208 International Journal on Advances in Security, vol 4 no 3 & 4, year 2011, http://www.iariajournals.org/security/ 2011, © Copyright by authors, Published under agreement with IARIA - www.iaria.org Advancement Towards Secure Authentication in the Session Initiation Protocol Lars Strand Norwegian Computing Center / University of Oslo Oslo, Norway Email: lars.strand@nr.no Wolfgang Leister Norwegian Computing Center Oslo, Norway Email: wolfgang.leister@nr.no Abstract—The Digest Access Authentication method used in the voice over IP signaling protocol, SIP, is weak. This authen- tication method is the only method with mandatory support and widespread adoption in the industry. At the same time, this authentication method is vulnerable to a serious real-world attack. This poses a threat to VoIP industry installations and solutions. In this paper, we propose a solution that counters attacks on this wide-spread authentication method. We also propose a two-step migration towards a stronger authentication in SIP. We add support for a Password Authenticated Key Exchange algorithm that can function as a drop-in replacement for the widely adopted Digest Access Authentication mechanism. This new authentication mechanism adds support for mutual authentication, is considered stronger and can rely on the same shared password used by the digest authentication. A long-term solution is to replace the authentication scheme in SIP with a security abstraction layer. Two such security frameworks are in- troduced, discussed and evaluated: the Generic Security Services Application Program Interface and the Simple Authentication and Security Layer, which both enable SIP to transparently support and use more secure authentication methods in a unified and generic way. Index Terms—SIP, authentication, Digest Access Authentica- tion, PAKE, SASL. I. I NTRODUCTION Considering the growing market share for Voice over IP (VoIP) technologies, VoIP services need to be stable and secure for the benefit of both users and service providers. Authentication methods are an important part of this and need to be thoroughly examined. We base our current work on a conference article [1], where we analyzed and implemented an attack on the Digest Access Authentication used in the Session Initiation Protocol (SIP) and proposed a correction to mitigate this attack. Since there is a need for better authentication methods in SIP, we add support for a security abstraction layer in SIP [2] and propose a migration strategy towards a secure authentication in SIP [3]. The importance of analyzing and improving the SIP au- thentication methods comes from the fact that there has been a steady increase in the number of VoIP users since 2002, as well as a decrease in the number of PSTN installations [4]. With two billion users worldwide having access to the Internet by the end of 2010 [5], the VoIP growth potential is huge. For example, at the end of 2009, 29.1 % of the private land-line phone market in Norway used VoIP. VoIP is the emerging technology that will eventually take over from the traditional Public Switched Telephone Network (PSTN) [6] due to VoIP’s improved flexibility and function- ality, such as improved sound quality (“HD sound”) using wideband codecs like G.722 [7], instant messaging (IM), presence, mobility support, and secure calls. VoIP also reduces maintenance and administration costs since it brings conver- gence to voice, video and data traffic over the IP infrastructure. Although there exist several competing network protocols that are capable of delivering VoIP, the Session Initiation Protocol (SIP) [8] and the Real-time Transport Protocol (RTP) [9] developed by the IETF have become the de facto industry standard. These two protocols fulfill two different functions – SIP is used for signaling, e.g., responsible for setting up, modifying and tearing down multimedia sessions, while RTP transports the actual media stream (voice). Although the SIP protocol is flexible and rich in functionality [10], several vulnerabilities and security attacks have been found [11]–[13]. Securing a SIP-based VoIP system has proven challenging and the reasons are multi-faceted: • The scale and complexity of the SIP protocol specifica- tion, with primary focus on functionality rather than a sound security design [14]. • SIP usage of intermediaries, expected communication between nodes with no trust at all, and its user-to-user operation make security far from trivial [8, page 232]. • A large number of threats against VoIP systems have been identified [15]. Several security mechanisms for coun- termeasures have been proposed, but no single security mechanism is suited to address all these security threats concerning VoIP and SIP [16], [17]. • Since the SIP and RTP protocols share the same infras- tructure as traditional data networks, they also inherit the security problems of data communication. • VoIP services have strict requirements to the network performance with respect to Quality of Service since it is a duplex communication with low tolerance for latency, packet loss and saturation. Introducing strong security mechanisms might affect network performance [18]. PSTN is a mature and stable technology providing 99.999% uptime [19], and users will expect VoIP to perform at similar