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Apple Reference Image
Apple (Hacker News, MacRumors):
Our solution hinges on splitting the Apple Reference Image process into two phases: creating a secure digital negative, and developing that negative into a reference image. Each phase receives our strongest protections.
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Apple Reference Image provides both a lower bound and an upper bound on capture time from Apple’s cryptographic timestamp service, and we guarantee the photo was taken between the two bounds. On a regular heartbeat, the device requests a cryptographic timestamp token, and retains the most recent one it has received. Globally this happens on average every 15 minutes, though the interval depends on local network conditions. This provides a proven lower bound timestamp for the photographic capture. After capture, the device requests a second timestamp to use as an upper bound, and both timestamps are embedded and signed with the sensor data.
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When the user chooses to create a reference image, the device uploads the digital negative to PCC, which runs the processing steps needed to render the image — including demosaicing, tone mapping, and compression — in a highly secure, private, and verifiable environment. Experts can verify that PCC doesn’t alter a digital negative during development: they can examine the software that does the work.
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Finally, as no security system is perfect, we created a revocation system that can revoke individual photos, as well as all photos from a specific sensor.
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We built Apple Reference Image to avoid using an explicit, public credential for photographers, and to avoid even implicit public association between different photos taken by the same sensor. The final reference image is instead signed by Apple’s signing service, after validation by PCC.
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