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Mr. Zimmermann, 52, does not see those fearing government surveillance — or trying to evade it — as the primary market. 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.

While Mr. Zimmerman is giving away his software so far, his goal is to attract VoIP software and hardware developers to license his technology and embed it in their products.

Voice Encryption May Draw U.S. Scrutiny
By JOHN MARKOFF
SAN FRANCISCO, May 21 — Philip R. Zimmermann wants to protect online privacy. Who could object to that?

He has found out once already. Trained as a computer scientist, he developed a program in 1991 called Pretty Good Privacy, or PGP, for scrambling and unscrambling e-mail messages. It won a following among privacy rights advocates and human rights groups working overseas — and a three-year federal criminal investigation into whether he had violated export restrictions on cryptographic software. The case was dropped in 1996, and Mr. Zimmermann, who lives in Menlo Park, Calif., started PGP Inc. to sell his software commercially.

Now he is again inviting government scrutiny. On Sunday, he released a free Windows software program, Zfone, that encrypts a computer-to-computer voice conversation so both parties can be confident that no one is listening in. It became available earlier this year to Macintosh and Linux users of the system known as voice-over-Internet protocol, or VoIP.

What sets Zfone apart from comparable systems is that it does not require a web of computers to hold the keys, or long numbers, used in most encryption schemes. Instead, it performs the key exchange inside the digital voice channel while the call is being set up, so no third party has the keys.

Zfone's introduction comes as reports continue to emerge about the government's electronic surveillance efforts. A lawsuit by the Electronic Frontier Foundation, a privacy rights group, contends that AT&T has given the National Security Agency real-time access to Internet communications.