Function secure_audio_answer 

pub fn secure_audio_answer(
    session_id: u64,
    host: &str,
    port: u16,
    payload_type: u8,
    telephone_event_pt: Option<u8>,
    crypto: &str,
) -> String

audio_answer for an RTP/SAVP offer: the answer uses the secure profile and carries crypto, the value of our a=crypto: line (the tag and suite of the offer line we accept, and our own key).