(* Title: HOL/Auth/OtwayRees_Bad
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1996 University of Cambridge
Inductive relation "otway" for the Otway-Rees protocol.
The FAULTY version omitting encryption of Nonce NB, as suggested on page 247 of
Burrows, Abadi and Needham. A Logic of Authentication.
Proc. Royal Soc. 426 (1989)
This file illustrates the consequences of such errors. We can still prove
impressive-looking properties such as Spy_not_see_encrypted_key, yet the
protocol is open to a middleperson attack. Attempting to prove some key lemmas
indicates the possibility of this attack.
*)
open OtwayRees_Bad;
proof_timing:=true;
HOL_quantifiers := false;
(*Weak liveness: there are traces that reach the end*)
goal thy
"!!A B. [| A ~= B; A ~= Server; B ~= Server |] \
\ ==> EX K. EX NA. EX evs: otway. \
\ Says B A {|Nonce NA, Crypt {|Nonce NA, Key K|} (shrK A)|} \
\ : set_of_list evs";
by (REPEAT (resolve_tac [exI,bexI] 1));
by (rtac (otway.Nil RS otway.OR1 RS otway.OR2 RS otway.OR3 RS otway.OR4) 2);
by (ALLGOALS (simp_tac (!simpset setsolver safe_solver)));
by (REPEAT_FIRST (resolve_tac [refl, conjI]));
by (ALLGOALS (fast_tac (!claset addss (!simpset setsolver safe_solver))));
result();
(**** Inductive proofs about otway ****)
(*The Spy can see more than anybody else, except for their initial state*)
goal thy
"!!evs. evs : otway ==> \
\ sees lost A evs <= initState lost A Un sees lost Spy evs";
by (etac otway.induct 1);
by (ALLGOALS (fast_tac (!claset addDs [sees_Says_subset_insert RS subsetD]
addss (!simpset))));
qed "sees_agent_subset_sees_Spy";
(*Nobody sends themselves messages*)
goal thy "!!evs. evs : otway ==> ALL A X. Says A A X ~: set_of_list evs";
by (etac otway.induct 1);
by (Auto_tac());
qed_spec_mp "not_Says_to_self";
Addsimps [not_Says_to_self];
AddSEs [not_Says_to_self RSN (2, rev_notE)];
(** For reasoning about the encrypted portion of messages **)
goal thy "!!evs. Says A' B {|N, Agent A, Agent B, X|} : set_of_list evs ==> \
\ X : analz (sees lost Spy evs)";
by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1);
qed "OR2_analz_sees_Spy";
goal thy "!!evs. Says S B {|N, X, X'|} : set_of_list evs ==> \
\ X : analz (sees lost Spy evs)";
by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1);
qed "OR4_analz_sees_Spy";
goal thy "!!evs. Says B' A {|N, Crypt {|N,K|} K'|} : set_of_list evs ==> \
\ K : parts (sees lost Spy evs)";
by (fast_tac (!claset addSEs partsEs
addSDs [Says_imp_sees_Spy RS parts.Inj]) 1);
qed "Reveal_parts_sees_Spy";
(*OR2_analz... and OR4_analz... let us treat those cases using the same
argument as for the Fake case. This is possible for most, but not all,
proofs: Fake does not invent new nonces (as in OR2), and of course Fake
messages originate from the Spy. *)
bind_thm ("OR2_parts_sees_Spy",
OR2_analz_sees_Spy RS (impOfSubs analz_subset_parts));
bind_thm ("OR4_parts_sees_Spy",
OR4_analz_sees_Spy RS (impOfSubs analz_subset_parts));
val parts_Fake_tac =
forward_tac [OR2_parts_sees_Spy] 4 THEN
forward_tac [OR4_parts_sees_Spy] 6 THEN
forward_tac [Reveal_parts_sees_Spy] 7;
(** Theorems of the form X ~: parts (sees lost Spy evs) imply that NOBODY
sends messages containing X! **)
(*Spy never sees another agent's shared key! (unless it is leaked at start)*)
goal thy
"!!evs. [| evs : otway; A ~: lost |] \
\ ==> Key (shrK A) ~: parts (sees lost Spy evs)";
by (etac otway.induct 1);
by parts_Fake_tac;
by (Auto_tac());
(*Deals with Fake message*)
by (best_tac (!claset addDs [impOfSubs analz_subset_parts,
impOfSubs Fake_parts_insert]) 1);
qed "Spy_not_see_shrK";
bind_thm ("Spy_not_analz_shrK",
[analz_subset_parts, Spy_not_see_shrK] MRS contra_subsetD);
Addsimps [Spy_not_see_shrK, Spy_not_analz_shrK];
(*We go to some trouble to preserve R in the 3rd and 4th subgoals
As usual fast_tac cannot be used because it uses the equalities too soon*)
val major::prems =
goal thy "[| Key (shrK A) : parts (sees lost Spy evs); \
\ evs : otway; \
\ A:lost ==> R \
\ |] ==> R";
by (rtac ccontr 1);
by (rtac ([major, Spy_not_see_shrK] MRS rev_notE) 1);
by (swap_res_tac prems 2);
by (ALLGOALS (fast_tac (!claset addIs prems)));
qed "Spy_see_shrK_E";
bind_thm ("Spy_analz_shrK_E",
analz_subset_parts RS subsetD RS Spy_see_shrK_E);
AddSEs [Spy_see_shrK_E, Spy_analz_shrK_E];
(*** Future keys can't be seen or used! ***)
(*Nobody can have SEEN keys that will be generated in the future.
