src/HOL/Auth/Yahalom2.ML
author paulson
Fri Sep 19 18:27:31 1997 +0200 (1997-09-19)
changeset 3686 4b484805b4c4
parent 3683 aafe719dff14
child 3730 6933d20f335f
permissions -rw-r--r--
First working version with Oops event for session keys
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(*  Title:      HOL/Auth/Yahalom2
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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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Inductive relation "yahalom" for the Yahalom protocol, Variant 2.
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This version trades encryption of NB for additional explicitness in YM3.
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From page 259 of
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  Burrows, Abadi and Needham.  A Logic of Authentication.
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  Proc. Royal Soc. 426 (1989)
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*)
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open Yahalom2;
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proof_timing:=true;
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HOL_quantifiers := false;
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(*A "possibility property": there are traces that reach the end*)
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goal thy 
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 "!!A B. [| A ~= B; A ~= Server; B ~= Server |]   \
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\        ==> EX X NB K. EX evs: yahalom.          \
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\               Says A B {|X, Crypt K (Nonce NB)|} : set evs";
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by (REPEAT (resolve_tac [exI,bexI] 1));
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by (rtac (yahalom.Nil RS yahalom.YM1 RS yahalom.YM2 RS yahalom.YM3 RS 
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          yahalom.YM4) 2);
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by possibility_tac;
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result();
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(**** Inductive proofs about yahalom ****)
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(*Nobody sends themselves messages*)
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goal thy "!!evs. evs: yahalom ==> ALL A X. Says A A X ~: set evs";
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by (etac yahalom.induct 1);
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by (Auto_tac());
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qed_spec_mp "not_Says_to_self";
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Addsimps [not_Says_to_self];
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AddSEs   [not_Says_to_self RSN (2, rev_notE)];
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(** For reasoning about the encrypted portion of messages **)
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(*Lets us treat YM4 using a similar argument as for the Fake case.*)
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goal thy "!!evs. Says S A {|NB, Crypt (shrK A) Y, X|} : set evs ==> \
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\                X : analz (spies evs)";
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by (blast_tac (!claset addSDs [Says_imp_spies RS analz.Inj]) 1);
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qed "YM4_analz_spies";
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bind_thm ("YM4_parts_spies",
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          YM4_analz_spies RS (impOfSubs analz_subset_parts));
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(*Relates to both YM4 and Oops*)
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goal thy "!!evs. Says S A {|NB, Crypt (shrK A) {|B,K,NA|}, X|} : set evs ==> \
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\                K : parts (spies evs)";
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by (blast_tac (!claset addSEs partsEs
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                       addSDs [Says_imp_spies RS parts.Inj]) 1);
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qed "YM4_Key_parts_spies";
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(*For proving the easier theorems about X ~: parts (spies evs).*)
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fun parts_spies_tac i = 
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    forward_tac [YM4_Key_parts_spies] (i+6) THEN
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    forward_tac [YM4_parts_spies] (i+5)     THEN
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    prove_simple_subgoals_tac  i;
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(*Induction for regularity theorems.  If induction formula has the form
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   X ~: analz (spies evs) --> ... then it shortens the proof by discarding
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   needless information about analz (insert X (spies evs))  *)
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fun parts_induct_tac i = 
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    etac yahalom.induct i
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    THEN 
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    REPEAT (FIRSTGOAL analz_mono_contra_tac)
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    THEN  parts_spies_tac i;
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(** Theorems of the form X ~: parts (spies evs) imply that NOBODY
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    sends messages containing X! **)
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(*Spy never sees another agent's shared key! (unless it's bad at start)*)
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goal thy 
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 "!!evs. evs : yahalom ==> (Key (shrK A) : parts (spies evs)) = (A : bad)";
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by (parts_induct_tac 1);
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by (Fake_parts_insert_tac 1);
