Hi,

Many people seem interested in a downloadable geolocation database, so that 
they can locate themselves without having to send any data over a network. I 
imagine this would not be the raw uploads, but something processed to contain 1 
entry per wifi hotspot with its average location. If a hotspot had been seen in 
multiple locations it would not be included, as it probably a mobile phone or 
public transport and would not be useful.

This privacy risk from this is small, if eve is trying locate alice, she would 
need to know the BSSID of alice's hotspot, which probably means knowing its 
location. http://www.advogato.org/person/zeenix/diary/326.html gives an example 
where this is still a problem (though filtering out moving hotspots means that 
the new location would not be in the database, assuming that the old one was).

The web API prevents these information leaks be requiring that a query contains 
multiple hotspots. Now to find a hotspot
you need to know several other hotspots that it is nearby.

I suggest that something similar is done with the database download, by only 
giving weak information about each hotspot. For example either its latitude or 
longitude (depending on something deterministic such as the last bit of the 
BSSID). Now an individual hotspot can't be located, but a group known to be 
close together can. There are a few alternatives, for example a hot spot could 
either be coarse (integer part of coordinates) or fine (fractional part of 
coordinates), but I am not sure this is any better.

Sam

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