src/HOL/Auth/Yahalom.thy
author wenzelm
Mon Aug 31 21:28:08 2015 +0200 (2015-08-31)
changeset 61070 b72a990adfe2
parent 59807 22bc39064290
child 61830 4f5ab843cf5b
permissions -rw-r--r--
prefer symbols;
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(*  Title:      HOL/Auth/Yahalom.thy
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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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*)
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section{*The Yahalom Protocol*}
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theory Yahalom imports Public begin
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text{*From page 257 of
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  Burrows, Abadi and Needham (1989).  A Logic of Authentication.
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  Proc. Royal Soc. 426
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This theory has the prototypical example of a secrecy relation, KeyCryptNonce.
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*}
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inductive_set yahalom :: "event list set"
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  where
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         (*Initial trace is empty*)
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   Nil:  "[] \<in> yahalom"
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         (*The spy MAY say anything he CAN say.  We do not expect him to
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           invent new nonces here, but he can also use NS1.  Common to
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           all similar protocols.*)
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 | Fake: "[| evsf \<in> yahalom;  X \<in> synth (analz (knows Spy evsf)) |]
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          ==> Says Spy B X  # evsf \<in> yahalom"
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         (*A message that has been sent can be received by the
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           intended recipient.*)
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 | Reception: "[| evsr \<in> yahalom;  Says A B X \<in> set evsr |]
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               ==> Gets B X # evsr \<in> yahalom"
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         (*Alice initiates a protocol run*)
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 | YM1:  "[| evs1 \<in> yahalom;  Nonce NA \<notin> used evs1 |]
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          ==> Says A B {|Agent A, Nonce NA|} # evs1 \<in> yahalom"
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         (*Bob's response to Alice's message.*)
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 | YM2:  "[| evs2 \<in> yahalom;  Nonce NB \<notin> used evs2;
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             Gets B {|Agent A, Nonce NA|} \<in> set evs2 |]
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          ==> Says B Server 
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                  {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
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                # evs2 \<in> yahalom"
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         (*The Server receives Bob's message.  He responds by sending a
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            new session key to Alice, with a packet for forwarding to Bob.*)
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 | YM3:  "[| evs3 \<in> yahalom;  Key KAB \<notin> used evs3;  KAB \<in> symKeys;
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             Gets Server 
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                  {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
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               \<in> set evs3 |]
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          ==> Says Server A
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                   {|Crypt (shrK A) {|Agent B, Key KAB, Nonce NA, Nonce NB|},
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                     Crypt (shrK B) {|Agent A, Key KAB|}|}
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                # evs3 \<in> yahalom"
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 | YM4:  
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       --{*Alice receives the Server's (?) message, checks her Nonce, and
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           uses the new session key to send Bob his Nonce.  The premise
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           @{term "A \<noteq> Server"} is needed for @{text Says_Server_not_range}.
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           Alice can check that K is symmetric by its length.*}
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         "[| evs4 \<in> yahalom;  A \<noteq> Server;  K \<in> symKeys;
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             Gets A {|Crypt(shrK A) {|Agent B, Key K, Nonce NA, Nonce NB|}, X|}
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                \<in> set evs4;
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             Says A B {|Agent A, Nonce NA|} \<in> set evs4 |]
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          ==> Says A B {|X, Crypt K (Nonce NB)|} # evs4 \<in> yahalom"
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         (*This message models possible leaks of session keys.  The Nonces
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           identify the protocol run.  Quoting Server here ensures they are
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           correct.*)
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 | Oops: "[| evso \<in> yahalom;  
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             Says Server A {|Crypt (shrK A)
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                                   {|Agent B, Key K, Nonce NA, Nonce NB|},
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                             X|}  \<in> set evso |]
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          ==> Notes Spy {|Nonce NA, Nonce NB, Key K|} # evso \<in> yahalom"
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definition KeyWithNonce :: "[key, nat, event list] => bool" where
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  "KeyWithNonce K NB evs ==
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     \<exists>A B na X. 
