Nidal Aboudagga Mohamed Eltoweissy Jean-Jacques Quisquater Two-level signature delegation for mobile first responder authentication. 16-23 2007 CollaborateCom http://dx.doi.org/10.1109/COLCOM.2007.4553803 conf/colcom/2007 db/conf/colcom/colcom2007.html#AboudaggaEQ07