This has to be proved anew for each protocol description,
but should go by similar reasoning every time. Hardest case is the
standard Fake rule.
The Union over C is essential for the induction! *)
goal thy "!!evs. evs : otway ==> \
\ length evs <= length evs' --> \
\ Key (newK evs') ~: (UN C. parts (sees lost C evs))";
by (etac otway.induct 1);
by parts_Fake_tac;
(*auto_tac does not work here, as it performs safe_tac first*)
by (ALLGOALS Asm_simp_tac);
by (REPEAT_FIRST (best_tac (!claset addDs [impOfSubs analz_subset_parts,
impOfSubs parts_insert_subset_Un,
Suc_leD]
addss (!simpset))));
val lemma = result();
(*Variant needed for the main theorem below*)
goal thy
"!!evs. [| evs : otway; length evs <= length evs' |] \
\ ==> Key (newK evs') ~: parts (sees lost C evs)";
by (fast_tac (!claset addDs [lemma]) 1);
qed "new_keys_not_seen";
Addsimps [new_keys_not_seen];
(*Another variant: old messages must contain old keys!*)
goal thy
"!!evs. [| Says A B X : set_of_list evs; \
\ Key (newK evt) : parts {X}; \
\ evs : otway \
\ |] ==> length evt < length evs";
by (rtac ccontr 1);
by (dtac leI 1);
by (fast_tac (!claset addSDs [new_keys_not_seen, Says_imp_sees_Spy]
addIs [impOfSubs parts_mono]) 1);
qed "Says_imp_old_keys";
(*** Future nonces can't be seen or used! [proofs resemble those above] ***)
goal thy "!!evs. evs : otway ==> \
\ length evs <= length evs' --> \
\ Nonce (newN evs') ~: (UN C. parts (sees lost C evs))";
by (etac otway.induct 1);
(*auto_tac does not work here, as it performs safe_tac first*)
by (ALLGOALS (asm_simp_tac (!simpset addsimps [parts_insert2]
addcongs [disj_cong])));
by (REPEAT_FIRST (fast_tac (!claset
addSEs partsEs
addSDs [Says_imp_sees_Spy RS parts.Inj]
addDs [impOfSubs analz_subset_parts,
impOfSubs parts_insert_subset_Un,
Suc_leD]
addss (!simpset))));
val lemma = result();
(*Variant needed for the main theorem below*)
goal thy
"!!evs. [| evs : otway; length evs <= length evs' |] \
\ ==> Nonce (newN evs') ~: parts (sees lost C evs)";
by (fast_tac (!claset addDs [lemma]) 1);
qed "new_nonces_not_seen";
Addsimps [new_nonces_not_seen];
(*Another variant: old messages must contain old nonces!*)
goal thy
"!!evs. [| Says A B X : set_of_list evs; \
\ Nonce (newN evt) : parts {X}; \
\ evs : otway \
\ |] ==> length evt < length evs";
by (rtac ccontr 1);
by (dtac leI 1);
by (fast_tac (!claset addSDs [new_nonces_not_seen, Says_imp_sees_Spy]
addIs [impOfSubs parts_mono]) 1);
qed "Says_imp_old_nonces";
(*Nobody can have USED keys that will be generated in the future.
