# A Robust Mutual Authentication with a Key Agreement Scheme for Session Initiation Protocol
## 1 Introduction  
Session initiation protocol (SIP) is an application layer control protocol proposed and studied by the Internet Engineering Task Force (IETF) on the Internet Protocol (IP) network for multimedia communication. SIP is used to create, modify, and terminate one or more participants' session processes. It supports five aspects in establishing and maintaining the termination of a multimedia session: User location, user effectiveness, user ability, session establishment, and session management. An important feature of SIP is that it does not define the type of a session to establish, but only defines how to manage a session. Because of this flexibility, SIP can be used in many applications and services including interactive games; music and video on demand; and voice, video, and Web conferences. SIP reuses a Multipurpose Internet Mail Extensions (MIME) type description as an e-mail client, so that the conversational related applications can be automatically activated. Moreover, SIP reuses several existing mature Internet services and protocols, such as Domain Name System (DNS), Real-timeTransport Protocol (RTP), Resource Reservation Protocol (RSVP), and so on. Therefore, since many parts of the infrastructure are in place or ready for use, there is no need to introduce new services to support the SIP infrastructure.  
Although users enjoy the services provided by SIP, security has emerged as a major issue because the transmitted data usually contains people's sensitive and private information. To guarantee a secure communication in SIP, a secure authentication with a key agreement scheme should be executed before the communication begins. For this reason, many related schemes for SIP have been proposed [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11] in the past few years.  
In 2014, [PERSON] et al. [1] proposed a flexible smart card based authentication scheme for SIP and claimed that it has strong security. However, [PERSON] et al. [2] pointed out that [PERSON] et al.'s scheme is vulnerable to a DOS attack, and that it could become more secure by adding a few modifications. They then proposed an improved SIP scheme [2]. Unfortunately, [PERSON] et al. [3] later found that the scheme of [PERSON] et al. cannot resist a user impersonation attack. To overcome this weakness, [PERSON] et al. proposed a new efficient and secure scheme based on ECC [3]. Very recently, [PERSON] et al. [4] demonstrated that the scheme of [PERSON] et al. is vulnerable to a server spoofing attack, a DOS attack, a privilege insider attack, and that it cannot achieve user anonymity. To mitigate these weaknesses, they proposed a new scheme for SIP using ECC.  
In this paper, we analyze the security of Lin et al.'s anonymous authentication and key agreement SIP scheme. We show that their scheme cannot withstand an offline password guessing attack nor a stolen memory device attack. Furthermore, Lin et al.'s scheme lacks a verification mechanism for a wrong password and the password updating process is not efficient. To overcome these flaws and inefficiencies, we propose a robust mutual authentication with a key agreement scheme.  
This paper is organized as follows: In Section 2, a review of Lin et al.'s scheme is presented. In Section 3, the flaws and inefficiencies of their scheme are described. In Section 4, a SIP scheme is introduced and described in detail. A security analysis of the proposed scheme is given in Section 5. In Section 6, an automatic cryptographic protocol tool, ProVerif, is used to simulate the proposed scheme. A performance analysis is given in Section 7. Lastly, conclusions and our findings are given in Section 8.