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.
