// Needham Schroeder Symmetric Key 
  
 A, B, S :         principal      
 Na, Nb :          nonce          
 Kas, Kbs, Kab :   key            
 dec :             nonce -> nonce 
  
  
 1.   A -> S  :   A, B, Na                     
 2.   S -> A  :   {Na, B, Kab, {Kab, A}Kbs}Kas 
 3.   A -> B  :   {Kab,A}Kbs                   
 4.   B -> A  :   {Nb}Kab                      
 5.   A -> B  :   {dec(Nb)}Kab                 
   
 // Security Protocols Open Repository 
 // http://www.lsv.ens-cachan.fr/spore 
   
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