src/HOL/Auth/Event.thy
author paulson
Wed, 17 Sep 1997 16:37:27 +0200
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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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Datatype of events; function "sees"; freshness
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"lost" 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  (*Messages--could add another constructor for agent knowledge*)
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  event = Says  agent agent msg
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        | Notes agent       msg
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consts  
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  lost :: agent set        (*compromised agents*)
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  sees :: [agent, event list] => msg set
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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_lost     "Spy: lost"
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  Server_not_lost "Server ~: lost"
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consts  
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  sees1 :: [agent, event] => msg set
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primrec sees1 event
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           (*Spy reads all traffic whether addressed to him or not*)
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  sees1_Says  "sees1 A (Says A' B X)  = (if A:{B,Spy} then {X} else {})"
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  sees1_Notes "sees1 A (Notes A' X)   = (if A=A' | A=Spy & A':lost then {X}
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					 else {})"
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primrec sees list
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  sees_Nil  "sees A []       = initState A"
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  sees_Cons "sees A (ev#evs) = sees1 A ev Un sees A evs"
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constdefs
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  (*Set of items that might be visible to somebody: complement of the set
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        of fresh items*)
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  used :: event list => msg set
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    "used evs == parts (UN B. sees B evs)"
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end