src/HOL/Auth/Yahalom.ML

setloop split_tac -> addsplits

(*  Title:      HOL/Auth/Yahalom
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1996  University of Cambridge

Inductive relation "yahalom" for the Yahalom protocol.

From page 257 of
  Burrows, Abadi and Needham.  A Logic of Authentication.
  Proc. Royal Soc. 426 (1989)
*)

open Yahalom;

proof_timing:=true;
HOL_quantifiers := false;
Pretty.setdepth 25;


(*A "possibility property": there are traces that reach the end*)
goal thy 
 "!!A B. [| A ~= B; A ~= Server; B ~= Server |]   \
\        ==> EX X NB K. EX evs: yahalom.     \
\               Says A B {|X, Crypt K (Nonce NB)|} : set evs";
by (REPEAT (resolve_tac [exI,bexI] 1));
by (rtac (yahalom.Nil RS yahalom.YM1 RS yahalom.YM2 RS yahalom.YM3 RS 
          yahalom.YM4) 2);
by possibility_tac;
result();


(**** Inductive proofs about yahalom ****)

(*Nobody sends themselves messages*)
goal thy "!!evs. evs: yahalom ==> ALL A X. Says A A X ~: set evs";
by (etac yahalom.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 **)

(*Lets us treat YM4 using a similar argument as for the Fake case.*)
goal thy "!!evs. Says S A {|Crypt (shrK A) Y, X|} : set evs ==> \
\                X : analz (spies evs)";
by (blast_tac (!claset addSDs [Says_imp_spies RS analz.Inj]) 1);
qed "YM4_analz_spies";

bind_thm ("YM4_parts_spies",
          YM4_analz_spies RS (impOfSubs analz_subset_parts));

(*Relates to both YM4 and Oops*)
goal thy "!!evs. Says S A {|Crypt (shrK A) {|B,K,NA,NB|}, X|} : set evs ==> \
\                K : parts (spies evs)";
by (blast_tac (!claset addSEs partsEs
                      addSDs [Says_imp_spies RS parts.Inj]) 1);
qed "YM4_Key_parts_spies";

(*For proving the easier theorems about X ~: parts (spies evs).*)
fun parts_spies_tac i = 
    forward_tac [YM4_Key_parts_spies] (i+6) THEN
    forward_tac [YM4_parts_spies] (i+5)     THEN
    prove_simple_subgoals_tac  i;

(*Induction for regularity theorems.  If induction formula has the form
   X ~: analz (spies evs) --> ... then it shortens the proof by discarding
   needless information about analz (insert X (spies evs))  *)
fun parts_induct_tac i = 
    etac yahalom.induct i
    THEN 
    REPEAT (FIRSTGOAL analz_mono_contra_tac)
    THEN  parts_spies_tac i;


(** Theorems of the form X ~: parts (spies evs) imply that NOBODY
    sends messages containing X! **)

(*Spy never sees another agent's shared key! (unless it's bad at start)*)
goal thy 
 "!!evs. evs : yahalom ==> (Key (shrK A) : parts (spies evs)) = (A : bad)";
by (parts_induct_tac 1);
by (Fake_parts_insert_tac 1);
by (Blast_tac 1);
qed "Spy_see_shrK";
Addsimps [Spy_see_shrK];

goal thy 
 "!!evs. evs : yahalom ==> (Key (shrK A) : analz (spies evs)) = (A : bad)";
by (auto_tac(!claset addDs [impOfSubs analz_subset_parts], !simpset));
qed "Spy_analz_shrK";
Addsimps [Spy_analz_shrK];

goal thy  "!!A. [| Key (shrK A) : parts (spies evs);       \
\                  evs : yahalom |] ==> A:bad";
by (blast_tac (!claset addDs [Spy_see_shrK]) 1);
qed "Spy_see_shrK_D";

bind_thm ("Spy_analz_shrK_D", analz_subset_parts RS subsetD RS Spy_see_shrK_D);
AddSDs [Spy_see_shrK_D, Spy_analz_shrK_D];


