TP-capable CUCM or enterprise domain with a SIP trunk that does not offer SRTP as a service. Chapter 8: Interworking 157 Business-to-business interconnect: The public segment between the two enterprises are protected with encryption while neither of the enterprises themselves have de- ployed encryption. Numerous combinations of TLS and SRTP interworking might be desirable. Often only the signaling is encrypted (TLS), but the media is not. The opposite is technically possi- ble-which is to encrypt the media (SRTP) but not the signaling-but this makes no sense to deploy because it is does not produce any security. The SRTP keys are exchanged in the signaling streams, SO unless the signaling path is secured, there is no point in encrypting the media. Figure 8-6 illustrates five different TLS/SRTP interworking scenarios: Server Server Server Server Server SIP/H.323 Figure 8-6 TLS/SRTP Interworking Options Both legs TLS; SRTP passthrough (a): The signaling of both legs of the call is en- crypted with TLS and are terminated on the border element. The SRTP packets are passed through as is. The border element rewrites the IP headers of the SRTP packets, but does not examine the encrypted payload. The border element passes through the SRTP keys between the endpoints, but it is not involved in key management or de- cryption or encryption of the media. Both legs TLS; no media encryption (b): The signaling of both legs of the call is encrypted with TLS and is terminated on the border element. The media is not encrypted, and the RTP packets are passed through as is. One leg TLS; no media encryption (c): The signaling of one leg of the call is encrypted with TLS and is terminated on the border element. The media is not encrypted, and the RTP packets are passed through as is. 158 SIP Trunking One leg TLS and SRTP; no encryption on the other leg (d): The signaling of one leg of the call is encrypted with TLS and is terminated on the border element. The media of the same leg is encrypted with SRTP and terminated (that is, decrypt