author  nipkow 
Fri, 24 Nov 2000 16:49:27 +0100  
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permissions  rwrr 
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(* Title: HOL/Auth/Event 
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ID: $Id$ 
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Author: Lawrence C Paulson, Cambridge University Computer Laboratory 
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Copyright 1996 University of Cambridge 
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Theory of events for security protocols 
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3683  8 
Datatype of events; function "spies"; freshness 
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3683  10 
"bad" agents have been broken by the Spy; their private keys and internal 
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stores are visible to him 
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*) 
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Event = Message + List + 
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consts (*Initial states of agents  parameter of the construction*) 
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initState :: agent => msg set 
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datatype 
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event = Says agent agent msg 
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 Gets agent msg 
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 Notes agent msg 
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consts 
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bad :: agent set (*compromised agents*) 
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knows :: agent => event list => msg set 
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(*"spies" is retained for compability's sake*) 
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syntax 
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spies :: event list => msg set 
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translations 
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"spies" => "knows Spy" 
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rules 
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(*Spy has access to his own key for spoof messages, but Server is secure*) 
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Spy_in_bad "Spy: bad" 
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Server_not_bad "Server ~: bad" 

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primrec 
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knows_Nil "knows A [] = initState A" 
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knows_Cons 
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"knows A (ev # evs) = 
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(if A = Spy then 
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(case ev of 
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Says A' B X => insert X (knows Spy evs) 
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 Gets A' X => knows Spy evs 
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 Notes A' X => 
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if A' : bad then insert X (knows Spy evs) else knows Spy evs) 
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else 
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(case ev of 
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Says A' B X => 
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if A'=A then insert X (knows A evs) else knows A evs 
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 Gets A' X => 
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if A'=A then insert X (knows A evs) else knows A evs 
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 Notes A' X => 
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if A'=A then insert X (knows A evs) else knows A evs))" 
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(* 
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Case A=Spy on the Gets event 
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enforces the fact that if a message is received then it must have been sent, 
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therefore the oops case must use Notes 
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*) 
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consts 
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(*Set of items that might be visible to somebody: 

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complement of the set of fresh items*) 

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used :: event list => msg set 

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primrec 
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used_Nil "used [] = (UN B. parts (initState B))" 
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used_Cons "used (ev # evs) = 

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(case ev of 

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Says A B X => parts {X} Un (used evs) 

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 Gets A X => used evs 
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 Notes A X => parts {X} Un (used evs))" 
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end 