I think the idea of using a ring for this purpose is that you never need remove it. It would have to be pretty durable, of course, but then you'd basically lea

I think the idea of using a ring for this purpose is that you never need remove it. It would have to be pretty durable, of course, but then you'd basically leave it on while swimming, sleeping, etc. (like the standard approach to a wedding band). Having it work by proximity makes me a bit more uneasy, though -- I imagine someone brushing by your hand in the subway and authenticating with your bank then & there.