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       Says Server A {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB|}, X|} 
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         \<in> set evs"
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declare Says_imp_analz_Spy [dest]
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declare parts.Body  [dest]
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declare Fake_parts_insert_in_Un  [dest]
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declare analz_into_parts [dest]
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text{*A "possibility property": there are traces that reach the end*}
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lemma "[| A \<noteq> Server; K \<in> symKeys; Key K \<notin> used [] |]
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      ==> \<exists>X NB. \<exists>evs \<in> yahalom.
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             Says A B {|X, Crypt K (Nonce NB)|} \<in> set evs"
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apply (intro exI bexI)
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apply (rule_tac [2] yahalom.Nil
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                    [THEN yahalom.YM1, THEN yahalom.Reception,
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                     THEN yahalom.YM2, THEN yahalom.Reception,
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                     THEN yahalom.YM3, THEN yahalom.Reception,
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                     THEN yahalom.YM4])
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apply (possibility, simp add: used_Cons)
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done
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subsection{*Regularity Lemmas for Yahalom*}
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lemma Gets_imp_Says:
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     "[| Gets B X \<in> set evs; evs \<in> yahalom |] ==> \<exists>A. Says A B X \<in> set evs"
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by (erule rev_mp, erule yahalom.induct, auto)
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text{*Must be proved separately for each protocol*}
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lemma Gets_imp_knows_Spy:
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     "[| Gets B X \<in> set evs; evs \<in> yahalom |]  ==> X \<in> knows Spy evs"
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by (blast dest!: Gets_imp_Says Says_imp_knows_Spy)
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lemmas Gets_imp_analz_Spy = Gets_imp_knows_Spy [THEN analz.Inj]
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declare Gets_imp_analz_Spy [dest]
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text{*Lets us treat YM4 using a similar argument as for the Fake case.*}
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lemma YM4_analz_knows_Spy:
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     "[| Gets A {|Crypt (shrK A) Y, X|} \<in> set evs;  evs \<in> yahalom |]
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      ==> X \<in> analz (knows Spy evs)"
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by blast
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lemmas YM4_parts_knows_Spy =
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       YM4_analz_knows_Spy [THEN analz_into_parts]
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text{*For Oops*}
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lemma YM4_Key_parts_knows_Spy:
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     "Says Server A {|Crypt (shrK A) {|B,K,NA,NB|}, X|} \<in> set evs
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      ==> K \<in> parts (knows Spy evs)"
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  by (metis parts.Body parts.Fst parts.Snd  Says_imp_knows_Spy parts.Inj)
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text{*Theorems of the form @{term "X \<notin> parts (knows Spy evs)"} imply 
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that NOBODY sends messages containing X! *}
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text{*Spy never sees a good agent's shared key!*}
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lemma Spy_see_shrK [simp]:
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     "evs \<in> yahalom ==> (Key (shrK A) \<in> parts (knows Spy evs)) = (A \<in> bad)"
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by (erule yahalom.induct, force,
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    drule_tac [6] YM4_parts_knows_Spy, simp_all, blast+)
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lemma Spy_analz_shrK [simp]:
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     "evs \<in> yahalom ==> (Key (shrK A) \<in> analz (knows Spy evs)) = (A \<in> bad)"
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by auto
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lemma Spy_see_shrK_D [dest!]:
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     "[|Key (shrK A) \<in> parts (knows Spy evs);  evs \<in> yahalom|] ==> A \<in> bad"
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by (blast dest: Spy_see_shrK)
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text{*Nobody can have used non-existent keys!
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    Needed to apply @{text analz_insert_Key}*}
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lemma new_keys_not_used [simp]:
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    "[|Key K \<notin> used evs; K \<in> symKeys; evs \<in> yahalom|]
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     ==> K \<notin> keysFor (parts (spies evs))"
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apply (erule rev_mp)
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apply (erule yahalom.induct, force,
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       frule_tac [6] YM4_parts_knows_Spy, simp_all)
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txt{*Fake*}
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apply (force dest!: keysFor_parts_insert, auto)
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done
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text{*Earlier, all protocol proofs declared this theorem.
