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Security Bits — 19 April 2020

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Deep Dive — Apple & Google’s Privacy-Protecting COVID-19 Contact Tracing API

Apple and Google have partnered to develop on an API for tracking close personal contacts in an attempt to fight the COVID-19 pandemic. Unlike other solutions rolled out by some more authoritarian governments, this is a de-centralised solution designed from the ground up to prevent it’s use for tracking.

The solution makes use low energy bluetooth rather than GPS for recording close physical contacts, and it uses a combination of public-key crypto and one-way hashing functions to generate anonymous ephemeral tokens that each participating phone broadcasts. As users move around their phone records the ephemeral tokens for all phones it comes close to, and keeps a lot for a few weeks. If a participating user gets tested positive, they can choose to instruct their phone to tell a server that they have tested positive, and upload all their ephemeral keys. Participating devices will periodically check to see if any of the known-infected keys are in their cache, and if they are, alert the user that they have potentially been exposed.

The key point is that the tokens change regularly, so the same phone does not have the same token for long. This means you can’t use the tokens to track people. Also, the tokens cycle in sync with the randomisation of Bluetooth MAC addresses, so the tokens can’t be used to un-do the tracking protection provided by MAC address randomisation.

Apple and Google insist their API will always be opt-in, and that there will need to be some kind of validation of diagnoses to avoid trolling. There will not be a single global server all phones check for positive tokens, instead, separate countries or regions will run their own servers, and the operators of those servers will put in the appropriate safeguards to validate positive diagnoses.

Initially this will be available as an API developers can incorporate into 3rd-party apps, but Apple and Google plan to add the functionality into iOS and Android in the coming months.

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