author | paulson |
Tue, 05 Nov 1996 11:20:52 +0100 | |
changeset 2160 | ad4382e546fc |
parent 2131 | 3106a99d30a5 |
child 2166 | d276a806cc10 |
permissions | -rw-r--r-- |
2002 | 1 |
(* Title: HOL/Auth/OtwayRees_Bad |
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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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The FAULTY version omitting encryption of Nonce NB, as suggested on page 247 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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This file illustrates the consequences of such errors. We can still prove |
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2032 | 13 |
impressive-looking properties such as Spy_not_see_encrypted_key, yet the |
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protocol is open to a middleperson attack. Attempting to prove some key lemmas |
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indicates the possibility of this attack. |
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*) |
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open OtwayRees_Bad; |
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proof_timing:=true; |
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HOL_quantifiers := false; |
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(*Weak liveness: 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 {|Nonce NA, Crypt {|Nonce NA, Key K|} (shrK A)|} \ |
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\ : set_of_list evs"; |
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by (REPEAT (resolve_tac [exI,bexI] 1)); |
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2032 | 31 |
by (rtac (otway.Nil RS otway.OR1 RS otway.OR2 RS otway.OR3 RS otway.OR4) 2); |
2002 | 32 |
by (ALLGOALS (simp_tac (!simpset setsolver safe_solver))); |
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by (REPEAT_FIRST (resolve_tac [refl, conjI])); |
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by (ALLGOALS (fast_tac (!claset addss (!simpset setsolver safe_solver)))); |
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result(); |
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(**** Inductive proofs about otway ****) |
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(*The Spy can see more than anybody else, except for their initial state*) |
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goal thy |
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"!!evs. evs : otway ==> \ |
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2052 | 43 |
\ sees lost A evs <= initState lost A Un sees lost Spy evs"; |
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by (etac otway.induct 1); |
2002 | 45 |
by (ALLGOALS (fast_tac (!claset addDs [sees_Says_subset_insert RS subsetD] |
2032 | 46 |
addss (!simpset)))); |
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qed "sees_agent_subset_sees_Spy"; |
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2002 | 48 |
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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_of_list evs"; |
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2032 | 52 |
by (etac otway.induct 1); |
2002 | 53 |
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 A' B {|N, Agent A, Agent B, X|} : set_of_list evs ==> \ |
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2052 | 62 |
\ X : analz (sees lost Spy evs)"; |
2032 | 63 |
by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1); |
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qed "OR2_analz_sees_Spy"; |
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2002 | 65 |
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goal thy "!!evs. Says S B {|N, X, X'|} : set_of_list evs ==> \ |
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2052 | 67 |
\ X : analz (sees lost Spy evs)"; |
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by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1); |
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qed "OR4_analz_sees_Spy"; |
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2002 | 70 |
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goal thy "!!evs. Says Server B {|NA, X, Crypt {|NB,K|} K'|} : set_of_list evs \ |
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\ ==> K : parts (sees lost Spy evs)"; |
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2002 | 73 |
by (fast_tac (!claset addSEs partsEs |
2032 | 74 |
addSDs [Says_imp_sees_Spy RS parts.Inj]) 1); |
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qed "Oops_parts_sees_Spy"; |
2002 | 76 |
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(*OR2_analz... and OR4_analz... let 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: Fake does not invent new nonces (as in OR2), and of course Fake |
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messages originate from the Spy. *) |
2002 | 81 |
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bind_thm ("OR2_parts_sees_Spy", |
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OR2_analz_sees_Spy RS (impOfSubs analz_subset_parts)); |
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bind_thm ("OR4_parts_sees_Spy", |
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OR4_analz_sees_Spy RS (impOfSubs analz_subset_parts)); |
