It's worth checking out: Seeing-Is-Believing: Using Camera Phones For Human-Verifiable Authentication Jonathan M. McCune, Adrian Perrig, Michael K. Reiter -- to appear at IEEE Security and Privacy (Oakland 2005) Talking To Strangers: Authentication in Ad-Hoc Wireless Networks Dirk Balfanz, D. K. Smetters, Paul Stewart and H. Chi Wong -- ISOC NDSS 2002 -Ekr