NEW YORK — Even as the U.S. government is embroiled in a debate over the legality of wiretapping, the fastest-growing technology for Internet calls appears to have the potential to make eavesdropping a thing of the past.

Skype, the Internet calling service recently acquired by eBay Inc., provides free voice calls and instant messaging between users. Unlike other Internet voice services, Skype calls are encrypted. That apparently makes them impossible to snoop on, though the company leaves the issue somewhat open to question.

Skype is certainly not the first application for encrypted communications on the Internet. Secure e-mail and instant messaging programs have been available for years at little or no cost.

But to a large extent, Internet users haven't felt a need for privacy that outweighed the extra effort needed to use encryption. In particular, e-mail programs such as Pretty Good Privacy have been considered too cumbersome by many.

And because such applications have had limited popularity, their mere use can draw attention. With Skype, however, criminals, terrorists and other people who really want to keep their communications private are indistinguishable from those who just want to call their mothers.

"Skype became popular not because it was secure, but because it was easy to use," said Bruce Schneier, technology chief at Counterpane Internet Security Inc.

Luxembourg-based Skype was founded by the Swedish and Estonian entrepreneurs who created the Kazaa file-sharing network, which has been the subject of several court actions by the music industry.

Skype's software for personal computers is distributed for free. Members pay nothing to talk to each other over PCs but pay fees to connect to people who are using telephones. Skype software also is being built into cell-phone-like devices that will work near wireless Internet "hot spots."

While still somewhat marginal in the United States, Skype had 75 million registered users worldwide at the end of 2005. Typically, 3 million to 4 million users are online at the same time.

Skype calls whip around the Internet encrypted with "keys," which essentially are very long numbers. Skype keys are 256 bits long — twice as long as the 128-bit keys used to send credit-card numbers over the Internet. The security is much more than doubled — in theory, Skype's 256-bit keys would take trillions of times longer to crack than 128-bit keys, which are themselves regarded as practically impossible to break by current means.

"It is a pretty secure form of communication, which if you're talking to your mistress you really appreciate. But if al-Qaida is talking over Skype, you have probably a different view," said Monty Bannerman, chief executive of Verso Technologies Inc., which makes equipment for Internet service providers, including software that can identify and block Skype calls.

Security experts doubt Skype is as secure as it seems, because the company hasn't made its technology open to review.

But according to Schneier, if Skype's encryption is weaker than believed, it still would stymie the broad eavesdropping the National Security Agency is reputed to be performing.