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||
2002 | 87 |
val parts_Fake_tac = |
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forward_tac [OR2_parts_sees_Spy] 4 THEN |
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forward_tac [OR4_parts_sees_Spy] 6 THEN |
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2131 | 90 |
forward_tac [Oops_parts_sees_Spy] 7; |
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(*For proving the easier theorems about X ~: parts (sees lost Spy evs) *) |
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fun parts_induct_tac i = SELECT_GOAL |
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(DETERM (etac otway.induct 1 THEN parts_Fake_tac THEN |
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(*Fake message*) |
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TRY (best_tac (!claset addDs [impOfSubs analz_subset_parts, |
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impOfSubs Fake_parts_insert] |
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addss (!simpset)) 2)) THEN |
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(*Base case*) |
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fast_tac (!claset addss (!simpset)) 1 THEN |
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ALLGOALS Asm_simp_tac) i; |
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2002 | 102 |
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(** Theorems of the form X ~: parts (sees lost Spy evs) imply that NOBODY |
2002 | 105 |
sends messages containing X! **) |
106 |
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2131 | 107 |
(*Spy never sees another agent's shared key! (unless it's lost at start)*) |
2002 | 108 |
goal thy |
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"!!evs. evs : otway \ |
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\ ==> (Key (shrK A) : parts (sees lost Spy evs)) = (A : lost)"; |
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by (parts_induct_tac 1); |
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2002 | 112 |
by (Auto_tac()); |
2131 | 113 |
qed "Spy_see_shrK"; |
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Addsimps [Spy_see_shrK]; |
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2002 | 115 |
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goal thy |
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"!!evs. evs : otway \ |
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\ ==> (Key (shrK A) : analz (sees lost Spy evs)) = (A : lost)"; |
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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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2002 | 122 |
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goal thy "!!A. [| Key (shrK A) : parts (sees lost Spy evs); \ |
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\ evs : otway |] ==> A:lost"; |
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by (fast_tac (!claset addDs [Spy_see_shrK]) 1); |
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qed "Spy_see_shrK_D"; |
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2002 | 127 |
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2131 | 128 |
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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2002 | 130 |
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(*** Future keys can't be seen or used! ***) |
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(*Nobody can have SEEN keys that will be generated in the future. *) |
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goal thy "!!evs. evs : otway ==> \ |
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2002 | 136 |
\ length evs <= length evs' --> \ |
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\ Key (newK evs') ~: parts (sees lost Spy evs)"; |
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by (parts_induct_tac 1); |
2002 | 139 |
by (REPEAT_FIRST (best_tac (!claset addDs [impOfSubs analz_subset_parts, |
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impOfSubs parts_insert_subset_Un, |
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Suc_leD] |
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addss (!simpset)))); |
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qed_spec_mp "new_keys_not_seen"; |
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Addsimps [new_keys_not_seen]; |
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2160 | 146 |
(*Variant: old messages must contain old keys!*) |
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goal thy |
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"!!evs. [| Says A B X : set_of_list evs; \ |
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\ Key (newK evt) : parts {X}; \ |
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\ evs : otway \ |
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\ |] ==> length evt < length evs"; |
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by (rtac ccontr 1); |
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by (dtac leI 1); |
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by (fast_tac (!claset addSDs [new_keys_not_seen, Says_imp_sees_Spy] |
