1 Secure VoIP: call establishment and media protection Johan Bilien, Erik Eliasson, Joachim Orrblad, Jon-Olov Vatn Telecommunication Systems Laboratory Royal Institute of Technology (KTH) Stockholm, Sweden

2 Protecting the signaling –encryption and integrity protection –hop-by-hop –protection of privacy Protecting the media –encryption and integrity protection –end-to-end –at network (IPSec ESP) or application layer (SRTP) Authenticated Key Exchange (AKE) –provides key to protect the media –allows callee policies, such as filtering of spam Requirements for secure VoIP UA P P

3 AKE for Secure VoIP Which protocol? –IKE (RFC 2409) widely deployed and acknowledged –MIKEY (RFC 3830) specifically designed for protection of multimedia services MIKEY profile defined for SRTP How to combine the AKE and the SIP signaling? –“out-of-band”, performed in additional messages, or –integrated, carried in the SIP messages

4 Performance metrics Ringing delay (RD) –from sending the INVITE to receiving the ringing notification –includes caller authentication Media clipping (MC) –media transmission is hindered by ongoing cryptographic processing Ghost ringing –the caller cancels the call after the callee started ringing INVITE 180 Ringing RD 200 OK RTP MC

5 IKE and SIP signaling IKE performed “out of band” SIP preconditions (RFC 3312) extended for IKE setup INVITE / IPSec required UPDATE IKE 183 Session in progress 200 OK (UPDATE) 200 OK (INVITE)

6 MIKEY and SIP signaling MIKEY integrated with SIP / SDP Without reliable provisional responses –Processing of the MIKEY response in the 200 OK creates media clipping INVITE / MIKEY Init 200 OK / MIKEY Response With reliable provisional responses –The MIKEY response is sent reliably in a provisional response –The security association is complete before the 200 OK is sent, thus avoiding media clipping 200 OK INVITE / MIKEY Init 183 / MIKEY Response PRACK

7 Implementation Signaling protection using TLS Media protection –SRTP AKE using MIKEY in the SDP offer-answer –IPSEC – ESP AKE using MIKEY in a separate MIME payload proposed MIKEY profile for ESP No reliable provisional response Open source (LGPL and GPL)

8 Secure call setup - delays Bob Alice INVITE/MIKEY Init Invite processing SIP Processing MIKEY verify, Policy check Callee Transmit Clipping Create MIKEY Reply Session key gen. (Update IPSec DBs) Packetization delay Ringing delay Create MIKEY Init SIP processing Caller Transmit Clipping: SIP Processing MIKEY verify, policy check Session key gen. (Update IPSec DBs) Packetization Delay Bob 180 Ringing 200 OK/MIKEY Reply DIAL OFF HOOK a2 a3 a4 a1 b1 RTP Media b2 b3 Caller Reception Clipping

9 Measurements

10 Conclusions and future work In all the measured cases, the ringing delay is not significant for a human person (~ 75 ms) The key exchange for SRTP results in a short transmit clipping on both sides (~170 ms) The use of IPSec results in a major media clipping on both sides (~ 800 ms). We believe this to be a Linux IPSec implementation issue. Adding support for reliable provisional responses, to carry the MIKEY response, would cancel those clippings. We recommend the use of SRTP for media protection, TLS for signaling protection, and an authenticated key exchange based on MIKEY.