(*Nobody can have used non-existent keys!  Needed to apply analz_insert_Key*)
goal thy "!!evs. evs : yahalom ==>          \
\         Key K ~: used evs --> K ~: keysFor (parts (spies evs))";
by (parts_induct_tac 1);
(*YM4: Key K is not fresh!*)
by (blast_tac (!claset addSEs spies_partsEs) 3);
(*YM3*)
by (Blast_tac 2);
(*Fake*)
by (best_tac
      (!claset addIs [impOfSubs analz_subset_parts]
               addDs [impOfSubs (analz_subset_parts RS keysFor_mono),
                      impOfSubs (parts_insert_subset_Un RS keysFor_mono)]
               addss (!simpset)) 1);
qed_spec_mp "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];


(*Describes the form of K when the Server sends this message.  Useful for
  Oops as well as main secrecy property.*)
goal thy 
 "!!evs. [| Says Server A {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|} \
\             : set evs;                                                   \
\           evs : yahalom |]                                          \
\        ==> K ~: range shrK";
by (etac rev_mp 1);
by (etac yahalom.induct 1);
by (ALLGOALS Asm_simp_tac);
by (Blast_tac 1);
qed "Says_Server_message_form";


(*For proofs involving analz.*)
val analz_spies_tac = 
    forward_tac [YM4_analz_spies] 6 THEN
    forward_tac [Says_Server_message_form] 7 THEN
    assume_tac 7 THEN REPEAT ((etac exE ORELSE' hyp_subst_tac) 7);


(****
 The following is to prove theorems of the form

  Key K : analz (insert (Key KAB) (spies evs)) ==>
  Key K : analz (spies evs)

 A more general formula must be proved inductively.
****)

(** Session keys are not used to encrypt other session keys **)

goal thy  
 "!!evs. evs : yahalom ==>                                 \
\  ALL K KK. KK <= Compl (range shrK) -->                       \
\            (Key K : analz (Key``KK Un (spies evs))) = \
\            (K : KK | Key K : analz (spies evs))";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (REPEAT_FIRST (resolve_tac [allI, impI]));
by (REPEAT_FIRST (rtac analz_image_freshK_lemma));
by (ALLGOALS (asm_simp_tac analz_image_freshK_ss));
(*Fake*) 
by (spy_analz_tac 2);
(*Base*)
by (Blast_tac 1);
qed_spec_mp "analz_image_freshK";

goal thy
 "!!evs. [| evs : yahalom;  KAB ~: range shrK |] ==>             \
\        Key K : analz (insert (Key KAB) (spies evs)) =       \
\        (K = KAB | Key K : analz (spies evs))";
by (asm_simp_tac (analz_image_freshK_ss addsimps [analz_image_freshK]) 1);
qed "analz_insert_freshK";


(*** The Key K uniquely identifies the Server's  message. **)

goal thy 
 "!!evs. evs : yahalom ==>                                     \
\      EX A' B' na' nb' X'. ALL A B na nb X.                        \
\          Says Server A                                            \
\           {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|}        \
\          : set evs --> A=A' & B=B' & na=na' & nb=nb' & X=X'";
by (etac yahalom.induct 1);
by (ALLGOALS (asm_simp_tac (!simpset addsimps [all_conj_distrib])));
by (ALLGOALS Clarify_tac);
by (ex_strip_tac 2);
by (Blast_tac 2);
(*Remaining case: YM3*)
by (expand_case_tac "K = ?y" 1);
by (REPEAT (ares_tac [refl,exI,impI,conjI] 2));
(*...we assume X is a recent message and handle this case by contradiction*)
by (blast_tac (!claset addSEs spies_partsEs
                       delrules [conjI]    (*no split-up to 4 subgoals*)) 1);
val lemma = result();

goal thy 
"!!evs. [| Says Server A                                                 \
\            {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|} : set evs; \
\          Says Server A'                                                \
\            {|Crypt (shrK A') {|Agent B', Key K, na', nb'|}, X'|} : set evs; \
\          evs : yahalom |]                                    \
\       ==> A=A' & B=B' & na=na' & nb=nb'";
by (prove_unique_tac lemma 1);
qed "unique_session_keys";


