Patent ID: 7240366

Claim:
A computer-readable medium having computer-executable instructions for performing steps by a first Session Initiation Protocol (SIP) node to communicate with a second SIP node, the steps comprising: under the control of the first SIP node, constructing a SIP request message; signing a selected portion of the SIP request message using a private key of a user of the first SIP node to produce a digital signature; including the digital signature in the SIP request message; generating a symmetric key for data encryption; encrypting the symmetric key with a public key of a user of the second SIP node; signing the encrypted symmetric key with the private key of the user of the first SIP node to establish a shared secret in an authenticated and secure fashion for subsequent messages; including the encrypted symmetric key and the signature in the SIP request message; and sending the SIP request message to the second SIP node; and under the control of the second SIP node, receiving the SIP request message sent by the first SIP node; retrieving the digital signature from the SIP request message; obtaining a certificate of a user identified in the SIP request message; verifying the digital signature contained in the SIP request message using a public key contained in the obtained certificate of the user; if the digital signature is verified, determining whether the user identification contained in the certificate identifies the user identified in the SIP request message; and if the user identification contained in the certificate identifies the user identified in the SIP request message, authenticating the user as the sender of the SIP request message.