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10 Apr 2020 : Initial observations on the joint Google/Apple “privacy-safe contact tracing” specification #

Apple and Google today announced a joint protocol to support contact tracing using BLE. You can read their respective posts about it on the Apple Newsroom and Google blog.

The posts offer some context, but the real meat can be found in a series of specification documents. The specs provide enough information about how the system will work to allow a decent understanding, albeit with some caveats.

With so much potential for misuse, and given that mistrust could lead to some people choosing not to use the system, it's great that Google and Apple are apparently taking privacy and interoperability so seriously. But I'm a natural sceptic, so whenever a company claims to be taking privacy seriously, I like to apply a few tests.
  1. Are the specs and implementation details (ideally sourcecode) freely and openly available?
  2. Is interoperability with other software and devices supported.
  3. Based on the information available, is there a more privacy-preserving approach that the company could have gone with, but chose not to?
The answers to these appear to be "yes" (but not the sourcecode), "mostly" and "no". It's quite unusual, even for companies like Apple that make bold claims about privacy, to satisfy any one of these, let alone more than one, so this is genuinely very encouraging. Based on the specs released so-far, it seems that this has been a good-faith attempt to achieve both protection and privacy.

The catch is that the API defined by the specs provides only half of a full implementation. Apple and Google are providing an API for generating and capturing BLE beacons. They don't say what should happen to those beacons once they've been captured. Presumably this is because they expect this part of the system to be implemented by a third-party, most likely a regional public health authority (or, even more likely, a company that a health authority has subcontracted to).

Again, this makes sense, since different regions may want to implement their own client and server software to do this. In fact, by delegating this part of the system, Google and Apple strengthen their claim that they're acting in good faith. They're essentially encouraging public health authorities and their subcontractors to live up to the same privacy standards.

Apart from the privacy issues, my other main interest is in having the same system work on operating systems other than iOS and Android. My specific interest is for Sailfish OS, but there are other smartphone operating systems that people use, and locking users of alternative operating systems out of something like this would be a terrible result both for the operating system and for all users.

Delegation of the server and app portions to health authorities unfortunately makes it highly unlikely that alternative operating systems will be able to hook into the system. For this to happen, the health authority servers would also need to provide a public API. Google and Apple leave this part completely open, and the likelihood that health authorities will provide an API is unfortunately very slim.

I'd urge any organisation planning to develop the client software and servers for a fully working system to prove me wrong. Otherwise alternative operating system users like me could be left unable to access the benefits of the system. This reduces its utility for those users to nill, but it also reduces the effectiveness of the system for all users, independent of which operating system they use, because it increases the false negative rate.

There's one other aspect of the specification that intrigues me. In the overview slide deck it states that "Alice’s phone periodically downloads the broadcast beacon keys of everyone who has tested positive for COVID-19 in her region." (my emphasis). This implies some form of region-locking that's not covered by the spec. Presumably this is because the servers will be run by regional health authorities and so the user will install an app that applies to their particular region. There are many reasons why this is a good idea, not least because otherwise the amount of data a user would have to download to their device each day would be prohibitive. But there is a downside too. It essentially means that users travelling across regions won't be protected. If they interact with someone from a different region who tests positive, this interaction won't be flagged up by the system.

The spec is still very new and no doubt more details will emerge over the coming days and weeks. I'll be interested to see how it pans out, and also interested to see whether this can be implemented on devices like my Sailfish OS phone.
Reference to region-locking, taken from the overview slide deck


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