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by (Blast_tac 1);
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qed "Spy_see_shrK";
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Addsimps [Spy_see_shrK];
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goal thy 
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 "!!evs. evs : yahalom ==> (Key (shrK A) : analz (spies evs)) = (A : bad)";
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by (auto_tac(!claset addDs [impOfSubs analz_subset_parts], !simpset));
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qed "Spy_analz_shrK";
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Addsimps [Spy_analz_shrK];
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goal thy  "!!A. [| Key (shrK A) : parts (spies evs);       \
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\                  evs : yahalom |] ==> A:bad";
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by (blast_tac (!claset addDs [Spy_see_shrK]) 1);
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qed "Spy_see_shrK_D";
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bind_thm ("Spy_analz_shrK_D", analz_subset_parts RS subsetD RS Spy_see_shrK_D);
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AddSDs [Spy_see_shrK_D, Spy_analz_shrK_D];
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(*Nobody can have used non-existent keys!  Needed to apply analz_insert_Key*)
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goal thy "!!evs. evs : yahalom ==>          \
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\         Key K ~: used evs --> K ~: keysFor (parts (spies evs))";
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by (parts_induct_tac 1);
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(*YM4: Key K is not fresh!*)
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by (blast_tac (!claset addSEs spies_partsEs) 3);
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(*YM3*)
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by (blast_tac (!claset addss (!simpset)) 2);
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(*Fake*)
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by (best_tac
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      (!claset addIs [impOfSubs analz_subset_parts]
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               addDs [impOfSubs (analz_subset_parts RS keysFor_mono),
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                      impOfSubs (parts_insert_subset_Un RS keysFor_mono)]
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               addss (!simpset)) 1);
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qed_spec_mp "new_keys_not_used";
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bind_thm ("new_keys_not_analzd",
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          [analz_subset_parts RS keysFor_mono,
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           new_keys_not_used] MRS contra_subsetD);
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Addsimps [new_keys_not_used, new_keys_not_analzd];
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(*Describes the form of K when the Server sends this message.  Useful for
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  Oops as well as main secrecy property.*)
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goal thy 
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 "!!evs. [| Says Server A {|nb', Crypt (shrK A) {|Agent B, Key K, na|}, X|} \
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\            : set evs;                                            \
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\           evs : yahalom |]                                       \
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\        ==> K ~: range shrK & A ~= B";
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by (etac rev_mp 1);
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by (etac yahalom.induct 1);
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by (ALLGOALS Asm_simp_tac);
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qed "Says_Server_message_form";
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(*For proofs involving analz.*)
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val analz_spies_tac = 
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    dtac YM4_analz_spies 6 THEN
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    forward_tac [Says_Server_message_form] 7 THEN
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    assume_tac 7 THEN
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    REPEAT ((etac conjE ORELSE' hyp_subst_tac) 7);
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(****
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 The following is to prove theorems of the form
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          Key K : analz (insert (Key KAB) (spies evs)) ==>
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          Key K : analz (spies evs)
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 A more general formula must be proved inductively.
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****)
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(** Session keys are not used to encrypt other session keys **)
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goal thy  
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 "!!evs. evs : yahalom ==>                                  \
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\  ALL K KK. KK <= Compl (range shrK) -->                   \
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\            (Key K : analz (Key``KK Un (spies evs))) =  \
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\            (K : KK | Key K : analz (spies evs))";
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by (etac yahalom.induct 1);
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by analz_spies_tac;
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by (REPEAT_FIRST (resolve_tac [allI, impI]));
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by (REPEAT_FIRST (rtac analz_image_freshK_lemma ));
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by (ALLGOALS (asm_simp_tac analz_image_freshK_ss));
