4.1 Peer Clients Require Encryption
Alice sends Bob an INVITE request for an audio call requesting encryption by including the a=encryption:required attribute and the SDP field k= with a suitable key.
Bob accepts the call with a 200 OK with the a=encryption:required attribute and the k= field.
Alice sends an ACK.
Alice and Bob start sending media encrypted with the keys negotiated using the k= field.

Figure 1: Audio call requesting encryption
The following is an example of SDP signaling in an SIP INVITE request that is sent across the wire.
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v=0 o=- 0 0 IN IP4 11.22.33.44 s=session c=IN IP4 11.22.33.44 b=CT:1000 t=0 0 m=audio 5050 RTP/AVP 97 111 112 6 0 8 4 5 3 101 k=base64: Y6UN8SAFnqNTI61uN+1II3dqRk0spxbtqfuv5EYSYSM a=rtpmap:97 red/8000 a=rtpmap:111 SIREN/16000 a=fmtp:111 bitrate=16000 a=rtpmap:112 G7221/16000 a=fmtp:112 bitrate=24000 a=rtpmap:6 DVI4/16000 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:4 G723/8000 a=rtpmap:5 DVI4/8000 a=rtpmap:3 GSM/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-16 a=encryption:required m=video 5036 RTP/AVP 34 31 k=base64: Y6UN8SAFnqNTI61uN+1II3dqRk0spxbtqfuv5EYSYSM a=rtpmap:34 H263/90000 a=rtpmap:31 H261/90000 a=encryption:required
The following is an example of SDP signaling in the 200 OK response that is received across the wire in response to the INVITE.
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v=0 o=- 0 0 IN IP4 11.22.33.55 s=session c=IN IP4 11.22.33.55 b=CT:1000 t=0 0 m=audio 5050 RTP/AVP 97 111 112 6 0 8 4 5 3 101 k=base64: h6KmqIeIFj3rz4iiMAsYQju5EEitwAiveVu7RuHhev8 a=rtpmap:97 red/8000 a=rtpmap:111 SIREN/16000 a=fmtp:111 bitrate=16000 a=rtpmap:112 G7221/16000 a=fmtp:112 bitrate=24000 a=rtpmap:6 DVI4/16000 a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:4 G723/8000 a=rtpmap:5 DVI4/8000 a=rtpmap:3 GSM/8000 a=rtpmap:101 telephone-event/8000 a=fmtp:101 0-16 a=encryption:required m=video 5036 RTP/AVP 34 31 k=base64: h6KmqIeIFj3rz4iiMAsYQju5EEitwAiveVu7RuHhev8 a=rtpmap:34 H263/90000 a=rtpmap:31 H261/90000 a=encryption:required