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The session initiation protocol (SIP) is a powerful signaling protocol that controls communication on the Internet, establishing, maintaining, and terminating the sessions. The services that are enabled by SIP are equally applicable in the world of mobile and ubiquitous computing. In 2009, Tsai proposed an efficient nonce-based authentication scheme for SIP. The current paper, however, demonstrates that Tsai's authentication scheme is still vulnerable to offline password guessing attacks, Denning-Sacco attacks, and stolen-verifier attacks, and does not provide perfect forward secrecy. We also propose a new secure and efficient authentication scheme based on the elliptic curve discrete logarithm problem for SIP in order to overcome such security problems.

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