Stream or Block Cipher for Securing VoIP?

When the network is homogeneous, choosing the cipher type for a packet telephony application is simple. It is clear that stream ciphers perform better than block ciphers over landline, circuit-switched networks, since loss is negligible in these networks but corruption is not. Likewise, it is also clear that block ciphers perform better than stream ciphers over landline, packet-switched networks, since corruption is negligible in these networks but loss is not [9]. However, the choice of cipher is not so clear for a heterogeneous internetwork containing a mix of packet and circuit-switched networks. Additionally, this issue becomes even more confounded when heterogeneous internetwork also consists of wireless links. Existing encryption systems will degrade performance in a heterogeneous internetwork because such internetworks have appreciable loss and corruption. Thus, the error properties would degrade the subjective quality of the packet telephony application. In this paper we present an experimental results of comparing block and stream ciphers when used to secure VoIP in terms of end-to-end delay and subjective quality of perceived voice. We proposed a new technique, which provides automatic synchronization of stream ciphers on a per packet basis, without the overhead of an initialization vector in packet headers or without maintaining any state of past-encrypted data. We show that this technique mitigates the trade-off between subjective quality and confidentiality.

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