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  But only a few proofs need it, e.g. Yahalom and Kerberos IV.*}
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lemma new_keys_not_analzd:
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 "[|K \<in> symKeys; evs \<in> yahalom; Key K \<notin> used evs|]
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  ==> K \<notin> keysFor (analz (knows Spy evs))"
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by (blast dest: new_keys_not_used intro: keysFor_mono [THEN subsetD])
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text{*Describes the form of K when the Server sends this message.  Useful for
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  Oops as well as main secrecy property.*}
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lemma Says_Server_not_range [simp]:
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     "[| Says Server A {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|}
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           \<in> set evs;   evs \<in> yahalom |]
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      ==> K \<notin> range shrK"
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by (erule rev_mp, erule yahalom.induct, simp_all)
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subsection{*Secrecy Theorems*}
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(****
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 The following is to prove theorems of the form
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  Key K \<in> analz (insert (Key KAB) (knows Spy evs)) ==>
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  Key K \<in> analz (knows Spy evs)
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 A more general formula must be proved inductively.
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****)
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text{* Session keys are not used to encrypt other session keys *}
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lemma analz_image_freshK [rule_format]:
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 "evs \<in> yahalom ==>
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   \<forall>K KK. KK <= - (range shrK) -->
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          (Key K \<in> analz (Key`KK Un (knows Spy evs))) =
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          (K \<in> KK | Key K \<in> analz (knows Spy evs))"
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apply (erule yahalom.induct,
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       drule_tac [7] YM4_analz_knows_Spy, analz_freshK, spy_analz, blast)
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apply (simp only: Says_Server_not_range analz_image_freshK_simps)
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apply safe
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done
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lemma analz_insert_freshK:
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     "[| evs \<in> yahalom;  KAB \<notin> range shrK |] ==>
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      (Key K \<in> analz (insert (Key KAB) (knows Spy evs))) =
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      (K = KAB | Key K \<in> analz (knows Spy evs))"
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by (simp only: analz_image_freshK analz_image_freshK_simps)
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text{*The Key K uniquely identifies the Server's  message.*}
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lemma unique_session_keys:
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     "[| Says Server A
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          {|Crypt (shrK A) {|Agent B, Key K, na, nb|}, X|} \<in> set evs;
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        Says Server A'
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          {|Crypt (shrK A') {|Agent B', Key K, na', nb'|}, X'|} \<in> set evs;
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        evs \<in> yahalom |]
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     ==> A=A' & B=B' & na=na' & nb=nb'"
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apply (erule rev_mp, erule rev_mp)
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apply (erule yahalom.induct, simp_all)
