We consider the problem of secret key distribution in a sensor network with multiple scattered sensor nodes and a mobile device that can be used to bootstrap the network. Our main contribution is a practical scheme that relies on network coding to provide a robust and low-complexity solution for sharing secret keys among sensor nodes. In spite of its role as a key enabler for this approach, the mobile node only has access to encrypted version of the keys. In contrast with probabilistic key pre-distribution schemes our method assures secure connectivity with probability one, requiring only a modest amount of memory — initially each sensor node stores only one key per secured link.