The U.S. agency best known for eavesdropping on telephone calls had a hand in the development of Microsoft's Vista operating system, Microsoft confirmed Tuesday.

The National Security Agency (NSA) stepped in to help Microsoft develop a configuration of its next-generation operating system that would meet U.S. Department of Defense (DOD) requirements, said NSA Spokesman Ken White.

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This is not the first time the secretive agency has been brought in to consult with private industry on operating system security, White said, but it is the first time the NSA has worked with a vendor prior to the release of an operating system.

By getting involved early in the process, the NSA helped Microsoft ensure that it was delivering a product that was both secure and compatible with existing government software, he said.

"This allows us to ensure that the off-the-shelf security configuration that the DOD customer receives is at a level that meets our standards," White said. "It just makes a lot more sense to be involved up-front, than it does to have the tail wag the dog."

The NSA's involvement in Vista was first reported Tuesday by The Washington Post .

The NSA has provided guidance on how best to secure Microsoft's Windows XP and Windows 2000 operating systems in the past. The agency is also credited with reviewing the Vista Security Guide published on Microsoft's Web site.

Microsoft declined to allow its executives to be interviewed for this story. But in a statement the company said that it asked a number of entities and government agencies to review Vista, including the NSA, the North Atlantic Treaty Organization (NATO), and the National Institute of Standards and Technology (NIST).

Still, the NSA's involvement in Vista raises red flags for some. "There could be some good reason for concern," said Marc Rotenberg, executive director of the Electronic Privacy Information Center (EPIC). "Some bells are going to go off when the government's spy agency is working with the private sector's top developer of operating systems."

Part of this concern may stem from the NSA's reported historical interest in gaining "back-door" access to encrypted data produced by products from U.S. computer companies like Microsoft.

In 1999, U.S. Congressman Curt Weldon said that "high level deal-making on access to encrypted data had taken place between the NSA and IBM and Microsoft," according to EPIC's Web site.

With Vista expected to eventually power the majority of the world's personal computers, it would be tempting for the government agency to push for a way to gain access to data on these systems, privacy advocates say.

The NSA provided guidance on Vista's security configuration, but it did not open any back doors to Windows, White said. "This is not the development of code here. This is the assisting in the development of a security configuration," he said.

While the NSA is best known for its surveillance activities, the work with Microsoft is being done in accordance with the NSA's second mandate: to protect the nation's information system, White said. "This is the other half of the NSA mission that you never hear much about," he said. "All you ever hear about is foreign signal intelligence. The other half is information assurance."