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txt{*YM3, by freshness, and YM4*}
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apply blast+
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done
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text{*Crucial secrecy property: Spy does not see the keys sent in msg YM3*}
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lemma secrecy_lemma:
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     "[| A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
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      ==> Says Server A
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            {|Crypt (shrK A) {|Agent B, Key K, na, nb|},
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              Crypt (shrK B) {|Agent A, Key K|}|}
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           \<in> set evs -->
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          Notes Spy {|na, nb, Key K|} \<notin> set evs -->
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          Key K \<notin> analz (knows Spy evs)"
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apply (erule yahalom.induct, force,
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       drule_tac [6] YM4_analz_knows_Spy)
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apply (simp_all add: pushes analz_insert_eq analz_insert_freshK, spy_analz)   --{*Fake*}
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apply (blast dest: unique_session_keys)+  --{*YM3, Oops*}
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done
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text{*Final version*}
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lemma Spy_not_see_encrypted_key:
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     "[| Says Server A
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            {|Crypt (shrK A) {|Agent B, Key K, na, nb|},
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              Crypt (shrK B) {|Agent A, Key K|}|}
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           \<in> set evs;
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         Notes Spy {|na, nb, Key K|} \<notin> set evs;
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         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
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      ==> Key K \<notin> analz (knows Spy evs)"
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by (blast dest: secrecy_lemma)
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subsubsection{* Security Guarantee for A upon receiving YM3 *}
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text{*If the encrypted message appears then it originated with the Server*}
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lemma A_trusts_YM3:
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     "[| Crypt (shrK A) {|Agent B, Key K, na, nb|} \<in> parts (knows Spy evs);
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         A \<notin> bad;  evs \<in> yahalom |]
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       ==> Says Server A
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            {|Crypt (shrK A) {|Agent B, Key K, na, nb|},
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              Crypt (shrK B) {|Agent A, Key K|}|}
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           \<in> set evs"
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apply (erule rev_mp)
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apply (erule yahalom.induct, force,
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       frule_tac [6] YM4_parts_knows_Spy, simp_all)
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txt{*Fake, YM3*}
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apply blast+
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done
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text{*The obvious combination of @{text A_trusts_YM3} with
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  @{text Spy_not_see_encrypted_key}*}
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lemma A_gets_good_key:
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     "[| Crypt (shrK A) {|Agent B, Key K, na, nb|} \<in> parts (knows Spy evs);