(** Crucial secrecy property: Spy does not see the keys sent in msg YM3 **)

goal thy 
 "!!evs. [| A ~: bad;  B ~: bad;  evs : yahalom |]         \
\        ==> Says Server A                                        \
\              {|Crypt (shrK A) {|Agent B, Key K, na, nb|},       \
\                Crypt (shrK B) {|Agent A, Key K|}|}              \
\             : set evs -->                                       \
\            Says A Spy {|na, nb, Key K|} ~: set evs -->          \
\            Key K ~: analz (spies evs)";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (ALLGOALS
    (asm_simp_tac 
     (!simpset addsimps [analz_insert_eq, analz_insert_freshK]
               addsplits [expand_if])));
(*Oops*)
by (blast_tac (!claset addDs [unique_session_keys]) 3);
(*YM3*)
by (blast_tac (!claset delrules [impCE]
                       addSEs spies_partsEs
                       addIs [impOfSubs analz_subset_parts]) 2);
(*Fake*) 
by (spy_analz_tac 1);
val lemma = result() RS mp RS mp RSN(2,rev_notE);


(*Final version*)
goal thy 
 "!!evs. [| Says Server A                                         \
\              {|Crypt (shrK A) {|Agent B, Key K, na, nb|},       \
\                Crypt (shrK B) {|Agent A, Key K|}|}              \
\             : set evs;                                          \
\           Says A Spy {|na, nb, Key K|} ~: set evs;              \
\           A ~: bad;  B ~: bad;  evs : yahalom |]         \
\        ==> Key K ~: analz (spies evs)";
by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1);
by (blast_tac (!claset addSEs [lemma]) 1);
qed "Spy_not_see_encrypted_key";


(** Security Guarantee for A upon receiving YM3 **)

(*If the encrypted message appears then it originated with the Server*)
goal thy
 "!!evs. [| Crypt (shrK A) {|Agent B, Key K, na, nb|} : parts (spies evs); \
\           A ~: bad;  evs : yahalom |]                          \
\         ==> Says Server A                                            \
\              {|Crypt (shrK A) {|Agent B, Key K, na, nb|},            \
\                Crypt (shrK B) {|Agent A, Key K|}|}                   \
\             : set evs";
by (etac rev_mp 1);
by (parts_induct_tac 1);
by (Fake_parts_insert_tac 1);
qed "A_trusts_YM3";


(** Security Guarantees for B upon receiving YM4 **)

(*B knows, by the first part of A's message, that the Server distributed 
  the key for A and B.  But this part says nothing about nonces.*)
goal thy 
 "!!evs. [| Crypt (shrK B) {|Agent A, Key K|} : parts (spies evs);   \
\           B ~: bad;  evs : yahalom |]                                \
\        ==> EX NA NB. Says Server A                                    \
\                        {|Crypt (shrK A) {|Agent B, Key K,             \
\                                           Nonce NA, Nonce NB|},       \
\                          Crypt (shrK B) {|Agent A, Key K|}|}          \
\                       : set evs";
by (etac rev_mp 1);
by (parts_induct_tac 1);
by (Fake_parts_insert_tac 1);
(*YM3*)
by (Blast_tac 1);
qed "B_trusts_YM4_shrK";

(*B knows, by the second part of A's message, that the Server distributed 
  the key quoting nonce NB.  This part says nothing about agent names. 
  Secrecy of NB is crucial.*)
goal thy 
 "!!evs. evs : yahalom                                             \
\        ==> Nonce NB ~: analz (spies evs) -->                  \
\            Crypt K (Nonce NB) : parts (spies evs) -->         \
\            (EX A B NA. Says Server A                             \
\                        {|Crypt (shrK A) {|Agent B, Key K,        \
\                                  Nonce NA, Nonce NB|},           \
\                          Crypt (shrK B) {|Agent A, Key K|}|}     \
\                       : set evs)";
by (parts_induct_tac 1);
by (ALLGOALS Clarify_tac);
(*YM3 & Fake*)
by (Blast_tac 2);
by (Fake_parts_insert_tac 1);
(*YM4*)
(*A is uncompromised because NB is secure*)
by (not_bad_tac "A" 1);
(*A's certificate guarantees the existence of the Server message*)
by (blast_tac (!claset addDs [Says_imp_spies RS parts.Inj RS parts.Fst RS
			      A_trusts_YM3]) 1);
bind_thm ("B_trusts_YM4_newK", result() RS mp RSN (2, rev_mp));


(**** Towards proving secrecy of Nonce NB ****)

