorization, and WWW-Authentication in the letterhead, similar to these of HTTP, for the authentication of the terminal system by means of a numerical signature. Rather, proxy-par-proxy authentication can be executed by using protocols of authentication of the transport layer or the network layer such that TLS or IPSEC. Data encryption is employed to insure the confidentiality of communication SIP, allowing only the destined client to decipher and read data. It is usual to use encryption algorithms such as the DES (Data Encryption Standard) and Advanced AES (Advanced Encryption Standard). SIP endorses two forms of cryptography: end-to-end and hop-by-hop. The end-to-end encryption pro- vides confidentiality for all information (some letterhead and the body of SIP message) that needs to be read by intermediate servers or proxy. The end- to-end encryption is executed by the mechanism S/MIME (Figure 21.10). On the contrary, the hop-by-hop encryption of whole SIP message can be employed in order to protect the information that would have to be accessed by intermediate entities, such as letterhead From, To and Via. Securing such information prevents malevolent users determining the calls or accessing the itinerary information. The hop-by-hop encryption ca