VoIP networks and FMC. FMC security considerations One way to enhance secure SIP communication is to mandate the use of Message Digest (MD5) authentication by all UAs and SIP proxy servers. This hardens the handshake between the SIP proxy server and the SIP UA by adding an authentication challenge. Call Center 2.1: INVITE Agent 1 1.INVITE Caller SIP-Proxy 3.200:OK 4.200:OK (CERT call center) 2.2: INVITE Agent 2 Figure 15.12 - Proxy-based SIP security Figure 15.12 shows authentication and proxy-based security. A single security certificate can be used to encrypt and sign a message for a related group of clients, for example, members of an organization, to make it more efficient. The SIP proxy takes the role of signing and encrypting SIP messages. The private key for security will reside centrally on the proxy server. This works well for a call center relying on FMC. In addition, the use of Secure Multipurpose Internet Mail Extensions (S/MIME) to encrypt information with an SIP message is another effective way to secure it. See https : 1/learn.microsoft. com/en-us/exchange/security-and-compliance/smime-exo/smime-exofor - more information on S/MIME and how Microsoft uses it for email security. Figure 15.13 shows the S/MIME-based security paradigm. SIP header SIP header fields fields Multipart/ MIME body signed Signature certificate Alice Figure 15.13 - S/MIME-based SIP security 5G Fixed Mobile Convergence The sending UA (Alice) creates multipart/signed S/MIME SIP messages. The UA uses the private key to sign the body of the MIME message. The receiving UA (Bob) decomposes the message get the public key and validate the authenticity of the message. If there are signs of tampering in the message, then remedial actions can be taken and a cybersecurity breach can be prevented. More details about securing SIP can be found in this research paper: https: //www.researchgate. het/publication/41392312_Towards_Public_Key_Infrastructure_less authentication_in_Session_Initiation_Protocol. By using a combination of SIP security me