(** Lemmas about the predicate KeyWithNonce **)

goalw thy [KeyWithNonce_def]
 "!!evs. Says Server A                                              \
\            {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB|}, X|} \
\          : set evs ==> KeyWithNonce K NB evs";
by (Blast_tac 1);
qed "KeyWithNonceI";

goalw thy [KeyWithNonce_def]
   "KeyWithNonce K NB (Says S A X # evs) =                                    \
\    (Server = S &                                                            \
\     (EX B n X'. X = {|Crypt (shrK A) {|Agent B, Key K, n, Nonce NB|}, X'|}) \
\    | KeyWithNonce K NB evs)";
by (Simp_tac 1);
by (Blast_tac 1);
qed "KeyWithNonce_Says";
Addsimps [KeyWithNonce_Says];

(*A fresh key cannot be associated with any nonce 
  (with respect to a given trace). *)
goalw thy [KeyWithNonce_def]
 "!!evs. Key K ~: used evs ==> ~ KeyWithNonce K NB evs";
by (blast_tac (!claset addSEs spies_partsEs) 1);
qed "fresh_not_KeyWithNonce";

(*The Server message associates K with NB' and therefore not with any 
  other nonce NB.*)
goalw thy [KeyWithNonce_def]
 "!!evs. [| Says Server A                                                \
\                {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB'|}, X|} \
\             : set evs;                                                 \
\           NB ~= NB';  evs : yahalom |]                            \
\        ==> ~ KeyWithNonce K NB evs";
by (blast_tac (!claset addDs [unique_session_keys]) 1);
qed "Says_Server_KeyWithNonce";


(*The only nonces that can be found with the help of session keys are
  those distributed as nonce NB by the Server.  The form of the theorem
  recalls analz_image_freshK, but it is much more complicated.*)


(*As with analz_image_freshK, we take some pains to express the property
  as a logical equivalence so that the simplifier can apply it.*)
goal thy  
 "!!evs. P --> (X : analz (G Un H)) --> (X : analz H)  ==> \
\        P --> (X : analz (G Un H)) = (X : analz H)";
by (blast_tac (!claset addIs [impOfSubs analz_mono]) 1);
val lemma = result();

goal thy 
 "!!evs. evs : yahalom ==>                                         \
\        (ALL KK. KK <= Compl (range shrK) -->                          \
\             (ALL K: KK. ~ KeyWithNonce K NB evs)   -->                \
\             (Nonce NB : analz (Key``KK Un (spies evs))) =     \
\             (Nonce NB : analz (spies evs)))";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (REPEAT_FIRST (resolve_tac [impI RS allI]));
by (REPEAT_FIRST (rtac lemma));
(*For Oops, simplification proves NBa~=NB.  By Says_Server_KeyWithNonce,
  we get (~ KeyWithNonce K NB evsa); then simplification can apply the
  induction hypothesis with KK = {K}.*)
by (ALLGOALS  (*17 seconds*)
    (asm_simp_tac 
     (analz_image_freshK_ss addsimps
        [all_conj_distrib, analz_image_freshK,
	 KeyWithNonce_Says, fresh_not_KeyWithNonce, 
	 imp_disj_not1,			      (*Moves NBa~=NB to the front*)
	 Says_Server_KeyWithNonce])));
(*Base*)
by (Blast_tac 1);
(*Fake*) 
by (spy_analz_tac 1);
(*YM4*)  (** LEVEL 7 **)
by (not_bad_tac "A" 1);
by (dtac (Says_imp_spies RS parts.Inj RS parts.Fst RS A_trusts_YM3) 1
    THEN REPEAT (assume_tac 1));
by (blast_tac (!claset addIs [KeyWithNonceI]) 1);
qed_spec_mp "Nonce_secrecy";


(*Version required below: if NB can be decrypted using a session key then it
  was distributed with that key.  The more general form above is required
  for the induction to carry through.*)
goal thy 
 "!!evs. [| Says Server A                                                 \
\            {|Crypt (shrK A) {|Agent B, Key KAB, na, Nonce NB'|}, X|}    \
\           : set evs;                                                    \
\           NB ~= NB';  KAB ~: range shrK;  evs : yahalom |]         \
\        ==> (Nonce NB : analz (insert (Key KAB) (spies evs))) =  \
\            (Nonce NB : analz (spies evs))";
by (asm_simp_tac (analz_image_freshK_ss addsimps 
		  [Nonce_secrecy, Says_Server_KeyWithNonce]) 1);
qed "single_Nonce_secrecy";


