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Sharing location data is always a choice on the part of a diagnosed user, with clear explanations provided about the purposes for which the data will be used.
Location data can only be shared with authenticated Health Authorities that have been approved by Path Check.
Location data is encrypted in transit, and at rest in the Health Authority’s database.
Prior to storage in the Health Authority databases, a Contact Tracer will review all provided data with the diagnosed user, and redact any information that either of them believes could reveal their identity.
Data is only stored in a databased owned and managed by the patient’s own Health Authority. There is no centralized store of location data.
The patient can also request that any other data point recorded is removed, for any reason they may have.
Data is only committed to the Health Authority database once the diagnosed user has given final consent regarding the set of data points stored.
Location data is stored as a set of places & times, with no data relating it back to any individual user, or correlating it to other location data points from the same user.
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The approach that we take is approximately that outlined in section 5 of this paper as an “Intermediary Implementation” https://arxiv.org/pdf/2003.14412v2.pdf, though with some variations in the detail.
In summary:
Data is published by Health Authorities, to allow Mobile Apps to check locally for matches with “points of concern”.
Prior to publication, each “point of concern” is mapped from an exact latitude and longitude, to a geohash, and a 5 minute time window. This information is then hashed, using a slow consistent one-way hash algorithm, and published only in that hashed form.A Mobile App can assess a
Each Health Authority publishes their data at a URL that they control, which is stored in an overall database of Health Authorities maintained by Path Check, and passed to the Mobile App when they register for Exposure Notifications from that Health Authority. This URL is not publicly advertised, or shown to users, but it can be easily determined by an attacker.
A Mobile App can assess a given location & time for exposure by computing the same geohash, time-window and hash, and checking for a match against the published data.
However, due Due to the use of a slow hash, it is expensive to check a large set of locations and/or times. And because the hash is one-way, it is impossible to reverse other than by computing the hashes for every possible point in the area & time period covered.
Below we provide details of the implementation, and the reasons for these. We then assess the level of protection that this scheme manages to provide.
Geohash tile size
We use 8 character Geohashes, which resolve to rectangular tiles that are 38m x 19m at the equator, and about 27m x 19m at 45 degrees north.
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