digest does not take into account specific SIP restrictions, i.e., some requests (e.g., ACKs) and responses must not be challenged. It is there- fore clear that certain HTTP digest limitations can be exploited for launching MITM and impersonations attacks. IP Security (IPSec) [23] is an independent and transparent network level protocol in the Internet architecture, providing services for the protection of IP packets against any possible attack like IP spoofing, session hijacking, traffic analysis and many more [24]. IPSec operates either in transport or in tunnel mode, offering integrity, data origin authentication and confidentiality services by utilizing Encapsulating Security Payload (ESP) and Authentication Header protocols. Consequently, IPSec could be utilized as an alternative safeguarding C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security mechanism for SIP, provided that some sort of trust (e.g., pre-shared keys) has been established between the communicating parties beforehand in a hop-by- hop fashion. However, as IPSec is implemented at the operating system level, most of the existing SIP clients do not support it. Thus, IPSec can be utilized to shield signaling traffic between SIP servers but not between servers and user agents. Transport Layer Security (TLS) [25] provides security services at the transport layer of the Internet architecture. Similarly to IPSec, TLS offers authentication, integrity, and confidentiality services among SIP entities, with- out mandating a pre-established trust relationship (as IPSec does) between the communicating parties. In addition, TLS is employed for providing end- to-end security through the support of the SIP Secure (SIPS) protocol. If implemented properly, the latter mechanism guarantees that the requested resource is reached securely. This is achieved by constructing TLS secure tun- nels among each hop in the path until the final recipient. However, until today no universal mechanism exists which can guarantee that TLS is ap