Session initiation protocol (SIP) is the most widely used application layer control protocol for creating, modifying, and terminating session processes. Many authentication schemes have been proposed for SIP aimed at providing secure communication. Recently, a new authentication and key agreement scheme for SIP has been proposed, and it was claimed that it could resist a variety of attacks. However, in this paper, we show that this scheme is vulnerable to an offline password guessing attack and a stolen memory device attack. Furthermore, we show that it lacks the verification mechanism for a wrong password, and that the password updating process is not efficient. To mitigate the flaws and inefficiencies of this scheme, we design a new robust mutual authentication with a key agreement scheme for SIP. A security analysis revealed that our proposed scheme was robust to several kinds of attacks. In addition, the proposed scheme was simulated by the automatic cryptographic protocol tool ProVerif. A performance analysis showed that our proposed scheme was superior to other related schemes.Transport 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, Zhang et al. [1] proposed a flexible smart card based authentication scheme for SIP and claimed that it has strong security. However, Irshad et al. [2] pointed out that Zhang 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, Arshad et al. [3] later found that the scheme of Irshad et al. cannot resist a user impersonation attack. To overcome this weakness, Arshad et al. proposed a new efficient and secure scheme based on ECC [3]. Very recently, Lin et al. [4]demonstrated that the scheme of Arshad 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 authen...