John G. Brainard Ari Juels Ronald L. Rivest Michael Szydlo Moti Yung Fourth-factor authentication: somebody you know. 168-178 2006 conf/ccs/2006usa ACM Conference on Computer and Communications Security http://doi.acm.org/10.1145/1180405.1180427 db/conf/ccs/ccs2006usa.html#BrainardJRSY06