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This specific one came out a few hours ago and is pretty auditable even if those two don't disclose their implementation for this tiny part.

Other than that many, in fact most of those I know to be active, tracing efforts are pretty open, the one you linked is definitely not the only one that currently publishes their source or plans to do so in the near future. There's even been others in this thread.

Agreed, most of them are completely open. Here's the best effort I have seen to document all the parallel efforts:

https://docs.google.com/document/d/16Kh4_Q_tmyRh0-v452wiul9o...

I believe the idea here is that the tracing is done at the platform level, where you can both have different capabilities with the wireless functions of the device, and can have different privacy constraints on usage compared to those of application sandboxing.

Also https://github.com/Corona-Trace