Steffen, AndreasKaufmann, DanielStricker, AndreasKnop, Jan vonHaverkamp, WilhelmJessen, Eike2019-10-112019-10-1120043-88579-384-9https://dl.gi.de/handle/20.500.12116/28593Ubiquitous worldwide broadband Internet access as well the coming of age of VoIP technology have made Voice-over-IP an increasingly attractive and useful network application. Currently the “human-readable” Session Initiation Protocol (SIP) which is based on a simple HTTP-like request/response exchange is steadily gaining headway against the considerably more complex ASN.1 encoded H.323 Multimedia ITU-T standard introduced by the telecom industry some years ago. Unfortunately little attention has been given to the security aspects involved in running a phone connection over the public Internet. This paper gives a comparative overview over the security mechanisms recommended by the SIP standard and presents a practical SIP implementation realized at the Zürcher Hochschule Winterthur (ZHW), based on S/MIME authentication and encryption of the session initiation and ensuing protection of the media channels using the Secure Real-time Transport Protocol (SRTP).enText/Conference Paper1617-5468