// Yahalom 
  
 A, B, S :         principal    
 Na, Nb :          number fresh 
 Kas, Kbs, Kab :   key          
  
 A knows :   A, B, S, Kas      
 B knows :   B, S, Kbs         
 S knows :   S, A, B, Kas, Kbs 
  
  
 1.   A -> B  :    A, Na                            
 2.   B -> S  :    B, {A, Na, Nb}Kbs                
 3.   S -> A  :    {B, Kab, Na, Nb}Kas, {A, Kab}Kbs 
 4.   A -> B  :    {A, Kab}Kbs, {Nb}Kab             
   
 // Security Protocols Open Repository 
 // http://www.lsv.ens-cachan.fr/spore 
   
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