Yes, Virginia, VoIP Can Be Secure

One of the biggest knocks on VoIP technology is its lack of security. But at the Interop show this week, a group of vendors and technicians has quietly been demonstrating secure, multivendor VoIP connections -- both from within the LAN and from remote users over a VPN.

The InteropLabs VoIP demo focuses on two major areas of security: protecting the SIP gateway from attack and supporting remote users connecting via a VPN. The demo shows that it is possible to support VoIP within the closed confines of a single enterprise, though it also exposes potential problems with network address port translation (NAPT) that will need to be solved in live deployments.

In the demo, border protection starts with a SIP-aware application layer gateway or deep packet inspection. During call setup, the phones negotiate the call parameters they are willing to accept and what UDP ports, or ephemeral ports, they will use for voice packets. A non-SIP-aware firewall can't handle the ephemeral ports, and the voice connection between the phones could never be completed.

As Craig Johnson, systems engineering manager for Check Point, explains, "the SIP firewall has to be session-aware so that when a call ends -- either through a hang-up or a time-out -- the ephemeral ports are closed. Otherwise, avenues for malicious activity like toll fraud are possible."

The SIP-aware firewall also can stop denial-of-service attacks, which bombard the SIP gateway with registration and call requests, effectively cutting off legitimate calls or sending malformed SIP packets to the SIP gateway.

In addition to supporting ephemeral ports, SIP-aware firewalls and routers -- especially in remote offices -- must also be aware of NAPT, rewriting both the UDP packets and the network information in the SIP packets. Unlike IPSec, SIP offers no standards for NAPT traversal. In SIP networks, NAPT is achieved through packet rewriting or by changing routes within the receiving phone's network. The former is the less intrusive option.

What About Encryption?

The InteropLabs VoIP demonstration used IPSec and an SSL VPN to encrypt the traffic between the remote user and central LAN, but it did not include encryption of SIP and voice traffic. The IETF has drafted some standards for encryption of SIP and RTP, and some products, including Sipera's IPCS310, support TLS and secure RTP. But vendors and other experts disagree on the immediate need for VoIP encryption.

Executives at Nortel acknowledge that encrypting VoIP eventually will be necessary. But as Aziz Khadbai, general manager of Converged Nortel Networks, observes, "VoIP installations today are within closed networks managed by a single entity. In the context of an internal network, the need for encryption is less critical than if the traffic passes over untrusted networks."

Robert Moskowitz, senior technical director at ICSA Labs, disagrees. "The CSI/FBI study shows that 80 percent of [VoIP] attacks are from insiders," he says. "Consider the ease of SIP hijacking and the ease of header manipulation. It's easy to become a man-in-the-middle and effectively wiretap all VoIP communications." Moskowitz cites an article on Voipinder entitled "Examining Two Well-Known Attacks on VoIP," which demonstrates such an attack.

Experts say the decision on whether to encrypt VoIP depends on how the VoIP travels over the network, the enterprise's need for security, and the state of the building's physical security.