Phil Zimmerman, who created the PGP encryption software, and faced a government investigation as a result, now offers a new program, Zfone, that provides end-to-end encryption of computer-to-computer (VoIP) phone calls, according to a story in yesterday’s New York Times.
One of the tricky technical problems in encrypting communications is key exchange: how to get the two parties to agree on a secret key that only they know. This is often done with a cumbersome “public key infrastructure” (PKI), which wouldn’t work well for this application. Zfone has a clever key exchange protocol that dispenses with the PKI and instead relies on the two people reading short character strings to each other over the voice connection. This will provide a reasonably secure shared secret key, as long as the two people recognize each others’ voices.
(Homework problem for security students: What does the string-reading accomplish? Based on just the information here, how do you think the Zfone key exchange protocol works?)
In the middle of the article is this interesting passage:
But Mr. Zimmermann, 52, does not see those fearing government surveillance — or trying to evade it — as the primary market [for Zfone]. The next phase of the Internet’s spyware epidemic, he contends, will be software designed to eavesdrop on Internet telephone calls made by corporate users.
“They will have entire digital jukeboxes of covertly acquired telephone conversations, and suddenly someone in Eastern Europe is going to be very wealthy,” he said.
Though the article doesn’t say so directly, this passage seems to imply that Zfone can protect against spyware-based eavesdropping. That’s not right.
One of the challenges in using encryption is that the datastream is not protected before it is encrypted at the source, or after it is decrypted at the destination. If you and I are having a Zfone-protected conversation, spyware on your computer could capture your voice before it is encrypted for transmission to me, and could also capture my voice after it is decrypted on your computer. Zfone is helpless against this threat, as are other VoIP encryption schemes.
All of this points to an interesting consequence of strong encryption. As more and more communications are strongly encrypted, would-be spies have less to gain from wiretapping and more to gain from injecting malware into their targets’ computers. Yet another reason to expect a future with even more malware.