  In medical emergencies, an instant and secure messaging is an important
service to provide quality healthcare services. A session initiation protocol
(SIP) is an IP-based multimedia and telephony communication protocol used to
provide instant messaging services. Thus, design of secure and efficient SIP
for quality medical services is an emerging problem. In this paper, we first
explore the security limitations of the existing SIPs proposed by Sureshkumar
et al. and Zhang et al. in the literature. Our analysis shows that most of the
existing schemes fail to protect the user credentials when unexpectedly the
session-specific ephemeral secrets revealed to an adversary by the session
exposure attacks. We then present a possible improvement over Sureshkumar et
al.'s scheme without increasing the computational cost. We compare the proposed
improvement for computational overheads and security features with the various
related existing schemes in the literature.
