The SIP (Session Initiation Protocol) is an application and presentation layer signalling protocol used for
initiating, continuing and terminating multimedia sessions for the end user. It gains much attention from the
researchers because it is exposed to several threats and has been identified with challenging vulnerabilities from time to time.
Consequently, the security of SIP is a crucial task, and different researchers have made many efforts
to divert attention towards its solution. But still, no one claims with conviction about a foolproof
secure mechanism for SIP. As users extensively use SIP services, the mutual authentication and key agreement
among the participants is an important issues. So, robust authentication and key agreement schemes are
mandatory for enhancing security, legitimacy and complexity. Therefore, we present an improved
three-factor authentication scheme that caters to all the weaknesses and known attacks in the Mishra et al. scheme. The
proposed scheme not only guarantees security but also ensures that performance can be made lightweight. As
performance and security contradict each other, the change in one inversely affects the other. The proposed
scheme has been analysed both formally using BAN (Burrows-Abadi-Needham) logic and ProVerif 1.93
Software verification toolkit, and informally using assumptions which show a delicate balance of security with
performance.