...very like new_keys_not_seen*)
goal thy "!!evs. evs : otway ==> \
\ length evs <= length evs' --> \
\ newK evs' ~: keysFor (UN C. parts (sees lost C evs))";
by (etac otway.induct 1);
by parts_Fake_tac;
by (ALLGOALS Asm_simp_tac);
(*OR1 and OR3*)
by (EVERY (map (fast_tac (!claset addDs [Suc_leD] addss (!simpset))) [4,2]));
(*Fake, OR2, OR4: these messages send unknown (X) components*)
by (REPEAT
(best_tac
(!claset addDs [impOfSubs (analz_subset_parts RS keysFor_mono),
impOfSubs (parts_insert_subset_Un RS keysFor_mono),
Suc_leD]
addEs [new_keys_not_seen RS not_parts_not_analz RSN(2,rev_notE)]
addss (!simpset)) 1));
val lemma = result();
goal thy
"!!evs. [| evs : otway; length evs <= length evs' |] \
\ ==> newK evs' ~: keysFor (parts (sees lost C evs))";
by (fast_tac (!claset addSDs [lemma] addss (!simpset)) 1);
qed "new_keys_not_used";
bind_thm ("new_keys_not_analzd",
[analz_subset_parts RS keysFor_mono,
new_keys_not_used] MRS contra_subsetD);
Addsimps [new_keys_not_used, new_keys_not_analzd];
(** Lemmas concerning the form of items passed in messages **)
(****
The following is to prove theorems of the form
Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) ==>
Key K : analz (sees lost Spy evs)
A more general formula must be proved inductively.
****)
(** Session keys are not used to encrypt other session keys **)
(*Describes the form of Key K when the following message is sent. The use of
"parts" strengthens the induction hyp for proving the Fake case. The
assumptions on A are needed to prevent its being a Faked message. (Based
on NS_Shared/Says_S_message_form) *)
goal thy
"!!evs. evs: otway ==> \
\ Crypt {|N, Key K|} (shrK A) : parts (sees lost Spy evs) & \
\ A ~: lost --> \
\ (EX evt:otway. K = newK evt)";
by (etac otway.induct 1);
by parts_Fake_tac;
by (Auto_tac());
(*Deals with Fake message*)
by (best_tac (!claset addDs [impOfSubs analz_subset_parts,
impOfSubs Fake_parts_insert]) 1);
val lemma = result() RS mp;
(*EITHER describes the form of Key K when the following message is sent,
OR reduces it to the Fake case.*)
goal thy
"!!evs. [| Says B' A {|N, Crypt {|N, Key K|} (shrK A)|} : set_of_list evs; \
\ evs : otway |] \
\ ==> (EX evt:otway. K = newK evt) | Key K : analz (sees lost Spy evs)";
by (case_tac "A : lost" 1);
by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]
addss (!simpset)) 1);
by (forward_tac [lemma] 1);
by (fast_tac (!claset addEs partsEs
addSDs [Says_imp_sees_Spy RS parts.Inj]) 1);
by (Fast_tac 1);
qed "Reveal_message_form";
(*Lemma for the trivial direction of the if-and-only-if*)
goal thy
"!!evs. (Key K : analz (Key``nE Un sEe)) --> \
\ (K : nE | Key K : analz sEe) ==> \
\ (Key K : analz (Key``nE Un sEe)) = (K : nE | Key K : analz sEe)";
by (fast_tac (!claset addSEs [impOfSubs analz_mono]) 1);
val lemma = result();
(*The equality makes the induction hypothesis easier to apply*)
goal thy
"!!evs. evs : otway ==> \
\ ALL K E. (Key K : analz (Key``(newK``E) Un (sees lost Spy evs))) = \
\ (K : newK``E | Key K : analz (sees lost Spy evs))";
by (etac otway.induct 1);
by (dtac OR2_analz_sees_Spy 4);
by (dtac OR4_analz_sees_Spy 6);
by (dtac Reveal_message_form 7);
by (REPEAT_FIRST (ares_tac [allI, lemma]));
by (REPEAT ((eresolve_tac [bexE, disjE] ORELSE' hyp_subst_tac) 7));
by (ALLGOALS (*Takes 28 secs*)
(asm_simp_tac
(!simpset addsimps ([insert_Key_singleton, insert_Key_image, pushKey_newK]
@ pushes)
setloop split_tac [expand_if])));
(** LEVEL 7 **)
(*Reveal case 2, OR4, OR2, Fake*)
by (EVERY (map spy_analz_tac [7,5,3,2]));
(*Reveal case 1, OR3, Base*)
by (Auto_tac());
qed_spec_mp "analz_image_newK";
goal thy
"!!evs. evs : otway ==> \
\ Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) = \
\ (K = newK evt | Key K : analz (sees lost Spy evs))";
