Why VOIP Needs Crypto
By Bruce Schneier
April 6, 2006
There are basically four ways to eavesdrop on a telephone call.
One, you can listen in on another phone extension. This is the method preferred by siblings everywhere. If you have the right access, it's the easiest. While it doesn't work for cell phones, cordless phones are vulnerable to a variant of this attack: A radio receiver set to the right frequency can act as another extension.
Two, you can attach some eavesdropping equipment to the wire with a pair of alligator clips. It takes some expertise, but you can do it anywhere along the phone line's path -- even outside the home. This used to be the way the police eavesdropped on your phone line. These days it's probably most often used by criminals. This method doesn't work for cell phones, either.
Three, you can eavesdrop at the telephone switch. Modern phone equipment includes the ability for someone to listen in this way. Currently, this is the preferred police method. It works for both land lines and cell phones. You need the right access, but if you can get it, this is probably the most comfortable way to eavesdrop on a particular person.
Four, you can tap the main trunk lines, eavesdrop on the microwave or satellite phone links, etc. It's hard to eavesdrop on one particular person this way, but it's easy to listen in on a large chunk of telephone calls. This is the sort of big-budget surveillance that organizations like the National Security Agency do best. They've even been known to use submarines to tap undersea phone cables.
That's basically the entire threat model for traditional phone calls. And when most people think about IP telephony -- voice over internet protocol, or VOIP -- that's the threat model they probably have in their heads.
Unfortunately, phone calls from your computer are fundamentally different from phone calls from your telephone. Internet telephony's threat model is much closer to the threat model for IP-networked computers than the threat model for telephony.
And we already know the threat model for IP. Data packets can be eavesdropped on anywhere along the transmission path. Data packets can be intercepted in the corporate network, by the internet service provider and along the backbone. They can be eavesdropped on by the people or organizations that own those computers, and they can be eavesdropped on by anyone who has successfully hacked into those computers. They can be vacuumed up by nosy hackers, criminals, competitors and governments.
It's comparable to threat No. 3 above, but with the scope vastly expanded.
My greatest worry is the criminal attacks. We already have seen how clever criminals have become over the past several years at stealing account information and personal data. I can imagine them eavesdropping on attorneys, looking for information with which to blackmail people. I can imagine them eavesdropping on bankers, looking for inside information with which to make stock purchases. I can imagine them stealing account information, hijacking telephone calls, committing identity theft. On the business side, I can see them engaging in industrial espionage and stealing trade secrets. In short, I can imagine them doing all the things they could never have done with the traditional telephone network.
This is why encryption for VOIP is so important. VOIP calls are vulnerable to a variety of threats that traditional telephone calls are not. Encryption is one of the essential security technologies for computer data, and it will go a long way toward securing VOIP.
The last time this sort of thing came up, the U.S. government tried to sell us something called "key escrow." Basically, the government likes the idea of everyone using encryption, as long as it has a copy of the key. This is an amazingly insecure idea for a number of reasons, mostly boiling down to the fact that when you provide a means of access into a security system, you greatly weaken its security.
A recent case in Greece demonstrated that perfectly: Criminals used a cell-phone eavesdropping mechanism already in place, designed for the police to listen in on phone calls. Had the call system been designed to be secure in the first place, there never would have been a backdoor for the criminals to exploit.
Fortunately, there are many VOIP-encryption products available. Skype has built-in encryption. Phil Zimmermann is releasing Zfone, an easy-to-use open-source product. There's even a VOIP Security Alliance.
Encryption for IP telephony is important, but it's not a panacea. Basically, it takes care of threats No. 2 through No. 4, but not threat No. 1. Unfortunately, that's the biggest threat: eavesdropping at the end points. No amount of IP telephony encryption can prevent a Trojan or worm on your computer -- or just a hacker who managed to get access to your machine -- from eavesdropping on your phone calls, just as no amount of SSL or e-mail encryption can prevent a Trojan on your computer from eavesdropping -- or even modifying -- your data.
So, as always, it boils down to this: We need secure computers and secure operating systems even more than we need secure transmission.
Photo of Bruce Schneier by Per Ervland.
Schneier on Security is a personal website. Opinions expressed are not necessarily those of Co3 Systems, Inc..