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The “time window” is a UTC timestamp in seconds, rounded down to the nearesrt nearest 5 minutes.
E.g. Time: 14 April 2020, 12:03:12pm (which rounds down to 12:00:00pm), which maps to 15868656001586865600000 (msecs)
The “nearby” time window is defined as follows:
If the timestamp is in the 1st half of the time window (e.g. 12:01pm) it is the preceding time winow window (begins 11:55am)
If the timestamp is in the 2nd half of the time window (e.g. 12:03pm), it is the following time winow window (begins 12:05pm)
The exact midpoint of the 5 min window (2:30) is ttreated treated as being in the 2nd half.
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A “cost” (N) that is to be determined. For initial implemention we use 2^14 2^12 = 163844096. For improved security we expect to move to 2^16,. 2^17 or 2^18 for production, but the exact value still needs to be determined.
A block size (r) of 8 - this is the default.
Parallelization (p) of 1 - this is the default.
A keylen (output) of 8 bytes = 16 hex digits.
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The 8 char geohash
Followed by the UTC timestamp of the beginning of the relevant 5 minute time window, in secondsmilliseconds
Example:
Latitude: 51.5019, Longitude: -0.1415
Time: 14 April 2020, 12:03:12pm (which rounds down to 12:00:00pm)
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8 character geohash: gcpuuz8u
UTC timestamp: 1586865600158686560000
Data string to encode: “gcpuuz8u1586865600” “gcpuuz8u1586865600000” which should hash to “0ed62968fef3dc0a““c9414c55812d796a“ (cost = 12)
Sending data to Safe Places for Redaction Processing
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| Code Block |
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"notification_threshold_percent":66, “notification_threshold_count”timeframe”:630 |
These two parameters work together to control the sensitivity of exposure notifications for this data. The Health Authority can turn these based on their experience and feedback from their community.
With the defaults, we trigger an exposure notification if we get more than 66% of location matches, across any 6 consecutive location data points 30 minute period (i.e. if we match 4 or more of 6 points).
Safe Paths logic for matching points of concern
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