Additional processing, such as compression and encryption, may increase VoIP network delay. If a stream cipher is used for encryption, very little delay is introduced if the key stream can be produced at least as fast as the voice data arrives. Block ciphers may generate more delay, which will vary with the algorithm used, but still introduce relatively little overhead. More significant delays are caused by computing HMAC hash values for authentication. In most applications, authentication and integrity are equally or more important than encryption, but with voice processing for human speakers some form of authentication is already built-in because parties recognize the person on the other end of the conversation. Even if the conversation is with a stranger, concern with source authentication applies primarily to call setup, rather than to the conversation. As a result of these considerations, some designers may limit HMAC use if performance is a problem.
The IPsec suite of protocols and encryption algorithms is the standard method for securing communications against unauthorized viewers over data networks. Accordingly, extending this protection to VoIP is both logical and practical, encrypting signal and voice packets on one end and decrypting them only when needed by their intended recipient. However, the nature of VoIP signaling protocols prevents such a simple scheme being used, as it becomes necessary for firewalls, routers, and some other network devices to read VoIP packets. Also, several factors, including the expansion of packet size, ciphering latency, possible packet loss, and lack of QoS urgency in the cryptographic engine itself, may cause a noticeable performance degradation in VoIP packet delivery. This once again highlights the tradeoff between security and voice quality and a need for speed. Fortunately, these difficulties are not insurmountable. Testing has shown that VoIPsec can be incorporated into a SIP network with roughly a three-second additional delay in call setup times, which is acceptable for many applications.
Finally, both H.323 and SIP provide additional security features and protocols, defined respectively in H.235v2/H.235v3 standards and in RFC 3261. In addition, specific protocols for media stream encryption and key management, enabling secure communication between H.323 and SIP-based clients, have been recently introduced. Secure Real-time Transport Protocol (SRTP) and Secure Real-time Transport Control Protocol (SRTCP) define a profile of RTP/RTCP intended to provide a framework for encryption, message authentication, and replay protection, achieving high throughput and low packet expansion.
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