SIP & RTP security for voice over IP

Abstract

Voice over Internet Protocol refers to sending voice and fax phone calls over data networks, particularly the Internet. This technology offers cost savings by making more efficient use of the existing network. Traditionally, voice and data were carried over separate networks optimized to suit the differing characteristics of voice and data traffic. With advances in technology, it is now possible to carry voice and data over the same networks whilst still catering for the different characteristics required by voice and data. VoIP has been developed for decades, allowed telephone calls or faxes to be transported over an IP data network. The IP network could be a local area network in an office, a wide area network linking the sites of a large international organization, a corporate intranet, the internet, any combination of the above. The explosive growth of the Internet makes IP the predominate networking protocol globally. As voice and data network technologies merge, the massive infrastructure of cost-saving can be made as the need for providing the separation of networks for voice and data eliminated. Most traditional phone networks use the Public Switched Telephone Network(PSTN). This system employs circuit-switched technology that requires a dedicated voice channel to be assigned to a particular conversation, sent in over this network. A VoIP system employs a packet-switched network, where the voice signal is digitized, compressed and packetized. This compressed digital message no longer requires a voice channel. Instead, a message can be sent across the same data lines that are used for the Intranet or Internet and a dedicated channels is no longer needed. The message can now share bandwidth with other messages in the network. Normal data traffic is carried between PC s, servers, printers, and other networked devices through a company s worldwide TCP/IP network. Each device on the network has an IP address, which is attached to every packet for routing. Voice-over-IP packets are no different. Users may use appliances such as Symbol s NetVision phone to talk to other IP phones or desktop PC-based phones located at company sites worldwide, provided that a voice-enabled network is installed at the site. Installation simply involves assigning an IP address to each wireless handset. VOIP lets you make toll-free long distance voice and fax calls over existing IP data networks instead of the public switched telephone network (PSTN). Today business that implement their own VOIP solution can dramatically cut long distance costs between two or more locations. This document describes the Secure Real-time Transport Protocol(SRTP), a profile of the Real-time Transport Protocol (RTP), which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, RTCP (the Real-time Transport Control Protocol) [RFC3350]. SRTP provides a framework for encryption and message authentication of RTP and RTCP streams. SRTP defines a set of default cryptographic transforms, and it allows new transforms to be introduced in the future. With appropriate key management, SRTP is secure for unicast and multicast RTP applications. SRTP can achieve high throughput and low packet expansion. SRTP proves to be a suitable protection for heterogeneous environments (mix of wired and wireless networks). To get such features, default transforms are described, based on an additive stream cipher for encryption, a keyed-hash based function for message authentication, and an "implicit" index for sequencing/synchronization based on the RTP sequence number for SRTP and an index number for Secure RTCP (SRTCP). All the packages are then transmitted by SRTP protocol and there is no signal of conversation caught by Wireshark. These packages are now encrypted and they can be protected from being eavesdropped and stolen now.