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(* Title: HOL/Auth/Yahalom
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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.
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From page 257 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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(*A "possibility property": there are traces that reach the end*)
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Goal "A ~= 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
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yahalom.YM1 RS yahalom.Reception RS
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yahalom.YM2 RS yahalom.Reception RS
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yahalom.YM3 RS yahalom.Reception RS yahalom.YM4) 2);
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by possibility_tac;
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result();
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Goal "[| Gets B X : set evs; evs : yahalom |] ==> EX A. Says A B X : set evs";
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by (etac rev_mp 1);
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by (etac yahalom.induct 1);
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by Auto_tac;
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qed "Gets_imp_Says";
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(*Must be proved separately for each protocol*)
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Goal "[| Gets B X : set evs; evs : yahalom |] ==> X : knows Spy evs";
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by (blast_tac (claset() addSDs [Gets_imp_Says, Says_imp_knows_Spy]) 1);
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qed"Gets_imp_knows_Spy";
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AddDs [Gets_imp_knows_Spy RS parts.Inj];
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fun g_not_bad_tac s =
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ftac Gets_imp_Says THEN' assume_tac THEN' not_bad_tac s;
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(**** Inductive proofs about yahalom ****)
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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 "[| Gets A {|Crypt (shrK A) Y, X|} : set evs; evs : yahalom |] \
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\ ==> X : analz (knows Spy evs)";
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by (blast_tac (claset() addSDs [Gets_imp_knows_Spy RS analz.Inj]) 1);
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qed "YM4_analz_knows_Spy";
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bind_thm ("YM4_parts_knows_Spy",
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YM4_analz_knows_Spy RS (impOfSubs analz_subset_parts));
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(*For proving the easier theorems about X ~: parts (knows Spy evs).*)
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fun parts_knows_Spy_tac i =
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EVERY
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[ftac YM4_parts_knows_Spy (i+6), assume_tac (i+6),
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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 (knows Spy evs) --> ... then it shortens the proof by discarding
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needless information about analz (insert X (knows Spy 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_knows_Spy_tac i;
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(** Theorems of the form X ~: parts (knows Spy 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 "evs : yahalom ==> (Key (shrK A) : parts (knows Spy 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 (ALLGOALS Blast_tac);
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qed "Spy_see_shrK";
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Addsimps [Spy_see_shrK];
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Goal "evs : yahalom ==> (Key (shrK A) : analz (knows Spy 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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AddSDs [Spy_see_shrK RSN (2, rev_iffD1),
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Spy_analz_shrK RSN (2, rev_iffD1)];
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(*Nobody can have used non-existent keys! Needed to apply analz_insert_Key*)
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Goal "evs : yahalom ==> \
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\ Key K ~: used evs --> K ~: keysFor (parts (knows Spy evs))";
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by (parts_induct_tac 1);
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(*Fake*)
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by (blast_tac (claset() addSDs [keysFor_parts_insert]) 1);
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(*YM2-4: Because Key K is not fresh, etc.*)
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by (REPEAT (blast_tac (claset() addSEs knows_Spy_partsEs) 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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(*For proofs involving analz.*)
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val analz_knows_Spy_tac =
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ftac YM4_analz_knows_Spy 7 THEN assume_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) (knows Spy evs)) ==>
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Key K : analz (knows Spy 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 "evs : yahalom ==> \
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\ ALL K KK. KK <= - (range shrK) --> \
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\ (Key K : analz (Key``KK Un (knows Spy evs))) = \
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\ (K : KK | Key K : analz (knows Spy evs))";
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by (etac yahalom.induct 1);
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by analz_knows_Spy_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 1);
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qed_spec_mp "analz_image_freshK";
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Goal "[| evs : yahalom; KAB ~: range shrK |] \
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\ ==> Key K : analz (insert (Key KAB) (knows Spy evs)) = \
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\ (K = KAB | Key K : analz (knows Spy 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 "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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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, 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 (ALLGOALS Clarify_tac);
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by (ex_strip_tac 2);
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by (Blast_tac 2);
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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 knows_Spy_partsEs
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delrules [conjI] (*no split-up to 4 subgoals*)) 1);
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val lemma = result();
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Goal "[| Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|} : set evs; \
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\ Says Server A' \
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\ {|Crypt (shrK A') {|Agent B', Key K, na', nb'|}, 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 "[| A ~: bad; B ~: bad; evs : yahalom |] \
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\ ==> Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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\ : set evs --> \
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\ Key K ~: analz (knows Spy evs)";
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by (etac yahalom.induct 1);
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by analz_knows_Spy_tac;
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by (ALLGOALS
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(asm_simp_tac
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(simpset() addsimps split_ifs @ pushes @
