src/HOL/Auth/Shared.thy
author paulson
Tue, 23 Sep 2003 15:41:33 +0200
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Removal of the Key_supply axiom (affects many possbility proofs) and minor changes
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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 symmetric-key protocols)
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Shared, long-term keys; initial states of agents
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*)
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theory Shared = Event:
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consts
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  shrK    :: "agent => key"  (*symmetric keys*);
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specification (shrK)
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  inj_shrK: "inj shrK"
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  --{*No two agents have the same long-term key*}
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   apply (rule exI [of _ "agent_case 0 (\<lambda>n. n + 2) 1"]) 
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   apply (simp add: inj_on_def split: agent.split) 
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   done
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text{*All keys are symmetric*}
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defs  all_symmetric_def: "all_symmetric == True"
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lemma isSym_keys: "K \<in> symKeys"	
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by (simp add: symKeys_def all_symmetric_def invKey_symmetric) 
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text{*Server knows all long-term keys; other agents know only their own*}
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primrec
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  initState_Server:  "initState Server     = Key ` range shrK"
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  initState_Friend:  "initState (Friend i) = {Key (shrK (Friend i))}"
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  initState_Spy:     "initState Spy        = Key`shrK`bad"
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subsection{*Basic properties of shrK*}
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(*Injectiveness: Agents' long-term keys are distinct.*)
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declare inj_shrK [THEN inj_eq, iff]
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lemma invKey_K [simp]: "invKey K = K"
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apply (insert isSym_keys)
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apply (simp add: symKeys_def) 
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done
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lemma analz_Decrypt' [dest]:
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     "[| Crypt K X \<in> analz H;  Key K  \<in> analz H |] ==> X \<in> analz H"
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by auto
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text{*Now cancel the @{text dest} attribute given to
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 @{text analz.Decrypt} in its declaration.*}
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declare analz.Decrypt [rule del]
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text{*Rewrites should not refer to  @{term "initState(Friend i)"} because
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  that expression is not in normal form.*}
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lemma keysFor_parts_initState [simp]: "keysFor (parts (initState C)) = {}"
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apply (unfold keysFor_def)
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apply (induct_tac "C", auto)
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done
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(*Specialized to shared-key model: no @{term invKey}*)
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lemma keysFor_parts_insert:
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     "[| K \<in> keysFor (parts (insert X G));  X \<in> synth (analz H) |] \
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\     ==> K \<in> keysFor (parts (G \<union> H)) | Key K \<in> parts H";
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by (force dest: Event.keysFor_parts_insert)  
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lemma Crypt_imp_keysFor: "Crypt K X \<in> H ==> K \<in> keysFor H"
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by (drule Crypt_imp_invKey_keysFor, simp)
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subsection{*Function "knows"*}
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(*Spy sees shared keys of agents!*)
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lemma Spy_knows_Spy_bad [intro!]: "A: bad ==> Key (shrK A) \<in> knows Spy evs"
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apply (induct_tac "evs")
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apply (simp_all (no_asm_simp) add: imageI knows_Cons split add: event.split)
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done
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(*For case analysis on whether or not an agent is compromised*)
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lemma Crypt_Spy_analz_bad: "[| Crypt (shrK A) X \<in> analz (knows Spy evs);  A: bad |]  
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      ==> X \<in> analz (knows Spy evs)"
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apply (force dest!: analz.Decrypt)
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done
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(** Fresh keys never clash with long-term shared keys **)
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(*Agents see their own shared keys!*)
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lemma shrK_in_initState [iff]: "Key (shrK A) \<in> initState A"
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by (induct_tac "A", auto)
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lemma shrK_in_used [iff]: "Key (shrK A) \<in> used evs"
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by (rule initState_into_used, blast)
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(*Used in parts_induct_tac and analz_Fake_tac to distinguish session keys
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  from long-term shared keys*)
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lemma Key_not_used [simp]: "Key K \<notin> used evs ==> K \<notin> range shrK"
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by blast
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lemma shrK_neq [simp]: "Key K \<notin> used evs ==> shrK B \<noteq> K"
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by blast
