[Users] Secure SIP messages

Hello, I have a few questions about how to secure SIP messages... IPSec or TLS can be used to guarantee privacy, integrity and authenticity of transmitted data. But SIP is only payload/data for these protocols. Digest Authentication does only guarantee authenticity... S/MIME seems to be interesting but I don't know any implementation in a SIP client. My questions: - Is there any mechanism that prevents me from corrupting or faking SIP messages? - Is it possible to create a kind of binding between the certificates used for TLS/IPSec and the SIP accounts? Thanks for your answers... Best regards, Steffen