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[analz_insert_eq, analz_insert_freshK])));
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(*YM3*)
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by (blast_tac (claset() delrules [impCE]
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addSEs knows_Spy_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 RSN(2,rev_notE);
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(*Final version*)
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Goal "[| Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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\ : set evs; \
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\ A ~: bad; B ~: bad; evs : yahalom |] \
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\ ==> Key K ~: analz (knows Spy evs)";
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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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Goal "[| Crypt (shrK A) {|Agent B, Key K, na, nb|} : parts (knows Spy evs); \
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\ A ~: bad; evs : yahalom |] \
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\ ==> Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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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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qed "A_trusts_YM3";
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(*The obvious combination of A_trusts_YM3 with Spy_not_see_encrypted_key*)
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Goal "[| Crypt (shrK A) {|Agent B, Key K, na, nb|} : parts (knows Spy evs); \
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\ A ~: bad; B ~: bad; evs : yahalom |] \
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\ ==> Key K ~: analz (knows Spy evs)";
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by (blast_tac (claset() addSDs [A_trusts_YM3, Spy_not_see_encrypted_key]) 1);
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qed "A_gets_good_key";
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(** Security Guarantees 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. But this part says nothing about nonces.*)
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Goal "[| Crypt (shrK B) {|Agent A, Key K|} : parts (knows Spy evs); \
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\ B ~: bad; evs : yahalom |] \
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\ ==> EX NA NB. Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, \
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\ Nonce NA, Nonce NB|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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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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(** Up to now, the reasoning is similar to standard Yahalom. Now the
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doubtful reasoning occurs. We should not be assuming that an unknown
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key is secure, but the model allows us to: there is no Oops rule to
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let session keys go.*)
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(*B knows, by the second part of A's message, that the Server distributed
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the key quoting nonce NB. This part says nothing about agent names.
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Secrecy of K is assumed; the valid Yahalom proof uses (and later proves)
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the secrecy of NB.*)
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Goal "evs : yahalom \
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\ ==> Key K ~: analz (knows Spy evs) --> \
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\ Crypt K (Nonce NB) : parts (knows Spy evs) --> \
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\ (EX A B NA. Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, \
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\ Nonce NA, Nonce NB|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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\ : set evs)";
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by (parts_induct_tac 1);
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by (ALLGOALS Clarify_tac);
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(*YM3 & Fake*)
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by (Blast_tac 2);
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by (Fake_parts_insert_tac 1);
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(*YM4*)
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(*A is uncompromised because NB is secure*)
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by (g_not_bad_tac "A" 1);
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(*A's certificate guarantees the existence of the Server message*)
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by (blast_tac (claset() addDs [Says_imp_knows_Spy RS parts.Inj RS parts.Fst RS
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A_trusts_YM3]) 1);
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bind_thm ("B_trusts_YM4_newK", result() RS mp RSN (2, rev_mp));
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(*B's session key guarantee from YM4. The two certificates contribute to a
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single conclusion about the Server's message. *)
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Goal "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|}, \
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\ Crypt K (Nonce NB)|} : set evs; \
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\ Says B Server \
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\ {|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|} \
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\ : set evs; \
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\ A ~: bad; B ~: bad; evs : yahalom |] \
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\ ==> EX na nb. Says Server A \
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\ {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, \
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\ Crypt (shrK B) {|Agent A, Key K|}|} \
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\ : set evs";
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by (etac (Gets_imp_knows_Spy RS parts.Inj RS MPair_parts) 1 THEN
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assume_tac 1 THEN dtac B_trusts_YM4_shrK 1);
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by (dtac B_trusts_YM4_newK 3);
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by (REPEAT_FIRST (eresolve_tac [asm_rl, exE]));
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by (etac Spy_not_see_encrypted_key 1 THEN REPEAT (assume_tac 1));
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by (ftac unique_session_keys 1 THEN REPEAT (assume_tac 1));
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by (blast_tac (claset() addDs []) 1);
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qed "B_trusts_YM4";
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(*The obvious combination of B_trusts_YM4 with Spy_not_see_encrypted_key*)
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Goal "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|}, \
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\ Crypt K (Nonce NB)|} : set evs; \
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\ Says B Server \
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\ {|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|} \
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\ : set evs; \
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\ A ~: bad; B ~: bad; evs : yahalom |] \
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\ ==> Key K ~: analz (knows Spy evs)";
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by (blast_tac (claset() addSDs [B_trusts_YM4, Spy_not_see_encrypted_key]) 1);
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qed "B_gets_good_key";
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(*** Authenticating B to A: these proofs are not considered.
