Location privacy is of utmost concern for location-based services. It
is the property that a person's location is revealed to other
entities, such as a service provider or the person's friends, only if
this release is strictly necessary and authorized by the person.  We
study how to achieve location privacy for a service that alerts people
of nearby friends. Here, location privacy guarantees that users of the
service can learn a friend's location if and only if the friend is
actually nearby.  We introduce three protocols---Louis, Lester and
Pierre---that provide location privacy for such a service. The key
advantage of our protocols is that they are distributed and do not
require a separate service provider that is aware of people's
locations. The evaluation of our sample implementation demonstrates
that the protocols are sufficiently fast to be practical.