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addIs [impOfSubs parts_mono]) 1); |
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qed "Says_imp_old_keys"; |
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||
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(*** Future nonces can't be seen or used! ***) |
2002 | 160 |
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goal thy "!!evs. evs : otway ==> \ |
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\ length evs <= length evs' --> \ |
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\ Nonce (newN evs') ~: parts (sees lost Spy evs)"; |
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by (parts_induct_tac 1); |
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by (REPEAT_FIRST |
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(fast_tac (!claset addSEs partsEs |
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addSDs [Says_imp_sees_Spy RS parts.Inj] |
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addDs [impOfSubs analz_subset_parts, |
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impOfSubs parts_insert_subset_Un, Suc_leD] |
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addss (!simpset)))); |
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qed_spec_mp "new_nonces_not_seen"; |
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2002 | 172 |
Addsimps [new_nonces_not_seen]; |
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(*Variant: old messages must contain old nonces!*) |
2002 | 175 |
goal thy |
2160 | 176 |
"!!evs. [| Says A B X : set_of_list evs; \ |
2002 | 177 |
\ Nonce (newN evt) : parts {X}; \ |
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\ evs : otway \ |
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\ |] ==> length evt < length evs"; |
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by (rtac ccontr 1); |
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by (dtac leI 1); |
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by (fast_tac (!claset addSDs [new_nonces_not_seen, Says_imp_sees_Spy] |
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addIs [impOfSubs parts_mono]) 1); |
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qed "Says_imp_old_nonces"; |
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(*Nobody can have USED keys that will be generated in the future. |
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...very like new_keys_not_seen*) |
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goal thy "!!evs. evs : otway ==> \ |
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\ length evs <= length evs' --> \ |
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2160 | 191 |
\ newK evs' ~: keysFor (parts (sees lost Spy evs))"; |
2131 | 192 |
by (parts_induct_tac 1); |
2002 | 193 |
(*OR1 and OR3*) |
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by (EVERY (map (fast_tac (!claset addDs [Suc_leD] addss (!simpset))) [4,2])); |
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(*Fake, OR2, OR4: these messages send unknown (X) components*) |
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2107
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
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by (REPEAT |
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
|
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(best_tac |
2002 | 198 |
(!claset addDs [impOfSubs (analz_subset_parts RS keysFor_mono), |
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impOfSubs (parts_insert_subset_Un RS keysFor_mono), |
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Suc_leD] |
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addEs [new_keys_not_seen RS not_parts_not_analz RSN(2,rev_notE)] |
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The new proof of the lemma for new_nonces_not_seen is faster
paulson
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addss (!simpset)) 1)); |
2160 | 203 |
qed_spec_mp "new_keys_not_used"; |
2002 | 204 |
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bind_thm ("new_keys_not_analzd", |
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2032 | 206 |
[analz_subset_parts RS keysFor_mono, |
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new_keys_not_used] MRS contra_subsetD); |
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2002 | 208 |
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Addsimps [new_keys_not_used, new_keys_not_analzd]; |
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2131 | 212 |
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(*** Proofs involving analz ***) |
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(*Describes the form of K and NA when the Server sends this message. Also |
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for Oops case.*) |
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goal thy |
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"!!evs. [| Says Server B \ |
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\ {|NA, X, Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \ |
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\ evs : otway |] \ |
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\ ==> (EX evt: otway. K = Key(newK evt)) & \ |