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declare shrK_neq [THEN not_sym, simp]
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subsection{*Fresh nonces*}
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lemma Nonce_notin_initState [iff]: "Nonce N \<notin> parts (initState B)"
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by (induct_tac "B", auto)
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lemma Nonce_notin_used_empty [simp]: "Nonce N \<notin> used []"
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apply (simp (no_asm) add: used_Nil)
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done
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subsection{*Supply fresh nonces for possibility theorems.*}
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(*In any trace, there is an upper bound N on the greatest nonce in use.*)
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lemma Nonce_supply_lemma: "\<exists>N. ALL n. N<=n --> Nonce n \<notin> used evs"
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apply (induct_tac "evs")
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apply (rule_tac x = 0 in exI)
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apply (simp_all (no_asm_simp) add: used_Cons split add: event.split)
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apply safe
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apply (rule msg_Nonce_supply [THEN exE], blast elim!: add_leE)+
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done
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lemma Nonce_supply1: "\<exists>N. Nonce N \<notin> used evs"
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by (rule Nonce_supply_lemma [THEN exE], blast)
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lemma Nonce_supply2: "\<exists>N N'. Nonce N \<notin> used evs & Nonce N' \<notin> used evs' & N \<noteq> N'"
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apply (cut_tac evs = evs in Nonce_supply_lemma)
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apply (cut_tac evs = "evs'" in Nonce_supply_lemma, clarify)
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apply (rule_tac x = N in exI)
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apply (rule_tac x = "Suc (N+Na)" in exI)
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apply (simp (no_asm_simp) add: less_not_refl3 le_add1 le_add2 less_Suc_eq_le)
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done
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lemma Nonce_supply3: "\<exists>N N' N''. Nonce N \<notin> used evs & Nonce N' \<notin> used evs' &  
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                    Nonce N'' \<notin> used evs'' & N \<noteq> N' & N' \<noteq> N'' & N \<noteq> N''"
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apply (cut_tac evs = evs in Nonce_supply_lemma)
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apply (cut_tac evs = "evs'" in Nonce_supply_lemma)
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apply (cut_tac evs = "evs''" in Nonce_supply_lemma, clarify)
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apply (rule_tac x = N in exI)
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apply (rule_tac x = "Suc (N+Na)" in exI)
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apply (rule_tac x = "Suc (Suc (N+Na+Nb))" in exI)
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apply (simp (no_asm_simp) add: less_not_refl3 le_add1 le_add2 less_Suc_eq_le)
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done
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lemma Nonce_supply: "Nonce (@ N. Nonce N \<notin> used evs) \<notin> used evs"
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apply (rule Nonce_supply_lemma [THEN exE])
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apply (rule someI, blast)
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done
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text{*Unlike the corresponding property of nonces, we cannot prove
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    @{term "finite KK ==> \<exists>K. K \<notin> KK & Key K \<notin> used evs"}.
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    We have infinitely many agents and there is nothing to stop their
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    long-term keys from exhausting all the natural numbers.  Instead,
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    possibility theorems must assume the existence of a few keys.*}
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subsection{*Tactics for possibility theorems*}
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ML
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{*
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val inj_shrK      = thm "inj_shrK";
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val isSym_keys    = thm "isSym_keys";
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val Nonce_supply = thm "Nonce_supply";
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val invKey_K = thm "invKey_K";
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val analz_Decrypt' = thm "analz_Decrypt'";
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val keysFor_parts_initState = thm "keysFor_parts_initState";
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val keysFor_parts_insert = thm "keysFor_parts_insert";
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val Crypt_imp_keysFor = thm "Crypt_imp_keysFor";
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val Spy_knows_Spy_bad = thm "Spy_knows_Spy_bad";
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val Crypt_Spy_analz_bad = thm "Crypt_Spy_analz_bad";
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val shrK_in_initState = thm "shrK_in_initState";
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val shrK_in_used = thm "shrK_in_used";
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val Key_not_used = thm "Key_not_used";
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val shrK_neq = thm "shrK_neq";
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val Nonce_notin_initState = thm "Nonce_notin_initState";
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val Nonce_notin_used_empty = thm "Nonce_notin_used_empty";
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val Nonce_supply_lemma = thm "Nonce_supply_lemma";
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val Nonce_supply1 = thm "Nonce_supply1";
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val Nonce_supply2 = thm "Nonce_supply2";
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val Nonce_supply3 = thm "Nonce_supply3";
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val Nonce_supply = thm "Nonce_supply";
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*}
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ML
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{*
