It's not what you know, but who you know: a social approach to last-resort authentication.

Stuart E. Schechter Serge Egelman Robert W. Reeder It's not what you know, but who you know: a social approach to last-resort authentication. 2009 SOUPS http://doi.acm.org/10.1145/1572532.1572581 conf/soups/2009 db/conf/soups/soups2009.html#SchechterER09