This paper reviews some security challenges currently faced by VoIP systems as well as their potential solutions. Particularly, it focuses on Zfone, a vendor-neutral security solution developed by PGP’s creator, Phil Zimmermann. Zfone is based on the Z Real-time Transport Protocol (ZRTP), which is an extension of the Real-time Transport Protocol (RTP). ZRTP offers a very simple and robust approach to providing protection against the most common type of VoIP threats. Basically, the protocol offers a mechanism to guarantee high entropy in a Diffie-Hellman key exchange by using a session key that is computed through the hashing several secrets, including a short authentication string that is read aloud by callers. The common shared secret is calculated and used only for one session at a time. However, the protocol allows for a part of the shared secret to be cached for future sessions. The mechanism provides for protection for man-in-the-middle, call hijack, spoofing, and other common types of attacks. Also, this paper explores the fact that VoIP security is a very complicated issue and that the technology is far from being inherently insecure as many people usually claim.
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