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They are irrelevant to showing the need for Oops. ***)
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(*** Authenticating A to B using the certificate Crypt K (Nonce NB) ***)
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(*Assuming the session key is secure, if both certificates are present then
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A has said NB. We can't be sure about the rest of A's message, but only
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NB matters for freshness.*)
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Goal "evs : yahalom \
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\ ==> Key K ~: analz (knows Spy evs) --> \
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\ Crypt K (Nonce NB) : parts (knows Spy evs) --> \
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\ Crypt (shrK B) {|Agent A, Key K|} : parts (knows Spy evs) --> \
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\ B ~: bad --> \
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\ (EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs)";
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by (parts_induct_tac 1);
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(*Fake*)
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by (Fake_parts_insert_tac 1);
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(*YM3: by new_keys_not_used we note that Crypt K (Nonce NB) could not exist*)
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by (fast_tac (claset() addSDs [Crypt_imp_keysFor] addss (simpset())) 1);
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(*YM4: was Crypt K (Nonce NB) the very last message? If not, use ind. hyp.*)
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332 |
by (asm_simp_tac (simpset() addsimps [ex_disj_distrib]) 1);
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|
333 |
(*yes: apply unicity of session keys*)
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334 |
by (g_not_bad_tac "Aa" 1);
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335 |
by (blast_tac (claset() addSEs [MPair_parts]
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336 |
addSDs [A_trusts_YM3, B_trusts_YM4_shrK]
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337 |
addDs [Says_imp_knows_Spy RS parts.Inj,
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|
338 |
unique_session_keys]) 1);
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|
339 |
qed_spec_mp "A_Said_YM3_lemma";
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340 |
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341 |
(*If B receives YM4 then A has used nonce NB (and therefore is alive).
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342 |
Moreover, A associates K with NB (thus is talking about the same run).
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343 |
Other premises guarantee secrecy of K.*)
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344 |
Goal "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|}, \
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345 |
\ Crypt K (Nonce NB)|} : set evs; \
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346 |
\ Says B Server \
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347 |
\ {|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|} \
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348 |
\ : set evs; \
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349 |
\ A ~: bad; B ~: bad; evs : yahalom |] \
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|
350 |
\ ==> EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs";
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7499
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351 |
by (ftac B_trusts_YM4 1);
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6400
|
352 |
by (REPEAT_FIRST assume_tac);
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|
353 |
by (etac (Gets_imp_knows_Spy RS parts.Inj RS MPair_parts) 1 THEN assume_tac 1);
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354 |
by (Clarify_tac 1);
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355 |
by (rtac A_Said_YM3_lemma 1);
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|
356 |
by (rtac Spy_not_see_encrypted_key 2);
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|
357 |
by (REPEAT_FIRST assume_tac);
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|
358 |
qed_spec_mp "YM4_imp_A_Said_YM3";
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