Patent ID: 7770007

Claim:
A method of a Session Initiation Protocol (“SIP”) client for providing mutual authentication between the SIP client and a SIP proxy, comprising: sending to the SIP proxy a first request; receiving from the SIP proxy a first challenge to the first request that includes a SIP proxy security context, the first challenge to the first request comprising an indication of an authentication mechanism comprising at least Kerberos and NTLM Protocol; in response to the SIP client receiving the first challenge, obtaining from a distribution center a session key of the SIP proxy and a Kerberos server ticket, the Kerberos server ticket encrypted with a key of the SIP proxy and including authentication data of the SIP client; and sending to the SIP proxy a second request signed using the session key the second request including a proxy authorization header with the Kerberos server ticket and a security context based on the SIP proxy security context; receiving from the SIP proxy a first response to the second request; and verifying that the first response was signed using the session key to authenticate the SIP proxy.