GAnGS: Gather, Authenticate ’n Group Securely (CMU-CyLab-08-007)

Mobile users share the same expectations as wired users: they want to communicate with other people, they expect the communication to be secure, and it should all be easy. However, mobility poses many challenges for security. Communication is often ad hoc, and the infrastructure may be untrusted. Secure communication relies on the distribution of authentic information among the communicating parties’ devices. This is a challenging problem because devices generally do not share pre-existing secrets. Current security protocols for distributing initial authentic information fail to consider the human element. Many protocols do not scale beyond a pair of devices, although people often need to communicate with a group. The few existing group protocols assume that users will always count the number of members and verify the list of members correctly. However, as group size increases, implementations of these protocols become more prone to human error. We present GAnGS, a fully-implemented system for exchanging authentic information between mobile devices when they are physically present in the same location. GAnGS is scalable, appropriate for two or more devices. We implement two user-friendly variants of GAnGS on Nokia N70 camera phones. The first variant, GAnGS-P, is based on an untrusted communication hub. The second variant, GAnGS-T, needs no infrastructure. Both variants use Bluetooth for peer-to-peer wireless communication during the information exchange.