// BAN modified version of CCITT X.509 (3) 
  
 A, B :     principal                  
 Na, Nb :   nonce                      
 Ya, Yb :   userdata                   
 Xa, Xb :   userdata                   
 PK, SK :   principal -> key (keypair) 
  
  
 1.   A -> B  :   A, {Na, B, Xa, {Ya}PK(B)}SK(A)     
 2.   B -> A  :   B, {Nb, A, Na, Xb, {Yb}PK(A)}SK(B) 
 3.   A -> B  :   A, {B, Nb}SK(A)                    
   
 // Security Protocols Open Repository 
 // http://www.lsv.ens-cachan.fr/spore 
   
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