How can you verify in software that a smartphone was at a particular location and interacted with a particular person?  For example, suppose a coffee shop has a rewards card that allows customers to get a free cup of coffee after ten visits.  You want to replace the physical punch card with a phone app, but how do you verify the user actually bought a cup of coffee? 


Madison, WI based startup Snowshoe has come up with a hardware key, which they call a snow stamp, that can be pressed against a smart phone to verify the users of the phone interacted with the bearer of that particular stamp at a particular time. 



This October, SnowShoe’s CEO, Claus Moberg, led the team that won the Disrupt SF Hackathon using this technology.  In this contest, participants have 24 hours to design a new product.  In one day and one night, they designed a device called Livebolt that fits over a door lock and actuates the lock in response to commands from a smartphone.  A homeowner could issue a command to unlock the door for someone during certain hours if that person’s phone were pressed against a stamp fixed near the door.  This prevents someone who is approved to enter from unlocking the door when their phone is not near the doorway. 



I had a chance to talk to Claus Moberg about the technical details.  Inside the surface of the enclosure that is pressed against the phone are pieces of aluminum.  Each stamp has a unique pattern of locations for the aluminum.  Snowshoe found by experimenting that aluminum is a good material to actuate a capacitive touchscreen.


The name Snowstamp comes from the saying that no two snowflakes are exactly alike.   Snowshoe empirically measured the minimum perturbation in stamp coordinates that a phone could reliably detect.  From this they were able to calculate that over a million unique patterns of locations are possible.  They wrote software that generates a unique set of coordinates for each stamp.  Each new stamp is machined to these coordinates. 


This is a neat technology waiting for some killer app that depends on verifying a phone is physically next to a key.  I am not sure that the rewards program or the Livebolt lock actuator will be such a killer app.  I am interested to hear killer app ideas or alternative ways to accomplish physical key verification on a phone.