by (asm_simp_tac (HOL_ss addsimps [pushKey_newK, analz_image_newK,
insert_Key_singleton]) 1);
by (Fast_tac 1);
qed "analz_insert_Key_newK";
(*Describes the form of K and NA when the Server sends this message.*)
goal thy
"!!evs. [| Says Server B \
\ {|NA, Crypt {|NA, K|} (shrK A), \
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \
\ evs : otway |] \
\ ==> (EX evt:otway. K = Key(newK evt)) & \
\ (EX i. NA = Nonce i) & \
\ (EX j. NB = Nonce j)";
by (etac rev_mp 1);
by (etac otway.induct 1);
by (ALLGOALS (fast_tac (!claset addIs [less_SucI] addss (!simpset))));
qed "Says_Server_message_form";
(*Crucial security property, but not itself enough to guarantee correctness!
The need for quantification over N, C seems to indicate the problem.
Omitting the Reveal message from the description deprives us of even
this clue. *)
goal thy
"!!evs. [| A ~: lost; B ~: lost; evs : otway; evt : otway |] \
\ ==> Says Server B \
\ {|NA, Crypt {|NA, Key K|} (shrK A), \
\ Crypt {|NB, Key K|} (shrK B)|} : set_of_list evs --> \
\ (ALL N C. Says C Spy {|N, Key K|} ~: set_of_list evs) --> \
\ Key K ~: analz (sees lost Spy evs)";
by (etac otway.induct 1);
by (dtac OR2_analz_sees_Spy 4);
by (dtac OR4_analz_sees_Spy 6);
by (dtac Reveal_message_form 7);
by (REPEAT_FIRST (eresolve_tac [asm_rl, bexE, disjE] ORELSE' hyp_subst_tac));
by (ALLGOALS
(asm_full_simp_tac
(!simpset addsimps ([analz_subset_parts RS contra_subsetD,
analz_insert_Key_newK] @ pushes)
setloop split_tac [expand_if])));
(** LEVEL 6 **)
(*Reveal case 1*)
by (Fast_tac 5);
(*OR3*)
by (fast_tac (!claset addSIs [parts_insertI]
addEs [Says_imp_old_keys RS less_irrefl]
addss (!simpset addsimps [parts_insert2])) 3);
(*Reveal case 2, OR4, OR2, Fake*)
by (rtac conjI 3);
by (REPEAT (spy_analz_tac 1));
val lemma = result() RS mp RS mp RSN(2,rev_notE);
(*WEAK VERSION: NEED TO ELIMINATE QUANTIFICATION OVER N, C!!*)
goal thy
"!!evs. [| Says Server B \
\ {|NA, Crypt {|NA, K|} (shrK A), \
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \
\ (ALL N C. Says C Spy {|N, K|} ~: set_of_list evs); \
\ A ~: lost; B ~: lost; evs : otway |] \
\ ==> K ~: analz (sees lost Spy evs)";
by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1);
by (fast_tac (!claset addSEs [lemma]) 1);
qed "Spy_not_see_encrypted_key";
(*** Attempting to prove stronger properties ***)
(** The Key K uniquely identifies the Server's message. **)
fun ex_strip_tac i = REPEAT (ares_tac [exI, conjI] i) THEN assume_tac (i+1);
goal thy
"!!evs. evs : otway ==> \
\ EX A' B' NA' NB'. ALL A B NA NB. \
\ Says Server B \
\ {|NA, Crypt {|NA, K|} (shrK A), \
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs --> \
\ A=A' & B=B' & NA=NA' & NB=NB'";
by (etac otway.induct 1);
by (ALLGOALS (asm_simp_tac (!simpset addsimps [all_conj_distrib])));
by (Step_tac 1);
(*Remaining cases: OR3 and OR4*)
by (ex_strip_tac 2);
by (Fast_tac 2);
by (expand_case_tac "K = ?y" 1);
by (REPEAT (ares_tac [refl,exI,impI,conjI] 2));
(*...we assume X is a very new message, and handle this case by contradiction*)
by (fast_tac (!claset addEs [Says_imp_old_keys RS less_irrefl]
delrules [conjI] (*prevent split-up into 4 subgoals*)
addss (!simpset addsimps [parts_insertI])) 1);
val lemma = result();
goal thy
"!!evs. [| Says Server B \
\ {|NA, Crypt {|NA, K|} (shrK A), \
\ Crypt {|NB, K|} (shrK B)|} \
\ : set_of_list evs; \
\ Says Server B' \
\ {|NA', Crypt {|NA', K|} (shrK A'), \
\ Crypt {|NB', K|} (shrK B')|} \
\ : set_of_list evs; \
\ evs : otway |] \
\ ==> A=A' & B=B' & NA=NA' & NB=NB'";
by (dtac lemma 1);
by (REPEAT (etac exE 1));
(*Duplicate the assumption*)
by (forw_inst_tac [("psi", "ALL C.?P(C)")] asm_rl 1);
by (fast_tac (!claset addSDs [spec]) 1);
qed "unique_session_keys";
(*Only OR1 can have caused such a part of a message to appear.