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         Notes Spy {|na, nb, Key K|} \<notin> set evs;
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         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
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      ==> Key K \<notin> analz (knows Spy evs)"
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  by (metis A_trusts_YM3 secrecy_lemma)
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subsubsection{* Security Guarantees for B upon receiving YM4 *}
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text{*B knows, by the first part of A's message, that the Server distributed
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  the key for A and B.  But this part says nothing about nonces.*}
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lemma B_trusts_YM4_shrK:
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     "[| Crypt (shrK B) {|Agent A, Key K|} \<in> parts (knows Spy evs);
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         B \<notin> bad;  evs \<in> yahalom |]
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      ==> \<exists>NA NB. Says Server A
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                      {|Crypt (shrK A) {|Agent B, Key K,
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                                         Nonce NA, Nonce NB|},
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                        Crypt (shrK B) {|Agent A, Key K|}|}
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                     \<in> set evs"
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apply (erule rev_mp)
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apply (erule yahalom.induct, force,
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       frule_tac [6] YM4_parts_knows_Spy, simp_all)
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txt{*Fake, YM3*}
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apply blast+
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done
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text{*B knows, by the second part of A's message, that the Server
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  distributed the key quoting nonce NB.  This part says nothing about
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  agent names.  Secrecy of NB is crucial.  Note that @{term "Nonce NB
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  \<notin> analz(knows Spy evs)"} must be the FIRST antecedent of the
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  induction formula.*}
wenzelm@17411
   303
paulson@14207
   304
lemma B_trusts_YM4_newK [rule_format]:
paulson@11251
   305
     "[|Crypt K (Nonce NB) \<in> parts (knows Spy evs);
paulson@11251
   306
        Nonce NB \<notin> analz (knows Spy evs);  evs \<in> yahalom|]
paulson@14207
   307
      ==> \<exists>A B NA. Says Server A
paulson@11251
   308
                      {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, Nonce NB|},
paulson@14207
   309
                        Crypt (shrK B) {|Agent A, Key K|}|}
paulson@11251
   310
                     \<in> set evs"
paulson@11251
   311
apply (erule rev_mp, erule rev_mp)
paulson@14207
   312
apply (erule yahalom.induct, force,
paulson@11251
   313
       frule_tac [6] YM4_parts_knows_Spy)
paulson@11251
   314
apply (analz_mono_contra, simp_all)
paulson@14207
   315
txt{*Fake, YM3*}
paulson@11251
   316
apply blast
paulson@11251
   317
apply blast
paulson@14207
   318
txt{*YM4.  A is uncompromised because NB is secure
paulson@14207
   319
  A's certificate guarantees the existence of the Server message*}
paulson@14207
   320
apply (blast dest!: Gets_imp_Says Crypt_Spy_analz_bad
paulson@14207
   321
             dest: Says_imp_spies
paulson@11251
   322
                   parts.Inj [THEN parts.Fst, THEN A_trusts_YM3])
paulson@11251
   323
done
paulson@11251
   324
paulson@11251
   325
paulson@14207
   326
subsubsection{* Towards proving secrecy of Nonce NB *}
paulson@11251
   327
paulson@14207
   328
text{*Lemmas about the predicate KeyWithNonce*}
paulson@11251
   329
paulson@14207
   330
lemma KeyWithNonceI:
paulson@14207
   331
 "Says Server A
paulson@14207
   332
          {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB|}, X|}
paulson@11251
   333
        \<in> set evs ==> KeyWithNonce K NB evs"