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(*Fake*) 
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by (spy_analz_tac 2);
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(*Base*)
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by (Blast_tac 1);
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qed_spec_mp "analz_image_freshK";
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goal thy
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 "!!evs. [| evs : yahalom;  KAB ~: range shrK |] ==>        \
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\        Key K : analz (insert (Key KAB) (spies evs)) =  \
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\        (K = KAB | Key K : analz (spies evs))";
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by (asm_simp_tac (analz_image_freshK_ss addsimps [analz_image_freshK]) 1);
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qed "analz_insert_freshK";
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(*** The Key K uniquely identifies the Server's  message. **)
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goal thy 
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 "!!evs. evs : yahalom ==>                                     \
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\      EX A' B' na' nb' X'. ALL A B na nb X.                   \
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\          Says Server A                                       \
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\           {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, X|}   \
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\          : set evs --> A=A' & B=B' & na=na' & nb=nb' & X=X'";
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by (etac yahalom.induct 1);
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by (ALLGOALS (asm_simp_tac (!simpset addsimps [all_conj_distrib])));
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by (Step_tac 1);
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(*Remaining case: YM3*)
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by (expand_case_tac "K = ?y" 1);
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by (REPEAT (ares_tac [refl,exI,impI,conjI] 2));
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(*...we assume X is a recent message and handle this case by contradiction*)
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by (blast_tac (!claset addSEs spies_partsEs
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                       delrules [conjI]    (*prevent split-up into 4 subgoals*)
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                       addss (!simpset addsimps [parts_insertI])) 1);
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val lemma = result();
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goal thy 
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"!!evs. [| Says Server A                                            \
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\            {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, X|} : set evs; \
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\          Says Server A'                                           \
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\            {|nb', Crypt (shrK A') {|Agent B', Key K, na'|}, X'|} : set evs; \
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\          evs : yahalom |]                                         \
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\       ==> A=A' & B=B' & na=na' & nb=nb'";
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by (prove_unique_tac lemma 1);
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qed "unique_session_keys";
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(** Crucial secrecy property: Spy does not see the keys sent in msg YM3 **)
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goal thy 
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 "!!evs. [| A ~: bad;  B ~: bad;  A ~= B;                     \
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\           evs : yahalom |]                                    \
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\        ==> Says Server A                                      \
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\              {|nb, Crypt (shrK A) {|Agent B, Key K, na|},     \
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\                    Crypt (shrK B) {|nb, Key K, Agent A|}|}    \
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\             : set evs -->                                     \
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\            Says A Spy {|na, nb, Key K|} ~: set evs -->        \
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\            Key K ~: analz (spies evs)";
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by (etac yahalom.induct 1);
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by analz_spies_tac;
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by (ALLGOALS
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    (asm_simp_tac 
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     (!simpset addsimps [analz_insert_eq, analz_insert_freshK]
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               setloop split_tac [expand_if])));
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(*Oops*)
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by (blast_tac (!claset addDs [unique_session_keys]) 3);
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(*YM3*)
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by (blast_tac (!claset delrules [impCE]
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                       addSEs spies_partsEs
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                       addIs [impOfSubs analz_subset_parts]) 2);
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(*Fake*) 
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by (spy_analz_tac 1);
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val lemma = result() RS mp RS mp RSN(2,rev_notE);
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(*Final version*)