(*** The Nonce NB uniquely identifies B's message. ***)

goal thy 
 "!!evs. evs : yahalom ==>                                            \
\   EX NA' A' B'. ALL NA A B.                                              \
\      Crypt (shrK B) {|Agent A, Nonce NA, nb|} : parts(spies evs) \
\      --> B ~: bad --> NA = NA' & A = A' & B = B'";
by (parts_induct_tac 1);
(*Fake*)
by (REPEAT (etac (exI RSN (2,exE)) 1)   (*stripping EXs makes proof faster*)
    THEN Fake_parts_insert_tac 1);
by (asm_simp_tac (!simpset addsimps [all_conj_distrib]) 1); 
(*YM2: creation of new Nonce.  Move assertion into global context*)
by (expand_case_tac "nb = ?y" 1);
by (REPEAT (resolve_tac [exI, conjI, impI, refl] 1));
by (blast_tac (!claset addSEs spies_partsEs) 1);
val lemma = result();

goal thy 
 "!!evs.[| Crypt (shrK B) {|Agent A, Nonce NA, nb|} : parts (spies evs); \
\          Crypt (shrK B') {|Agent A', Nonce NA', nb|} : parts (spies evs); \
\          evs : yahalom;  B ~: bad;  B' ~: bad |]  \
\        ==> NA' = NA & A' = A & B' = B";
by (prove_unique_tac lemma 1);
qed "unique_NB";


(*Variant useful for proving secrecy of NB: the Says... form allows 
  not_bad_tac to remove the assumption B' ~: bad.*)
goal thy 
 "!!evs.[| Says C D   {|X,  Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}    \
\            : set evs;          B ~: bad;                               \
\          Says C' D' {|X', Crypt (shrK B') {|Agent A', Nonce NA', nb|}|} \
\            : set evs;                                                   \
\          nb ~: analz (spies evs);  evs : yahalom |]        \
\        ==> NA' = NA & A' = A & B' = B";
by (not_bad_tac "B'" 1);
by (blast_tac (!claset addSDs [Says_imp_spies RS parts.Inj]
                       addSEs [MPair_parts]
                       addDs  [unique_NB]) 1);
qed "Says_unique_NB";


(** A nonce value is never used both as NA and as NB **)

goal thy 
 "!!evs. [| B ~: bad;  evs : yahalom  |]            \
\ ==> Nonce NB ~: analz (spies evs) -->           \
\     Crypt (shrK B') {|Agent A', Nonce NB, nb'|} : parts(spies evs) --> \
\     Crypt (shrK B)  {|Agent A, Nonce NA, Nonce NB|} ~: parts(spies evs)";
by (parts_induct_tac 1);
by (Fake_parts_insert_tac 1);
by (blast_tac (!claset addDs [Says_imp_spies RS analz.Inj]
                       addSIs [parts_insertI]
                       addSEs partsEs) 1);
bind_thm ("no_nonce_YM1_YM2", result() RS mp RSN (2,rev_mp) RSN (2,rev_notE));

(*The Server sends YM3 only in response to YM2.*)
goal thy 
 "!!evs. [| Says Server A                                                \
\            {|Crypt (shrK A) {|Agent B, k, na, nb|}, X|} : set evs;     \
\           evs : yahalom |]                                             \
\        ==> EX B'. Says B' Server                                       \
\                      {| Agent B, Crypt (shrK B) {|Agent A, na, nb|} |} \
\                   : set evs";
by (etac rev_mp 1);
by (etac yahalom.induct 1);
by (ALLGOALS Asm_simp_tac);
by (ALLGOALS Blast_tac);
qed "Says_Server_imp_YM2";