I'm not sure why A ~= B premise is needed: OtwayRees.ML doesn't need it.
Perhaps it's because OR2 has two similar-looking encrypted messages in
this version.*)
goal thy
"!!evs. [| A ~: lost; A ~= B; evs : otway |] \
\ ==> Crypt {|NA, Agent A, Agent B|} (shrK A) \
\ : parts (sees lost Spy evs) --> \
\ Says A B {|NA, Agent A, Agent B, \
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \
\ : set_of_list evs";
by (etac otway.induct 1);
by parts_Fake_tac;
by (ALLGOALS Asm_simp_tac);
(*Fake*)
by (best_tac (!claset addSDs [impOfSubs analz_subset_parts,
impOfSubs Fake_parts_insert]) 2);
by (Auto_tac());
qed_spec_mp "Crypt_imp_OR1";
(*This key property is FALSE. Somebody could make a fake message to Server
substituting some other nonce NA' for NB.*)
goal thy
"!!evs. [| A ~: lost; A ~= Spy; evs : otway |] \
\ ==> Crypt {|NA, Key K|} (shrK A) : parts (sees lost Spy evs) --> \
\ Says A B {|NA, Agent A, Agent B, \
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \
\ : set_of_list evs --> \
\ (EX B NB. Says Server B \
\ {|NA, \
\ Crypt {|NA, Key K|} (shrK A), \
\ Crypt {|NB, Key K|} (shrK B)|} \
\ : set_of_list evs)";
by (etac otway.induct 1);
by parts_Fake_tac;
by (ALLGOALS Asm_simp_tac);
(*Fake*)
by (best_tac (!claset addSDs [impOfSubs analz_subset_parts,
impOfSubs Fake_parts_insert]) 1);
(*OR1: it cannot be a new Nonce, contradiction.*)
by (fast_tac (!claset addSIs [parts_insertI]
addSEs partsEs
addEs [Says_imp_old_nonces RS less_irrefl]
addss (!simpset)) 1);
(*OR3 and OR4*) (** LEVEL 5 **)
(*OR4*)
by (REPEAT (Safe_step_tac 2));
by (REPEAT (best_tac (!claset addSDs [parts_cut]) 3));
by (fast_tac (!claset addSIs [Crypt_imp_OR1]
addEs partsEs
addDs [Says_imp_sees_Spy RS parts.Inj]) 2);
(*OR3*) (** LEVEL 8 **)
by (ALLGOALS (asm_simp_tac (!simpset addsimps [ex_disj_distrib])));
by (step_tac (!claset delrules [disjCI, impCE]) 1);
(*The hypotheses at this point suggest an attack in which nonce NA is used
in two different roles:
Says B' Server
{|Nonce NAa, Agent Aa, Agent A,
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK Aa), Nonce NA,
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK A)|}
: set_of_list evsa;
Says A B
{|Nonce NA, Agent A, Agent B,
Crypt {|Nonce NA, Agent A, Agent B|} (shrK A)|}
: set_of_list evsa
*)
writeln "GIVE UP!";
(*Thus the key property A_can_trust probably fails too.*)