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goal thy 
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 "!!evs. [| Says Server A                                    \
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\              {|nb, Crypt (shrK A) {|Agent B, Key K, na|},  \
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\                    Crypt (shrK B) {|nb, Key K, Agent A|}|} \
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\           : set evs;                                       \
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\           Says A Spy {|na, nb, Key K|} ~: set evs;         \
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\           A ~: bad;  B ~: bad;  evs : yahalom |]         \
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\        ==> Key K ~: analz (spies evs)";
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by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1);
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by (blast_tac (!claset addSEs [lemma]) 1);
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qed "Spy_not_see_encrypted_key";
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(** Security Guarantee for A upon receiving YM3 **)
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(*If the encrypted message appears then it originated with the Server.
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  May now apply Spy_not_see_encrypted_key, subject to its conditions.*)
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goal thy
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 "!!evs. [| Crypt (shrK A) {|Agent B, Key K, na|}                      \
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\            : parts (spies evs);                                   \
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\           A ~: bad;  evs : yahalom |]                               \
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\         ==> EX nb. Says Server A                                     \
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\                      {|nb, Crypt (shrK A) {|Agent B, Key K, na|},    \
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\                            Crypt (shrK B) {|nb, Key K, Agent A|}|}   \
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\                    : set evs";
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by (etac rev_mp 1);
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by (parts_induct_tac 1);
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by (Fake_parts_insert_tac 1);
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by (Blast_tac 1);
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qed "A_trusts_YM3";
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(** Security Guarantee for B upon receiving YM4 **)
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(*B knows, by the first part of A's message, that the Server distributed 
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  the key for A and B, and has associated it with NB. *)
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goal thy 
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 "!!evs. [| Crypt (shrK B) {|Nonce NB, Key K, Agent A|} : parts (spies evs); \
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\           B ~: bad;  evs : yahalom |]                             \
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\        ==> EX NA. Says Server A                                    \
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\                    {|Nonce NB,                                     \
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\                      Crypt (shrK A) {|Agent B, Key K, Nonce NA|},  \
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\                      Crypt (shrK B) {|Nonce NB, Key K, Agent A|}|} \
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\                       : set evs";
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by (etac rev_mp 1);
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by (parts_induct_tac 1);
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by (Fake_parts_insert_tac 1);
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(*YM3*)
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by (Blast_tac 1);
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qed "B_trusts_YM4_shrK";
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(*With this protocol variant, we don't need the 2nd part of YM4 at all:
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  Nonce NB is available in the first part.*)
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(*What can B deduce from receipt of YM4?  Stronger and simpler than Yahalom
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  because we do not have to show that NB is secret. *)
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goal thy 
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 "!!evs. [| Says A' B {|Crypt (shrK B) {|Nonce NB, Key K, Agent A|}, X|} \
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\             : set evs;                                                 \
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\           A ~: bad;  B ~: bad;  evs : yahalom |]                     \
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\        ==> EX NA. Says Server A                                        \
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\                    {|Nonce NB,                                         \
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\                      Crypt (shrK A) {|Agent B, Key K, Nonce NA|},      \
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\                      Crypt (shrK B) {|Nonce NB, Key K, Agent A|}|}     \
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\                   : set evs";
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by (etac (Says_imp_spies RS parts.Inj RS MPair_parts) 1);
paulson@3121
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by (blast_tac (!claset addSDs [B_trusts_YM4_shrK]) 1);
paulson@2323
   311
qed "B_trusts_YM4";
paulson@3432
   312
paulson@3432