(*A vital theorem for B, that nonce NB remains secure from the Spy.*)
goal thy 
 "!!evs. [| A ~: bad;  B ~: bad;  evs : yahalom |]  \
\ ==> Says B Server                                                    \
\          {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|} \
\     : set evs -->                                                    \
\     (ALL k. Says A Spy {|Nonce NA, Nonce NB, k|} ~: set evs) -->     \
\     Nonce NB ~: analz (spies evs)";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (ALLGOALS
    (asm_simp_tac 
     (!simpset addsimps [analz_insert_eq, analz_insert_freshK]
               addsplits [expand_if])));
(*Prove YM3 by showing that no NB can also be an NA*)
by (blast_tac (!claset addDs [Says_imp_spies RS parts.Inj]
	               addSEs [MPair_parts]
		       addDs  [no_nonce_YM1_YM2, Says_unique_NB]) 4
    THEN flexflex_tac);
(*YM2: similar freshness reasoning*) 
by (blast_tac (!claset addSEs partsEs
		       addDs  [Says_imp_spies RS analz.Inj,
			       impOfSubs analz_subset_parts]) 3);
(*YM1: NB=NA is impossible anyway, but NA is secret because it is fresh!*)
by (blast_tac (!claset addSIs [parts_insertI]
                       addSEs spies_partsEs) 2);
(*Fake*)
by (spy_analz_tac 1);
(** LEVEL 7: YM4 and Oops remain **)
by (ALLGOALS Clarify_tac);
(*YM4: key K is visible to Spy, contradicting session key secrecy theorem*) 
by (not_bad_tac "Aa" 1);
by (dtac (Says_imp_spies RS parts.Inj RS parts.Fst RS A_trusts_YM3) 1);
by (forward_tac [Says_Server_message_form] 3);
by (forward_tac [Says_Server_imp_YM2] 4);
by (REPEAT_FIRST (eresolve_tac [asm_rl, bexE, exE, disjE]));
(*  use Says_unique_NB to identify message components: Aa=A, Ba=B, NAa=NA *)
by (blast_tac (!claset addDs [Says_unique_NB, Spy_not_see_encrypted_key,
			      impOfSubs Fake_analz_insert]) 1);
(** LEVEL 14 **)
(*Oops case: if the nonce is betrayed now, show that the Oops event is 
  covered by the quantified Oops assumption.*)
by (full_simp_tac (!simpset addsimps [all_conj_distrib]) 1);
by (forward_tac [Says_Server_imp_YM2] 1 THEN assume_tac 1 THEN etac exE 1);
by (expand_case_tac "NB = NBa" 1);
(*If NB=NBa then all other components of the Oops message agree*)
by (blast_tac (!claset addDs [Says_unique_NB]) 1 THEN flexflex_tac);
(*case NB ~= NBa*)
by (asm_simp_tac (!simpset addsimps [single_Nonce_secrecy]) 1);
by (blast_tac (!claset addSEs [MPair_parts]
		       addDs  [Says_imp_spies RS parts.Inj, 
			       no_nonce_YM1_YM2 (*to prove NB~=NAa*) ]) 1);
bind_thm ("Spy_not_see_NB", result() RSN(2,rev_mp) RSN(2,rev_mp));


(*B's session key guarantee from YM4.  The two certificates contribute to a
  single conclusion about the Server's message.  Note that the "Says A Spy"
  assumption must quantify over ALL POSSIBLE keys instead of our particular K.
  If this run is broken and the spy substitutes a certificate containing an
  old key, B has no means of telling.*)
goal thy 
 "!!evs. [| Says B Server                                                   \
\             {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}   \
\             : set evs;                                                    \
\           Says A' B {|Crypt (shrK B) {|Agent A, Key K|},                  \
\                       Crypt K (Nonce NB)|} : set evs;                     \
\           ALL k. Says A Spy {|Nonce NA, Nonce NB, k|} ~: set evs;         \
\           A ~: bad;  B ~: bad;  evs : yahalom |]       \
\         ==> Says Server A                                                 \
\                     {|Crypt (shrK A) {|Agent B, Key K,                    \
\                               Nonce NA, Nonce NB|},                       \
\                       Crypt (shrK B) {|Agent A, Key K|}|}                 \
\               : set evs";
by (forward_tac [Spy_not_see_NB] 1 THEN REPEAT (assume_tac 1));
by (etac (Says_imp_spies RS parts.Inj RS MPair_parts) 1 THEN
    dtac B_trusts_YM4_shrK 1);
by (dtac B_trusts_YM4_newK 3);
by (REPEAT_FIRST (eresolve_tac [asm_rl, exE]));
by (forward_tac [Says_Server_imp_YM2] 1 THEN assume_tac 1);
by (dtac unique_session_keys 1 THEN REPEAT (assume_tac 1));
by (blast_tac (!claset addDs [Says_unique_NB]) 1);
qed "B_trusts_YM4";



(*** Authenticating B to A ***)

