I had a similar idea for transit cards a while ago. http://jimkeener.com/posts/public-transit-and-ring-signatures http://jimkeener.com/posts/public-transit-and-

I had a similar idea for transit cards a while ago. http://jimkeener.com/posts/public-transit-and-ring-signatures http://jimkeener.com/posts/public-transit-and-ring-signature... Right now, you can probably narrow someone's home and work pretty well.