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