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Facial recognition takes your most personal feature and turns it into data. Of course there will be questions about privacy.

In a letter (PDF), Democratic Sen. Al Franken of Minnesota decided to put several of them directly to Apple CEO Tim Cook, a day after the company announced that its iPhone X would unlock with Face ID. The phone scans your face and uses it as a password, relying on 30,000 unique dots it recognizes. Franken raised concerns about whether the tech giant could protect people's privacy with millions of faces being scanned.

Face ID has an uphill climb when it comes to gaining public confidence, thanks to past issues with facial recognition on other devices. Franken asked Cook to explain how the Face ID data is being stored, if a third-party app would be able to access face data, if the data would ever be stored remotely (instead of in the iPhone X's Secure Enclave), and how exactly the technology avoids being tricked by a mask or photograph.

Franken also asked if the camera associated with Face ID will always be on, and how Apple will deal with law enforcement requests for Face ID data. It's an important question, considering biometric data isn't protected by the Fifth Amendment. Last year, a judge ruled that a suspect had to unlock an iPhone using a fingerprint.

Apple has fought for user privacy in the past. The company went head-to-head with the FBI over the agency's demand that Apple unlock an iPhone belonging to one of the terrorists behind the San Bernardino, California, shootings.

Franken is the chairman of the Senate Judiciary Subcommittee on Privacy, Technology and the Law, and has questioned Apple's biometrics in the past. In 2013, he wrote a similar letter to Cook after Apple announced Touch ID, a fingerprint scanner, for its iPhones.

While promoting Face ID, Apple Marketing Chief Phil Schiller said the company used more than 1 billion pictures to train its facial recognition algorithm. Franken said he wants to know where those 1 billion photos came from.

Franken said he's expecting answers by Oct. 13.

Apple didn't respond to a request for comment.

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