Today, Session Initiation Protocol (SIP) is considered as predominant protocol for Voice over IP (VoIP) systems. When the client require to use SIP services, as a demand of accessing proxy server, authentication will be important challenge. SIP protocol presents SIP request authentication through a challenge-response scheme named HTTP Digest Authentication which is inherited from HTTP protocol, but this authentication protocol is vulnerable to different kind of known attacks. In this paper, it has been tried to make the authentication mechanism stronger and to resolve its weaknesses. In this regard, Elliptic Curve Cryptography (ECC) used along with an auxiliary ticket server, in which the results showed an increase in the authentication mechanism security and made it stronger against different kinds of attacks. Gained analysis results represent that the proposed method provides more security and less computational overhead, in compare with other methods.

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