How easy is it for the average internet user to make a phone call secure enough to frustrate the NSA's extrajudicial surveillance program?

Wired News took Phil Zimmermann's newest encryption software, Zfone, for a test drive and found it's actually quite easy, even if the program is still in beta.

Zimmermann, the man who released the PGP e-mail encryption program to the world in 1991 – only to face an abortive criminal prosecution from the government – has been trying for 10 years to give the world easy-to-use software to cloak internet phone calls.

On March 14, Zimmermann released a beta version of the widely anticipated Zfone. The software is currently available only for OS X (Tiger) and Linux, though a Windows version is due in April.

The open-source software manages cryptographic handshakes invisibly, and encrypts and decrypts voice calls as the traffic leaves and enters the computer. Operation is simple, and users don't have to agree in advance on an encryption key or type out long passcodes to make it work.

Would-be beta testers must provide Zimmermann with an e-mail address. That seems an odd requirement for a privacy product, but the process itself was painless, and an e-mail with a download code arrived immediately.

In our test, Zfone installed easily and quickly on OS X, though there were some mild hitches in actually getting it to work.

Zfone is designed to work with VoIP clients that use the industry standard SIP protocol, and has been tested with clients such as X-lite, Free World Dialup and Gizmo Project.

Following Zfone's instructions, Wired News was able to fairly quickly configure Gizmo Project to work with the software. But initial efforts to make phone calls with the system failed. Eventually, a little trial and error revealed that Zfone needed to be started before Gizmo Project, and that to see if a secure connection has been created, both Gizmo and Zfone's interface needed to be visible on the desktop.

Once that happens, and the caller on the other end also has Zfone installed, the interface cleanly indicates that the call is secure. It also displays two different three-character codes. One party reads his code, e.g. "CF8," while the other says hers, "TKP."

This bit of cloak-and-dagger isn't just fun, it helps prevents what is known as a man-in-the-middle attack, in which an eavesdropper sits between two callers, intercepting their cryptographic keys and then relaying the communications between them. If someone tries that with Zfone, the spoken codes won't match what the callers see on their screens.

Using Zfone didn't add any noticeable latency or distortion to calls made with Gizmo Project. Once it's up and running, you're simply talking on the phone.

But make no mistake: to eavesdroppers, Zfone is anything but routine. The protocol is based on SRTP, a system that uses the 256-bit AES cipher and adds to that a 3,000-bit key exchange that produces the codes callers can read off to one another. It has been submitted to IETF for approval as an internet standard, and by most accounts is strong enough to defy even the most sophisticated code-breaking technologies, from a hacker's packet sniffer to the acres of computers beneath Ft. Meade.

That makes Zfone the "most secure telephone system anyone has ever used," according to PGP Corporation's CTO Jon Callas, who worked with Zimmermann on the protocol

Of course, security is nice, but the value of an end-to-end crypto system is partially a function of its popularity. If you're the only one using the system, there's nobody to talk to.

The Gizmo Project ostensibly uses its own encryption for Gizmo-to-Gizmo calls, though the company won't reveal what algorithms they use. But primarily, Zfone is competing with the built-in crypto that comes with Skype, which is closed-source, uses its own proprietary protocols, and employs its own encryption scheme – which, significantly, is not available for inspection and peer-review (though some have evaluated (.pdf) it and others purportedly cracked it anyway).

Those are all troubling signs for a security system. But as a standard element in Skype's popular VoIP software, this unproven crypto has already achieved a market penetration that will likely elude Zimmerman's system.

So as nice as it is, unless Zfone is adopted by mainstream VoIP providers, it will probably occupy the same limited market niche as the hyper-secure PGP program that ruffled so many government feathers over a decade ago.

PGP didn't become standard e-mail fare outside of the community of geeks, cypherpunks and those with special privacy needs, like human rights workers and people living in countries where the government routinely spies on its citizens without oversight. Fortunately for Zimmerman, there are a lot more of us these days.