The updated Apple | Google COVID-19 exposure notification cryptography paper is here: https://covid19-static.cdn-apple.com/applications/covid19/current/static/contact-tracing/pdf/ExposureNotification-CryptographySpecificationv1.1.pdf … . Going to follow up with observations as I read it.
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And Apple | Google are careful not to leak the ciphertext byte. The bluetooth paper makes clear that you have to change the metadata at the same time as the key used to encrypt it (which is valid for a time interval).pic.twitter.com/nen8STOZx0
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On the whole, it looks ok to me if a bit precarious. If the Bluetooth MAC address changes and the key-rotations get out of phase, things go bad. If the app re-generates an ID when the power level changes, that's worse, though not the end of the world.
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This all reads like a well-integrated effort by professionals working across cryptographic, radio, and epidemiological boundaries to make smart trade-offs.
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Addendum: maybe the power theory isn't so silly. I forgot that the app also has to re-generate the IDs of every infected person to determine matches. AES will save a lot of power in that case.
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End of conversation
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