src/HOL/Auth/OtwayRees_AN.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/OtwayRees
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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 "otway" for the Otway-Rees protocol.
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Simplified version with minimal encryption but explicit messages
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From page 11 of
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  Abadi and Needham.  Prudent Engineering Practice for Cryptographic Protocols.
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  IEEE Trans. SE 22 (1), 1996
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*)
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open OtwayRees_AN;
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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 K. EX NA. EX evs: otway.                                      \
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\             Says B A (Crypt (shrK A) {|Nonce NA, Agent A, Agent B, Key K|}) \
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\             : set evs";
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by (REPEAT (resolve_tac [exI,bexI] 1));
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by (rtac (otway.Nil RS otway.OR1 RS otway.OR2 RS otway.OR3 RS otway.OR4) 2);
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by possibility_tac;
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result();
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(**** Inductive proofs about otway ****)
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(*Nobody sends themselves messages*)
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goal thy "!!evs. evs : otway ==> ALL A X. Says A A X ~: set evs";
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by (etac otway.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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goal thy "!!evs. Says S' B {|X, Crypt(shrK B) 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 "OR4_analz_spies";
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goal thy "!!evs. Says Server B {|X, Crypt K' {|NB, a, Agent B, K|}|} \
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\                  : set evs ==> K : parts (spies evs)";
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by (blast_tac (!claset addSEs spies_partsEs) 1);
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qed "Oops_parts_spies";
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(*OR4_analz_spies lets us treat those cases using the same 
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  argument as for the Fake case.  This is possible for most, but not all,
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  proofs, since Fake messages originate from the Spy. *)
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bind_thm ("OR4_parts_spies",
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          OR4_analz_spies RS (impOfSubs analz_subset_parts));
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(*For proving the easier theorems about X ~: parts (spies evs).*)
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fun parts_induct_tac i = 
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    etac otway.induct i			THEN 
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    forward_tac [Oops_parts_spies] (i+6) THEN
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    forward_tac [OR4_parts_spies]  (i+5) THEN
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    prove_simple_subgoals_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 : otway ==> (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 : otway ==> (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); evs : otway |] ==> 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!*)
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goal thy "!!evs. evs : otway ==>          \
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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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(*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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(*OR3*)
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by (Blast_tac 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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(*** Proofs involving analz ***)
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(*Describes the form of K and NA when the Server sends this message.*)
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goal thy 
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 "!!evs. [| Says Server B                                           \
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\              {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},    \
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\                Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}   \
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\             : set evs;                                            \
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\           evs : otway |]                                          \
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\        ==> K ~: range shrK & (EX i. NA = Nonce i) & (EX j. NB = Nonce j)";
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by (etac rev_mp 1);
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by (etac otway.induct 1);
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by (ALLGOALS Asm_simp_tac);
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by (Blast_tac 1);
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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 OR4_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 ((eresolve_tac [exE, 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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(*The equality makes the induction hypothesis easier to apply*)
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goal thy  
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 "!!evs. evs : otway ==>                                    \
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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 otway.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 : otway;  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 : otway ==>                                            \
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\      EX A' B' NA' NB'. ALL A B NA NB.                             \
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\       Says Server B                                               \
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\         {|Crypt (shrK A) {|NA, Agent A, Agent B, K|},             \
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\           Crypt (shrK B) {|NB, Agent A, Agent B, K|}|} : set evs  \
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\       --> A=A' & B=B' & NA=NA' & NB=NB'";
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by (etac otway.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 cases: OR3 and OR4*)
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by (ex_strip_tac 2);
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by (Blast_tac 2);
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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 splitup into 4 subgoals*)) 1);
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val lemma = result();
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goal thy 
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"!!evs. [| Says Server B                                           \
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\            {|Crypt (shrK A) {|NA, Agent A, Agent B, K|},         \
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\              Crypt (shrK B) {|NB, Agent A, Agent B, K|}|}        \
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\           : set evs;                                             \
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\          Says Server B'                                          \
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\            {|Crypt (shrK A') {|NA', Agent A', Agent B', K|},     \
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\              Crypt (shrK B') {|NB', Agent A', Agent B', K|}|}    \
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\           : set evs;                                             \
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\          evs : otway |]                                          \
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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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(**** Authenticity properties relating to NA ****)
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(*If the encrypted message appears then it originated with the Server!*)
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goal thy 
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 "!!evs. [| A ~: bad;  evs : otway |]                 \
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\ ==> Crypt (shrK A) {|NA, Agent A, Agent B, Key K|} : parts (spies evs) \
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\     --> (EX NB. Says Server B                                          \
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\                  {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},     \
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\                    Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}    \
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\                  : set evs)";
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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 (asm_simp_tac (!simpset addsimps [ex_disj_distrib])));
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(*OR3*)
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by (Blast_tac 1);
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qed_spec_mp "NA_Crypt_imp_Server_msg";
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(*Corollary: if A receives B's OR4 message then it originated with the Server.