   313
paulson@3432
   314
paulson@3432
   315
(*** Authenticating B to A ***)
paulson@3432
   316
paulson@3432
   317
(*The encryption in message YM2 tells us it cannot be faked.*)
paulson@3432
   318
goal thy 
paulson@3519
   319
 "!!evs. evs : yahalom                                                  \
paulson@3683
   320
\  ==> Crypt (shrK B) {|Agent A, Nonce NA|} : parts (spies evs) -->  \
paulson@3683
   321
\      B ~: bad -->                                                    \
paulson@3519
   322
\      (EX NB. Says B Server {|Agent B, Nonce NB,                       \
paulson@3519
   323
\                              Crypt (shrK B) {|Agent A, Nonce NA|}|}   \
nipkow@3465
   324
\         : set evs)";
paulson@3519
   325
by (parts_induct_tac 1);
paulson@3432
   326
by (Fake_parts_insert_tac 1);
paulson@3432
   327
(*YM2*)
paulson@3432
   328
by (Blast_tac 1);
paulson@3432
   329
bind_thm ("B_Said_YM2", result() RSN (2, rev_mp) RS mp);
paulson@3432
   330
paulson@3432
   331
(*If the server sends YM3 then B sent YM2, perhaps with a different NB*)
paulson@3432
   332
goal thy 
paulson@3519
   333
 "!!evs. evs : yahalom                                                   \
paulson@3432
   334
\  ==> Says Server A {|nb, Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, X|} \
paulson@3466
   335
\         : set evs -->                                                  \
paulson@3683
   336
\      B ~: bad -->                                                     \
paulson@3432
   337
\      (EX nb'. Says B Server {|Agent B, nb',                            \
paulson@3432
   338
\                               Crypt (shrK B) {|Agent A, Nonce NA|}|}   \
nipkow@3465
   339
\                 : set evs)";
paulson@3432
   340
by (etac yahalom.induct 1);
paulson@3432
   341
by (ALLGOALS Asm_simp_tac);
paulson@3432
   342
(*YM3*)
paulson@3432
   343
by (blast_tac (!claset addSDs [B_Said_YM2]
paulson@3432
   344
		       addSEs [MPair_parts]
paulson@3683
   345
		       addDs [Says_imp_spies RS parts.Inj]) 3);
paulson@3432
   346
(*Fake, YM2*)
paulson@3432
   347
by (ALLGOALS Blast_tac);
paulson@3450
   348
val lemma = result() RSN (2, rev_mp) RS mp |> standard;
paulson@3432
   349
paulson@3432
   350
(*If A receives YM3 then B has used nonce NA (and therefore is alive)*)
paulson@3432
   351
goal thy
paulson@3450
   352
 "!!evs. [| Says S A {|nb, Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, X|} \
paulson@3466
   353
\             : set evs;                                                    \
paulson@3683
   354
\           A ~: bad;  B ~: bad;  evs : yahalom |]                   \
paulson@3450
   355
\   ==> EX nb'. Says B Server                                               \
paulson@3450
   356
\                    {|Agent B, nb', Crypt (shrK B) {|Agent A, Nonce NA|}|} \
nipkow@3465
   357
\                 : set evs";
paulson@3450
   358
by (blast_tac (!claset addSDs [A_trusts_YM3, lemma]
paulson@3683
   359
		       addEs spies_partsEs) 1);
paulson@3432
   360
qed "YM3_auth_B_to_A";
paulson@3432
   361
paulson@3432
   362
paulson@3450
   363
(*** Authenticating A to B using the certificate Crypt K (Nonce NB) ***)
paulson@3450
   364
paulson@3450
   365
(*Assuming the session key is secure, if both certificates are present then
paulson@3432
   366
  A has said NB.  We can't be sure about the rest of A's message, but only
paulson@3432
   367
  NB matters for freshness.*)  
paulson@3432
   368
goal thy 
paulson@3519
   369
 "!!evs. evs : yahalom                                        \
paulson@3683
   370
\        ==> Key K ~: analz (spies evs) -->                \
paulson@3683
   371
\            Crypt K (Nonce NB) : parts (spies evs) -->    \
paulson@3519
   372
\            Crypt (shrK B) {|Nonce NB, Key K, Agent A|}      \
paulson@3683
   373
\              : parts (spies evs) -->                     \
paulson@3683
   374
\            B ~: bad -->                                    \
paulson@3683
   375
\            (EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs)";
paulson@3519
   376
by (parts_induct_tac 1);
paulson@3432
   377
(*Fake*)
paulson@3432
   378
by (Fake_parts_insert_tac 1);
paulson@3432
   379
(*YM3: by new_keys_not_used we note that Crypt K (Nonce NB) could not exist*)
paulson@3432
   380
by (fast_tac (!claset addSDs [Crypt_imp_invKey_keysFor] addss (!simpset)) 1); 
paulson@3450
   381
(*YM4: was Crypt K (Nonce NB) the very last message?  If not, use ind. hyp.*)
paulson@3450
   382
by (asm_simp_tac (!simpset addsimps [ex_disj_distrib]) 1);
paulson@3450
   383
(*yes: apply unicity of session keys*)
paulson@3683
   384
by (not_bad_tac "Aa" 1);
paulson@3432
   385
by (blast_tac (!claset addSEs [MPair_parts]
paulson@3432
   386
                       addSDs [A_trusts_YM3, B_trusts_YM4_shrK]
paulson@3683
   387
		       addDs  [Says_imp_spies RS parts.Inj,
paulson@3432
   388
			       unique_session_keys]) 1);
paulson@3450
   389
val lemma = normalize_thm [RSspec, RSmp] (result()) |> standard;
paulson@3432
   390
paulson@3432
   391
(*If B receives YM4 then A has used nonce NB (and therefore is alive).
paulson@3450
   392
  Moreover, A associates K with NB (thus is talking about the same run).
paulson@3432
   393
  Other premises guarantee secrecy of K.*)
paulson@3432
   394
goal thy 
paulson@3432
   395
 "!!evs. [| Says A' B {|Crypt (shrK B) {|Nonce NB, Key K, Agent A|},    \
paulson@3466
   396
\                       Crypt K (Nonce NB)|} : set evs;                 \
nipkow@3465
   397
\           (ALL NA. Says A Spy {|Nonce NA, Nonce NB, Key K|} ~: set evs); \
paulson@3683
   398
\           A ~: bad;  B ~: bad;  evs : yahalom |]                    \
nipkow@3465
   399
\        ==> EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs";
paulson@3683
   400
by (etac (Says_imp_spies RS parts.Inj RS MPair_parts) 1);
paulson@3450
   401
by (dtac B_trusts_YM4_shrK 1);
paulson@3432
   402
by (safe_tac (!claset));
paulson@3450
   403
by (rtac lemma 1);
paulson@3450
   404
by (rtac Spy_not_see_encrypted_key 2);
paulson@3432
   405
by (REPEAT_FIRST assume_tac);
paulson@3432
   406
by (ALLGOALS (blast_tac (!claset addSEs [MPair_parts]
paulson@3683
   407
			         addDs [Says_imp_spies RS parts.Inj])));
paulson@3432
   408
qed_spec_mp "YM4_imp_A_Said_YM3";