SRTP master key determination is through configurations at the end points? or through SDP exchanges?

Question

In case of a SRTP call setup through SIP session setup, is the master key exchanged between the two parties?
or they have got their master keys through configuration and then use KDF to get the encryption keys for the session?

Comments

Check if this is helpful (saying it is exchanged)
http://genesysguru.com/blog/blog/2011/09/14/implementing-secure-voice-using-secure-rtp-srtp/

Answer 1

Very old thread but let me try to answer it

SRTP master key is exchanged through SDP exchange. Follow the RFC 4568 to exchange the Master key b/w two parties. The Inline attribute supports to generate the Key.

Ex: Offer

Øv=0
Øo=sam ********** ********** IN IP4 10.47.16.5
Øt=********** **********
Øm=audio 49170 RTP/SAVP 0
Øc=IN IP4 168.2.17.12
Øa= sendrecv
Øa= 3ge2ae:requested
Ø a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz