// CCITT X.509 (1c) 
  
 A, B :     principal                      
 Na, Nb :   nonce                          
 Ta, Tb :   timestamp                      
 Ya, Yb :   userdata                       
 Xa, Xb :   userdata                       
 PK, SK :   principal -> key (keypair)     
 h :        userdata -> userdata (one-way) 
  
  
 1.   A -> B  :   A, {Ta, Na, B, Xa,   {Ya, {h(Ya)}SK(A)}PK(B)}SK(A) 
   
 // Security Protocols Open Repository 
 // http://www.lsv.ens-cachan.fr/spore 
   
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