[Freeswitch-users] how to encrypt the sip message?

one of my sip gateway using a encrypted SIP message. char * sip_encrypt(char* str) { ... return encrypted_str; } i should send encrypted to the gateway. the gateway also send me a encrypted SIP message, and i should decrypt it. then how can i do it in Freeswitch? where can i code insert sip_encrypt()? thanks -- Peng Yong