paulson@11251
   334
by (unfold KeyWithNonce_def, blast)
paulson@11251
   335
paulson@14207
   336
lemma KeyWithNonce_Says [simp]:
paulson@14207
   337
   "KeyWithNonce K NB (Says S A X # evs) =
paulson@11251
   338
      (Server = S &
paulson@14207
   339
       (\<exists>B n X'. X = {|Crypt (shrK A) {|Agent B, Key K, n, Nonce NB|}, X'|})
paulson@11251
   340
      | KeyWithNonce K NB evs)"
paulson@11251
   341
by (simp add: KeyWithNonce_def, blast)
paulson@11251
   342
paulson@11251
   343
paulson@14207
   344
lemma KeyWithNonce_Notes [simp]:
paulson@11251
   345
   "KeyWithNonce K NB (Notes A X # evs) = KeyWithNonce K NB evs"
paulson@11251
   346
by (simp add: KeyWithNonce_def)
paulson@11251
   347
paulson@14207
   348
lemma KeyWithNonce_Gets [simp]:
paulson@11251
   349
   "KeyWithNonce K NB (Gets A X # evs) = KeyWithNonce K NB evs"
paulson@11251
   350
by (simp add: KeyWithNonce_def)
paulson@11251
   351
paulson@14207
   352
text{*A fresh key cannot be associated with any nonce
paulson@14207
   353
  (with respect to a given trace). *}
paulson@14207
   354
lemma fresh_not_KeyWithNonce:
paulson@14207
   355
     "Key K \<notin> used evs ==> ~ KeyWithNonce K NB evs"
paulson@11251
   356
by (unfold KeyWithNonce_def, blast)
paulson@11251
   357
paulson@14207
   358
text{*The Server message associates K with NB' and therefore not with any
paulson@14207
   359
  other nonce NB.*}
paulson@14207
   360
lemma Says_Server_KeyWithNonce:
paulson@14207
   361
 "[| Says Server A {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB'|}, X|}
paulson@14207
   362
       \<in> set evs;
paulson@14207
   363
     NB \<noteq> NB';  evs \<in> yahalom |]
paulson@11251
   364
  ==> ~ KeyWithNonce K NB evs"
paulson@11251
   365
by (unfold KeyWithNonce_def, blast dest: unique_session_keys)
paulson@11251
   366
paulson@11251
   367
paulson@14207
   368
text{*The only nonces that can be found with the help of session keys are
paulson@11251
   369
  those distributed as nonce NB by the Server.  The form of the theorem
paulson@14207
   370
  recalls @{text analz_image_freshK}, but it is much more complicated.*}
paulson@11251
   371
paulson@11251
   372
paulson@14207
   373
text{*As with @{text analz_image_freshK}, we take some pains to express the 
paulson@14207
   374
  property as a logical equivalence so that the simplifier can apply it.*}
paulson@11251
   375
lemma Nonce_secrecy_lemma:
paulson@14207
   376
     "P --> (X \<in> analz (G Un H)) --> (X \<in> analz H)  ==>
paulson@11251
   377
      P --> (X \<in> analz (G Un H)) = (X \<in> analz H)"
paulson@11251
   378
by (blast intro: analz_mono [THEN subsetD])
paulson@11251
   379
paulson@11251
   380
lemma Nonce_secrecy:
paulson@14207
   381
     "evs \<in> yahalom ==>
paulson@14207
   382
      (\<forall>KK. KK <= - (range shrK) -->
paulson@14207
   383
           (\<forall>K \<in> KK. K \<in> symKeys --> ~ KeyWithNonce K NB evs)   -->
paulson@14207
   384
           (Nonce NB \<in> analz (Key`KK Un (knows Spy evs))) =
paulson@11251
   385
           (Nonce NB \<in> analz (knows Spy evs)))"
paulson@14207
   386
apply (erule yahalom.induct,
paulson@14207
   387
       frule_tac [7] YM4_analz_knows_Spy)
paulson@11251
   388
apply (safe del: allI impI intro!: Nonce_secrecy_lemma [THEN impI, THEN allI])
paulson@14207
   389
apply (simp_all del: image_insert image_Un
paulson@11251
   390
       add: analz_image_freshK_simps split_ifs
paulson@14207
   391
            all_conj_distrib ball_conj_distrib
paulson@11251
   392
            analz_image_freshK fresh_not_KeyWithNonce
paulson@11251
   393
            imp_disj_not1               (*Moves NBa\<noteq>NB to the front*)
paulson@11251
   394
            Says_Server_KeyWithNonce)
wenzelm@17411
   395
txt{*For Oops, simplification proves @{prop "NBa\<noteq>NB"}.  By
wenzelm@17411
   396
  @{term Says_Server_KeyWithNonce}, we get @{prop "~ KeyWithNonce K NB
wenzelm@17411
   397
  evs"}; then simplification can apply the induction hypothesis with
wenzelm@17411
   398
  @{term "KK = {K}"}.*}
paulson@14207
   399
txt{*Fake*}
paulson@11251
   400
apply spy_analz
paulson@14207
   401
txt{*YM2*}
paulson@14207
   402
apply blast
paulson@14207
   403
txt{*YM3*}
paulson@14207
   404
apply blast
paulson@14207
   405
txt{*YM4*}
wenzelm@59807
   406
apply (erule_tac V = "\<forall>KK. P KK" for P in thin_rl, clarify)
wenzelm@17411
   407