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\ (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 (fast_tac (!claset addss (!simpset)))); |
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qed "Says_Server_message_form"; |
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(*For proofs involving analz.*) |
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val analz_Fake_tac = |
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dtac OR2_analz_sees_Spy 4 THEN |
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dtac OR4_analz_sees_Spy 6 THEN |
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forward_tac [Says_Server_message_form] 7 THEN |
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assume_tac 7 THEN Full_simp_tac 7 THEN |
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REPEAT ((eresolve_tac [bexE, exE, conjE] ORELSE' hyp_subst_tac) 7); |
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2002 | 236 |
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237 |
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(**** |
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The following is to prove theorems of the form |
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2052 | 241 |
Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) ==> |
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Key K : analz (sees lost Spy evs) |
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2002 | 243 |
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A more general formula must be proved inductively. |
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****) |
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248 |
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(** Session keys are not used to encrypt other session keys **) |
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(*Lemma for the trivial direction of the if-and-only-if*) |
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goal thy |
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"!!evs. (Key K : analz (Key``nE Un sEe)) --> \ |
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\ (K : nE | Key K : analz sEe) ==> \ |
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\ (Key K : analz (Key``nE Un sEe)) = (K : nE | Key K : analz sEe)"; |
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by (fast_tac (!claset addSEs [impOfSubs analz_mono]) 1); |
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val lemma = result(); |
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||
259 |
||
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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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2052 | 263 |
\ ALL K E. (Key K : analz (Key``(newK``E) Un (sees lost Spy evs))) = \ |
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\ (K : newK``E | Key K : analz (sees lost Spy evs))"; |
|
2032 | 265 |
by (etac otway.induct 1); |
2131 | 266 |
by analz_Fake_tac; |
2002 | 267 |
by (REPEAT_FIRST (ares_tac [allI, lemma])); |
2131 | 268 |
by (ALLGOALS |
2002 | 269 |
(asm_simp_tac |
270 |
(!simpset addsimps ([insert_Key_singleton, insert_Key_image, pushKey_newK] |
|
2032 | 271 |
@ pushes) |
2002 | 272 |
setloop split_tac [expand_if]))); |
273 |
(** LEVEL 7 **) |
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2131 | 274 |
(*OR4, OR2, Fake*) |
275 |
by (EVERY (map spy_analz_tac [5,3,2])); |
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(*Oops, OR3, Base*) |
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by (REPEAT (fast_tac (!claset addIs [image_eqI] addss (!simpset)) 1)); |
|
2002 | 278 |
qed_spec_mp "analz_image_newK"; |
279 |
||
280 |
||
281 |
goal thy |
|
282 |
"!!evs. evs : otway ==> \ |
|
2052 | 283 |
\ Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) = \ |
284 |
\ (K = newK evt | Key K : analz (sees lost Spy evs))"; |
|
2002 | 285 |
by (asm_simp_tac (HOL_ss addsimps [pushKey_newK, analz_image_newK, |
2032 | 286 |
insert_Key_singleton]) 1); |
2002 | 287 |
by (Fast_tac 1); |
288 |
qed "analz_insert_Key_newK"; |
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289 |
||
290 |
||
2131 | 291 |
(*** The Key K uniquely identifies the Server's message. **) |
2002 | 292 |
|
293 |
goal thy |
|
2131 | 294 |
"!!evs. evs : otway ==> \ |
295 |
\ EX B' NA' NB' X'. ALL B NA NB X. \ |
|
296 |
\ Says Server B {|NA, X, Crypt {|NB, K|} (shrK B)|} : set_of_list evs --> \ |
|
297 |
\ B=B' & NA=NA' & NB=NB' & X=X'"; |
|
2032 | 298 |
by (etac otway.induct 1); |
2002 | 299 |
by (ALLGOALS (asm_simp_tac (!simpset addsimps [all_conj_distrib]))); |
300 |
by (Step_tac 1); |
|
301 |
(*Remaining cases: OR3 and OR4*) |
|
302 |
by (ex_strip_tac 2); |
|
303 |
by (Fast_tac 2); |
|
2107
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
|
304 |
by (expand_case_tac "K = ?y" 1); |
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
|
305 |
by (REPEAT (ares_tac [refl,exI,impI,conjI] 2)); |
2002 | 306 |
(*...we assume X is a very new message, and handle this case by contradiction*) |
307 |
by (fast_tac (!claset addEs [Says_imp_old_keys RS less_irrefl] |
|
2032 | 308 |
delrules [conjI] (*prevent split-up into 4 subgoals*) |
309 |
addss (!simpset addsimps [parts_insertI])) 1); |
|
2002 | 310 |
val lemma = result(); |
311 |
||
312 |
goal thy |
|
2131 | 313 |
"!!evs. [| Says Server B {|NA, X, Crypt {|NB, K|} (shrK B)|} \ |
