The encryption of Skype VoIP phone calls might not be as secure as you think.

It's possible the company keeps keys so law enforcement authorities can decrypt encrypted VoIP phone calls, a report says, but Skype won't say for sure one way or the other.

According to an online report, Austrian officials with legal authority to tap VoIP phone communications have no problem listening in on Skype calls, which are encrypted as a standard part of Skype service.

A Skype spokesman wouldn't say whether Skype keeps keys to decrypt calls. "Sorry, Skype does not comment on media speculation," says Skype vice president Chiam Haas.

It's virtually impossible to figure out for sure from independent research whether Skype keeps encryption keys or not, says David Endler, chairman of Voice Over IP Security Alliance and senior director of security research at Tipping Point.

"No one has shown it publicly," he says. "Skype is a closed software package, essentially a black box." The company has on rare occasions allowed outside researchers to examine and verify the security of its encryption, but not whether the keys that can crack the encryption can be retrieved, he says.

To allay fears that the calls might not be secure from law enforcement, Skype should open its platform to evaluation by trusted, credible industry experts, he says.

Endler says it's equally difficult to know whether commercial VoIP vendors leave open the possibility of turning encryption keys over to law enforcement.

In the United States, the Communications Assistance for Law Enforcement Act (CALEA) forbids requiring that vendors build in back-door decryption, says Jim Dempsey, vice president for public policy at the Center for Democracy & Technology. "CALEA expressly forbids requiring anyone to be able to decrypt anything," he says.

But that doesn't mean they don't build in key-retrieval anyway. Dempsey says there are no active proposals to force vendors to leave encryption back doors in their VoIP gear, but that could change. "Nothing in regulations is permanent," he says.

Endler says that attempts by researchers to learn more about how Skype works have been effectively blocked by measures put in place by Skype. "They've taken extreme measures to prevent reverse engineering of their client software," he says, more so than mainstream VoIP vendors.