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(*Omitting used_Says makes the tactic much faster: it leaves expressions
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    such as  Nonce ?N \<notin> used evs that match Nonce_supply*)
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fun gen_possibility_tac ss state = state |>
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   (REPEAT 
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    (ALLGOALS (simp_tac (ss delsimps [used_Says, used_Notes, used_Gets] 
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                         setSolver safe_solver))
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     THEN
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     REPEAT_FIRST (eq_assume_tac ORELSE' 
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                   resolve_tac [refl, conjI, Nonce_supply])))
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(*Tactic for possibility theorems (ML script version)*)
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fun possibility_tac state = gen_possibility_tac (simpset()) state
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(*For harder protocols (such as Recur) where we have to set up some
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  nonces and keys initially*)
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fun basic_possibility_tac st = st |>
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    REPEAT 
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    (ALLGOALS (asm_simp_tac (simpset() setSolver safe_solver))
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     THEN
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     REPEAT_FIRST (resolve_tac [refl, conjI]))
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*}
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subsection{*Specialized Rewriting for Theorems About @{term analz} and Image*}
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lemma subset_Compl_range: "A <= - (range shrK) ==> shrK x \<notin> A"
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by blast
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lemma insert_Key_singleton: "insert (Key K) H = Key ` {K} \<union> H"
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by blast
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lemma insert_Key_image: "insert (Key K) (Key`KK \<union> C) = Key`(insert K KK) \<union> C"
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by blast
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(** Reverse the normal simplification of "image" to build up (not break down)
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    the set of keys.  Use analz_insert_eq with (Un_upper2 RS analz_mono) to
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    erase occurrences of forwarded message components (X). **)
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lemmas analz_image_freshK_simps =
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       simp_thms mem_simps --{*these two allow its use with @{text "only:"}*}
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       disj_comms 
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       image_insert [THEN sym] image_Un [THEN sym] empty_subsetI insert_subset
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       analz_insert_eq Un_upper2 [THEN analz_mono, THEN [2] rev_subsetD]
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       insert_Key_singleton subset_Compl_range
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       Key_not_used insert_Key_image Un_assoc [THEN sym]
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(*Lemma for the trivial direction of the if-and-only-if*)
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lemma analz_image_freshK_lemma:
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     "(Key K \<in> analz (Key`nE \<union> H)) --> (K \<in> nE | Key K \<in> analz H)  ==>  
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         (Key K \<in> analz (Key`nE \<union> H)) = (K \<in> nE | Key K \<in> analz H)"
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by (blast intro: analz_mono [THEN [2] rev_subsetD])
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ML
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{*
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val analz_image_freshK_lemma = thm "analz_image_freshK_lemma";
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val analz_image_freshK_ss = 
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     simpset() delsimps [image_insert, image_Un]
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	       delsimps [imp_disjL]    (*reduces blow-up*)
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	       addsimps thms "analz_image_freshK_simps"
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*}
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(*Lets blast_tac perform this step without needing the simplifier*)
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lemma invKey_shrK_iff [iff]:
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     "(Key (invKey K) \<in> X) = (Key K \<in> X)"
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by auto
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(*Specialized methods*)
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method_setup analz_freshK = {*
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    Method.no_args
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     (Method.METHOD
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      (fn facts => EVERY [REPEAT_FIRST (resolve_tac [allI, ballI, impI]),
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                          REPEAT_FIRST (rtac analz_image_freshK_lemma),
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                          ALLGOALS (asm_simp_tac analz_image_freshK_ss)])) *}
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    "for proving the Session Key Compromise theorem"
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method_setup possibility = {*
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    Method.ctxt_args (fn ctxt =>
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        Method.METHOD (fn facts =>
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            gen_possibility_tac (Simplifier.get_local_simpset ctxt))) *}
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    "for proving possibility theorems"
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lemma knows_subset_knows_Cons: "knows A evs <= knows A (e # evs)"
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by (induct e, auto simp: knows_Cons)
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58573e7041b4 Separation of theory Event into two parts:
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end