End-to-end authentication capability based on public-key certificates is combined with the Session Initiation Protocol (SIP) to allow a SIP node that receives a SIP request message to authenticate the sender of request. The SIP request message is sent with a digital signature generated with a private key of the sender and may include a certificate of the sender. The SIP request message may also be encrypted with a public key of the recipient. After receiving the SIP request, the receiving SIP node obtains a certificate of the sender and authenticates the sender based on the digital signature. The digital signature may be included in an Authorization header of the SIP request, or in a multipart message body constructed according to the S/MIME standard.