Michael Backes Real-or-random Key Secrecy of the Otway-Rees Protocol via a Symbolic Security Proof. 111-145 2006 155 Electr. Notes Theor. Comput. Sci. http://dx.doi.org/10.1016/j.entcs.2005.11.054 db/journals/entcs/entcs155.html#Backes06