author  paulson 
Fri, 11 Jul 1997 13:26:15 +0200  
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permissions  rwrr 
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(* Title: HOL/Auth/Shared 
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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 Shared Keys (common to all symmetrickey protocols) 

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Shared, longterm keys; initial states of agents 
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*) 
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Shared = Event + Finite + 
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consts 

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shrK :: agent => key (*symmetric keys*) 
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rules 
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isSym_keys "isSymKey K" (*All keys are symmetric*) 
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inj_shrK "inj shrK" (*No two agents have the same longterm key*) 
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primrec initState agent 

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(*Server knows all longterm keys; other agents know only their own*) 
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initState_Server "initState lost Server = Key `` range shrK" 
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initState_Friend "initState lost (Friend i) = {Key (shrK (Friend i))}" 

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initState_Spy "initState lost Spy = Key``shrK``lost" 
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rules 
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(*Unlike the corresponding property of nonces, this cannot be proved. 
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We have infinitely many agents and there is nothing to stop their 
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longterm keys from exhausting all the natural numbers. The axiom 
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assumes that their keys are dispersed so as to leave room for infinitely 
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many fresh session keys. We could, alternatively, restrict agents to 
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an unspecified finite number.*) 
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Key_supply_ax "finite KK ==> EX K. K ~: KK & Key K ~: used evs" 
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end 