DTLS, DTLS-SRTP

ByMin Wang

May 4, 2016

DTLS:

Basically DTLS is to construct TLS over datagram (UDP, DCCP, etc.)

DTLS is similar to TLS intentionally except that DTLS has to solve two problems: packet lost and reordering.

DTLS-SRTP

DTLS-SRTP can be viewed in two equivalent ways: 
   as a new key management method for SRTP,
   and a new RTP-specific data format for DTLS.

         Client                                               Server

         ClientHello + use_srtp       -------->
                                              ServerHello + use_srtp
                                                        Certificate*
                                                  ServerKeyExchange*
                                                 CertificateRequest*
                                      <--------      ServerHelloDone
         Certificate*
         ClientKeyExchange
         CertificateVerify*
         [ChangeCipherSpec]
         Finished                     -------->
                                                  [ChangeCipherSpec]
                                      <--------             Finished
         SRTP packets                 <------->      SRTP packets

Image:Srtp key exchange dtls srtp.png

It does not transmit the keys in signalling messages but inband as part of the media stream using end-to-end encryption. Before the media stream starts both endpoints perform a mutual DTLS handshake on the media ports. The shared symmetric key that was established for the resulting DTLS session is then used to derive an SRTP key. Then the encrypted SRTP media stream is started.

References

http://wiki.innovaphone.com/index.php?title=Reference11r1:Concept_DTLS-SRTP

https://tools.ietf.org/html/rfc5764

http://stackoverflow.com/questions/15331294/difference-between-dtls-and-tls

http://security.stackexchange.com/questions/29172/what-changed-between-tls-and-dtls