(*The encryption in message YM2 tells us it cannot be faked.*)
goal thy 
 "!!evs. evs : yahalom                                            \
\  ==> Crypt (shrK B) {|Agent A, Nonce NA, nb|} : parts (spies evs) --> \
\      B ~: bad -->                                              \
\      Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}  \
\         : set evs";
by (parts_induct_tac 1);
by (Fake_parts_insert_tac 1);
bind_thm ("B_Said_YM2", result() RSN (2, rev_mp) RS mp);

(*If the server sends YM3 then B sent YM2*)
goal thy 
 "!!evs. evs : yahalom                                                      \
\  ==> Says Server A {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, nb|}, X|} \
\         : set evs -->                                                     \
\      B ~: bad -->                                                        \
\      Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}  \
\                 : set evs";
by (etac yahalom.induct 1);
by (ALLGOALS Asm_simp_tac);
(*YM4*)
by (Blast_tac 2);
(*YM3*)
by (best_tac (!claset addSDs [B_Said_YM2, Says_imp_spies RS parts.Inj]
		      addSEs [MPair_parts]) 1);
val lemma = result() RSN (2, rev_mp) RS mp |> standard;

(*If A receives YM3 then B has used nonce NA (and therefore is alive)*)
goal thy
 "!!evs. [| Says S A {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, nb|}, X|} \
\             : set evs;                                                    \
\           A ~: bad;  B ~: bad;  evs : yahalom |]                        \
\   ==> Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|} \
\         : set evs";
by (blast_tac (!claset addSDs [A_trusts_YM3, lemma]
		       addEs spies_partsEs) 1);
qed "YM3_auth_B_to_A";


(*** Authenticating A to B using the certificate Crypt K (Nonce NB) ***)

(*Assuming the session key is secure, if both certificates are present then
  A has said NB.  We can't be sure about the rest of A's message, but only
  NB matters for freshness.*)  
goal thy 
 "!!evs. evs : yahalom                                             \
\        ==> Key K ~: analz (spies evs) -->                     \
\            Crypt K (Nonce NB) : parts (spies evs) -->         \
\            Crypt (shrK B) {|Agent A, Key K|} : parts (spies evs) --> \
\            B ~: bad -->                                         \
\            (EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs)";
by (parts_induct_tac 1);
(*Fake*)
by (Fake_parts_insert_tac 1);
(*YM3: by new_keys_not_used we note that Crypt K (Nonce NB) could not exist*)
by (fast_tac (!claset addSDs [Crypt_imp_invKey_keysFor] addss (!simpset)) 1); 
(*YM4: was Crypt K (Nonce NB) the very last message?  If not, use ind. hyp.*)
by (asm_simp_tac (!simpset addsimps [ex_disj_distrib]) 1);
(*yes: apply unicity of session keys*)
by (not_bad_tac "Aa" 1);
by (blast_tac (!claset addSEs [MPair_parts]
                       addSDs [A_trusts_YM3, B_trusts_YM4_shrK]
		       addDs  [Says_imp_spies RS parts.Inj,
			       unique_session_keys]) 1);
val lemma = normalize_thm [RSspec, RSmp] (result()) |> standard;

(*If B receives YM4 then A has used nonce NB (and therefore is alive).
  Moreover, A associates K with NB (thus is talking about the same run).
  Other premises guarantee secrecy of K.*)
goal thy 
 "!!evs. [| Says B Server                                                   \
\             {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}   \
\             : set evs;                                                    \
\           Says A' B {|Crypt (shrK B) {|Agent A, Key K|},                  \
\                       Crypt K (Nonce NB)|} : set evs;                     \
\           (ALL NA k. Says A Spy {|Nonce NA, Nonce NB, k|} ~: set evs);    \
\           A ~: bad;  B ~: bad;  evs : yahalom |]       \
\        ==> EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs";
by (dtac B_trusts_YM4 1);
by (REPEAT_FIRST (eresolve_tac [asm_rl, spec]));
by (etac (Says_imp_spies RS parts.Inj RS MPair_parts) 1);
by (rtac lemma 1);
by (rtac Spy_not_see_encrypted_key 2);
by (REPEAT_FIRST assume_tac);
by (blast_tac (!claset addSEs [MPair_parts]
	       	       addDs [Says_imp_spies RS parts.Inj]) 1);
qed_spec_mp "YM4_imp_A_Said_YM3";