2002 | 314 |
\ : set_of_list evs; \ |
2131 | 315 |
\ Says Server B' {|NA',X',Crypt {|NB',K|} (shrK B')|} \ |
2002 | 316 |
\ : set_of_list evs; \ |
2131 | 317 |
\ evs : otway |] ==> X=X' & B=B' & NA=NA' & NB=NB'"; |
2032 | 318 |
by (dtac lemma 1); |
2002 | 319 |
by (REPEAT (etac exE 1)); |
320 |
(*Duplicate the assumption*) |
|
321 |
by (forw_inst_tac [("psi", "ALL C.?P(C)")] asm_rl 1); |
|
322 |
by (fast_tac (!claset addSDs [spec]) 1); |
|
323 |
qed "unique_session_keys"; |
|
324 |
||
325 |
||
2131 | 326 |
(*Crucial security property, but not itself enough to guarantee correctness!*) |
327 |
goal thy |
|
328 |
"!!evs. [| A ~: lost; B ~: lost; evs : otway; evt : otway |] \ |
|
329 |
\ ==> Says Server B \ |
|
330 |
\ {|NA, Crypt {|NA, Key K|} (shrK A), \ |
|
331 |
\ Crypt {|NB, Key K|} (shrK B)|} : set_of_list evs --> \ |
|
332 |
\ Says B Spy {|NA, NB, Key K|} ~: set_of_list evs --> \ |
|
333 |
\ Key K ~: analz (sees lost Spy evs)"; |
|
334 |
by (etac otway.induct 1); |
|
335 |
by analz_Fake_tac; |
|
336 |
by (ALLGOALS |
|
337 |
(asm_full_simp_tac |
|
338 |
(!simpset addsimps ([analz_subset_parts RS contra_subsetD, |
|
339 |
analz_insert_Key_newK] @ pushes) |
|
340 |
setloop split_tac [expand_if]))); |
|
341 |
(*OR3*) |
|
342 |
by (fast_tac (!claset addSIs [parts_insertI] |
|
343 |
addEs [Says_imp_old_keys RS less_irrefl] |
|
344 |
addss (!simpset addsimps [parts_insert2])) 3); |
|
345 |
(*OR4, OR2, Fake*) |
|
346 |
by (REPEAT_FIRST (resolve_tac [conjI, impI] ORELSE' spy_analz_tac)); |
|
347 |
(*Oops*) (** LEVEL 5 **) |
|
348 |
by (fast_tac (!claset delrules [disjE] |
|
349 |
addDs [unique_session_keys] addss (!simpset)) 1); |
|
350 |
val lemma = result() RS mp RS mp RSN(2,rev_notE); |
|
351 |
||
352 |
||
353 |
goal thy |
|
354 |
"!!evs. [| Says Server B \ |
|
355 |
\ {|NA, Crypt {|NA, K|} (shrK A), \ |
|
356 |
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \ |
|
357 |
\ Says B Spy {|NA, NB, K|} ~: set_of_list evs; \ |
|
358 |
\ A ~: lost; B ~: lost; evs : otway |] \ |
|
359 |
\ ==> K ~: analz (sees lost Spy evs)"; |
|
360 |
by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1); |
|
361 |
by (fast_tac (!claset addSEs [lemma]) 1); |
|
362 |
qed "Spy_not_see_encrypted_key"; |
|
363 |
||
364 |
||
365 |
(*** Attempting to prove stronger properties ***) |
|
366 |
||
2052 | 367 |
(*Only OR1 can have caused such a part of a message to appear. |
368 |
I'm not sure why A ~= B premise is needed: OtwayRees.ML doesn't need it. |
|
369 |
Perhaps it's because OR2 has two similar-looking encrypted messages in |
|
370 |
this version.*) |
|
2002 | 371 |
goal thy |
2131 | 372 |
"!!evs. [| A ~: lost; A ~= B; evs : otway |] \ |
373 |
\ ==> Crypt {|NA, Agent A, Agent B|} (shrK A) \ |
|
2052 | 374 |
\ : parts (sees lost Spy evs) --> \ |
2131 | 375 |
\ Says A B {|NA, Agent A, Agent B, \ |
2002 | 376 |
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \ |
377 |
\ : set_of_list evs"; |
|
2131 | 378 |
by (parts_induct_tac 1); |
379 |
by (Fast_tac 1); |
|
2002 | 380 |
qed_spec_mp "Crypt_imp_OR1"; |
381 |
||
382 |
||
2131 | 383 |
(*Crucial property: If the encrypted message appears, and A has used NA |
384 |
to start a run, then it originated with the Server!*) |
|
385 |
(*Only it is FALSE. Somebody could make a fake message to Server |
|
2002 | 386 |
substituting some other nonce NA' for NB.*) |
387 |
goal thy |
|
2052 | 388 |
"!!evs. [| A ~: lost; A ~= Spy; evs : otway |] \ |
389 |
\ ==> Crypt {|NA, Key K|} (shrK A) : parts (sees lost Spy evs) --> \ |
|
2131 | 390 |
\ Says A B {|NA, Agent A, Agent B, \ |
2052 | 391 |
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \ |
2131 | 392 |
\ : set_of_list evs --> \ |
393 |
\ (EX B NB. Says Server B \ |
|
394 |
\ {|NA, \ |
|
2052 | 395 |
\ Crypt {|NA, Key K|} (shrK A), \ |
396 |
\ Crypt {|NB, Key K|} (shrK B)|} \ |
|
2002 | 397 |
\ : set_of_list evs)"; |
2131 | 398 |
by (parts_induct_tac 1); |
2002 | 399 |
(*OR1: it cannot be a new Nonce, contradiction.*) |
400 |
by (fast_tac (!claset addSIs [parts_insertI] |
|
2032 | 401 |
addSEs partsEs |
402 |
addEs [Says_imp_old_nonces RS less_irrefl] |
|
403 |
addss (!simpset)) 1); |
|
2002 | 404 |
(*OR4*) |
405 |
by (REPEAT (Safe_step_tac 2)); |
|
2052 | 406 |
by (REPEAT (best_tac (!claset addSDs [parts_cut]) 3)); |
407 |
by (fast_tac (!claset addSIs [Crypt_imp_OR1] |
|
408 |
addEs partsEs |
|
409 |
addDs [Says_imp_sees_Spy RS parts.Inj]) 2); |
|
2131 | 410 |
(*OR3*) (** LEVEL 5 **) |
2002 | 411 |
by (ALLGOALS (asm_simp_tac (!simpset addsimps [ex_disj_distrib]))); |
2052 | 412 |
by (step_tac (!claset delrules [disjCI, impCE]) 1); |
2002 | 413 |
(*The hypotheses at this point suggest an attack in which nonce NA is used |
2052 | 414 |
in two different roles: |
415 |
Says B' Server |
|
416 |
{|Nonce NAa, Agent Aa, Agent A, |
|
417 |
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK Aa), Nonce NA, |
|
418 |
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK A)|} |
|
419 |
: set_of_list evsa; |
|
420 |
Says A B |
|
421 |
{|Nonce NA, Agent A, Agent B, |
|
422 |
Crypt {|Nonce NA, Agent A, Agent B|} (shrK A)|} |
|
423 |
: set_of_list evsa |
|
424 |
*) |
|
2131 | 425 |
writeln "GIVE UP! on NA_Crypt_imp_Server_msg"; |
2002 | 426 |
|
427 |
||
2052 | 428 |
(*Thus the key property A_can_trust probably fails too.*) |