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  Freshness may be inferred from nonce NA.*)
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goal thy 
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 "!!evs. [| Says B' A (Crypt (shrK A) {|NA, Agent A, Agent B, Key K|})  \
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\            : set evs;                                                 \
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\           A ~: bad;  evs : otway |]                                  \
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\        ==> EX NB. Says Server B                                       \
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\                    {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},  \
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\                      Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|} \
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\                   : set evs";
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by (blast_tac (!claset addSIs [NA_Crypt_imp_Server_msg]
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                      addEs  spies_partsEs) 1);
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qed "A_trusts_OR4";
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(** Crucial secrecy property: Spy does not see the keys sent in msg OR3
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    Does not in itself guarantee security: an attack could violate 
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    the premises, e.g. by having A=Spy **)
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goal thy 
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 "!!evs. [| A ~: bad;  B ~: bad;  evs : otway |]                 \
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\        ==> Says Server B                                         \
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\             {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},    \
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\               Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}   \
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\            : set evs -->                                         \
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\            Says B Spy {|NA, NB, Key K|} ~: set evs -->           \
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\            Key K ~: analz (spies evs)";
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by (etac otway.induct 1);
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by analz_spies_tac;
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by (ALLGOALS
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    (asm_simp_tac (!simpset addcongs [conj_cong, if_weak_cong] 
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                            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 addSDs [unique_session_keys]) 4);
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(*OR4*) 
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by (Blast_tac 3);
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(*OR3*)
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by (blast_tac (!claset 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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goal thy 
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 "!!evs. [| Says Server B                                           \
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\              {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},    \
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\                Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}   \
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\             : set evs;                                            \
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\           Says B Spy {|NA, NB, Key K|} ~: set evs;                \
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\           A ~: bad;  B ~: bad;  evs : otway |]                  \
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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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(**** Authenticity properties relating to NB ****)
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(*If the encrypted message appears then it originated with the Server!*)
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goal thy 
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 "!!evs. [| B ~: bad;  evs : otway |]                                 \
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\    ==> Crypt (shrK B) {|NB, Agent A, Agent B, Key K|} : parts (spies evs) \
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\        --> (EX NA. Says Server B                                          \
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\                     {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},     \
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\                       Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}    \
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\                     : set evs)";
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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 (asm_simp_tac (!simpset addsimps [ex_disj_distrib])));
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(*OR3*)
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by (Blast_tac 1);
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qed_spec_mp "NB_Crypt_imp_Server_msg";
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(*Guarantee for B: if it gets a well-formed certificate then the Server
paulson@2454
   315
  has sent the correct message in round 3.*)
paulson@2090
   316
goal thy 
paulson@3683
   317
 "!!evs. [| B ~: bad;  evs : otway;                                        \
paulson@2837
   318
\           Says S' B {|X, Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|} \
paulson@3466
   319
\            : set evs |]                                                   \
paulson@2106
   320
\        ==> EX NA. Says Server B                                           \
paulson@2284
   321
\                     {|Crypt (shrK A) {|NA, Agent A, Agent B, Key K|},     \
paulson@2284
   322
\                       Crypt (shrK B) {|NB, Agent A, Agent B, Key K|}|}    \
nipkow@3465
   323
\                     : set evs";
paulson@3102
   324
by (blast_tac (!claset addSIs [NB_Crypt_imp_Server_msg]
paulson@3683
   325
                       addEs  spies_partsEs) 1);
paulson@2331
   326
qed "B_trusts_OR3";