We propose a novel approach to establish crypto- graphic keys among mobile users and a networking infrastruc- ture. Our approach comes at a low cost and can either be used as an alternative to existing solutions or can be employed in a complementary way. Our basic observation is that users are often very mobile. As they interact with the infrastructure, each of them leaves a unique trace behind, known both to the users and the infrastructure. We can leverage this shared information to create shared secret keys, with little or no change of existing mobile communication systems. We show that we can achieve (almost passively) a rate of roughly 0.1 bits per second.