Apple has hired a noted computer security researcher who helped Microsoft lock down its Windows operating system.

Kristin Paget — formerly known as Chris Paget — now works on Apple’s security team. Just over five years ago, she was part of a small team of elite hackers brought in by Microsoft to lock down Windows Vista.

Criminals have long preferred to focus their attacks on Microsoft’s operating system — still the world’s most popular — but lately, there have been signs that they’re eying Apple’s Mac OS X too. And Apple, slowly, has been trying to make inroads into the security community. This summer, an Apple engineer spoke at the Black Hat security conference for the first time.

Reached via e-mail, Paget confirmed that she is now an Apple employee, but referred all questions to Apple’s public relations group. An Apple spokeswoman declined to comment on Paget.

Paget’s work at Microsoft had been similarly secretive. She’d been forbidden from speaking about it for five years after her work there ended.

But in 2011, the NDA expired, and she spilled the beans on her Vista hacking at the Black Hat Las Vegas conference. In short: Microsoft’s security team had expected Vista to be pretty clean when Paget got her hands on it, but they were wrong.

“We prevented a lot of bugs from shipping on Vista,” Paget said, according to a recording of her talk. “I’m proud of the number of bugs we found and helped get fixed.”

Paget and company’s bug-hunt was so successful, in fact, that it forced Microsoft to push back Vista’s ship date. When the work was done, the hackers received special T-shirts, signed by Microsoft Vice President of Windows Development Brian Valentine. They read: “I delayed Windows Vista.”

@radian Building things, ideally security-focussed hardware.I've done too much breaking of things, it's time to create for a change... — Kristin Paget (@KrisPaget) July 9, 2012

Until this past summer, Paget had been chief hacker at Recursion Ventures, a company that specializes in hardware security. When she left in July, she said she was looking for a break from bug-finding, hoping to find a job that involved building “security-focused hardware.”

“I’ve done too much breaking of things, it’s time to create for a change,” she said on Twitter.

She was hired in September as a core operating system security researcher at Apple, according to her Linkedin Profile.

Paget made headlines in 2010 when she built her own cellphone-intercepting base station at the Defcon hacker conference.

Back then, Paget was known as Chris. She switched genders last year.

Although Paget said she was “total Unix head” — and expressed a dislike of Windows during her 2011 Black Hat talk — she didn’t have anything to say about Apple.