txt{*If @{prop "A \<in> bad"} then @{term NBa} is known, therefore
wenzelm@17411
   408
  @{prop "NBa \<noteq> NB"}.  Previous two steps make the next step
wenzelm@17411
   409
  faster.*}
paulson@32367
   410
apply (metis A_trusts_YM3 Gets_imp_analz_Spy Gets_imp_knows_Spy KeyWithNonce_def
paulson@32367
   411
      Spy_analz_shrK analz.Fst analz.Snd analz_shrK_Decrypt parts.Fst parts.Inj)
paulson@11251
   412
done
paulson@11251
   413
paulson@11251
   414
paulson@14207
   415
text{*Version required below: if NB can be decrypted using a session key then
paulson@14207
   416
   it was distributed with that key.  The more general form above is required
paulson@14207
   417
   for the induction to carry through.*}
paulson@11251
   418
lemma single_Nonce_secrecy:
paulson@14207
   419
     "[| Says Server A
paulson@14207
   420
          {|Crypt (shrK A) {|Agent B, Key KAB, na, Nonce NB'|}, X|}
paulson@14207
   421
         \<in> set evs;
paulson@14207
   422
         NB \<noteq> NB';  KAB \<notin> range shrK;  evs \<in> yahalom |]
paulson@14207
   423
      ==> (Nonce NB \<in> analz (insert (Key KAB) (knows Spy evs))) =
paulson@11251
   424
          (Nonce NB \<in> analz (knows Spy evs))"
paulson@11251
   425
by (simp_all del: image_insert image_Un imp_disjL
paulson@11251
   426
             add: analz_image_freshK_simps split_ifs
paulson@13507
   427
                  Nonce_secrecy Says_Server_KeyWithNonce)
paulson@11251
   428
paulson@11251
   429
paulson@14207
   430
subsubsection{* The Nonce NB uniquely identifies B's message. *}
paulson@11251
   431
paulson@11251
   432
lemma unique_NB:
paulson@14207
   433
     "[| Crypt (shrK B) {|Agent A, Nonce NA, nb|} \<in> parts (knows Spy evs);
paulson@14207
   434
         Crypt (shrK B') {|Agent A', Nonce NA', nb|} \<in> parts (knows Spy evs);
paulson@14207
   435
        evs \<in> yahalom;  B \<notin> bad;  B' \<notin> bad |]
paulson@11251
   436
      ==> NA' = NA & A' = A & B' = B"
paulson@11251
   437
apply (erule rev_mp, erule rev_mp)
paulson@14207
   438
apply (erule yahalom.induct, force,
paulson@11251
   439
       frule_tac [6] YM4_parts_knows_Spy, simp_all)
paulson@14207
   440
txt{*Fake, and YM2 by freshness*}
paulson@11251
   441
apply blast+
paulson@11251
   442
done
paulson@11251
   443
paulson@11251
   444
paulson@14207
   445
text{*Variant useful for proving secrecy of NB.  Because nb is assumed to be
paulson@14207
   446
  secret, we no longer must assume B, B' not bad.*}
paulson@11251
   447
lemma Says_unique_NB:
paulson@14207
   448
     "[| Says C S   {|X,  Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}
paulson@14207
   449
           \<in> set evs;
paulson@14207
   450
         Gets S' {|X', Crypt (shrK B') {|Agent A', Nonce NA', nb|}|}
paulson@14207
   451
           \<in> set evs;
paulson@14207
   452
         nb \<notin> analz (knows Spy evs);  evs \<in> yahalom |]
paulson@11251
   453
      ==> NA' = NA & A' = A & B' = B"
paulson@14207
   454
by (blast dest!: Gets_imp_Says Crypt_Spy_analz_bad
paulson@11251
   455
          dest: Says_imp_spies unique_NB parts.Inj analz.Inj)
paulson@11251
   456
paulson@11251
   457
paulson@14207
   458
subsubsection{* A nonce value is never used both as NA and as NB *}
paulson@11251
   459
paulson@11251
   460
lemma no_nonce_YM1_YM2:
paulson@11251
   461
     "[|Crypt (shrK B') {|Agent A', Nonce NB, nb'|} \<in> parts(knows Spy evs);
paulson@11251
   462
        Nonce NB \<notin> analz (knows Spy evs);  evs \<in> yahalom|]
paulson@11251
   463
  ==> Crypt (shrK B)  {|Agent A, na, Nonce NB|} \<notin> parts(knows Spy evs)"
paulson@11251
   464
apply (erule rev_mp, erule rev_mp)
paulson@14207
   465
apply (erule yahalom.induct, force,
paulson@11251
   466
       frule_tac [6] YM4_parts_knows_Spy)
paulson@11251
   467
apply (analz_mono_contra, simp_all)
paulson@14207
   468
txt{*Fake, YM2*}
paulson@11251
   469
apply blast+
paulson@11251
   470
done
paulson@11251
   471
paulson@14207
   472
text{*The Server sends YM3 only in response to YM2.*}
paulson@11251
   473
lemma Says_Server_imp_YM2:
paulson@11251
   474
     "[| Says Server A {|Crypt (shrK A) {|Agent B, k, na, nb|}, X|} \<in> set evs;
paulson@14207
   475
         evs \<in> yahalom |]
paulson@14207
   476
      ==> Gets Server {| Agent B, Crypt (shrK B) {|Agent A, na, nb|} |}
paulson@11251
   477
             \<in> set evs"
paulson@14207
   478
by (erule rev_mp, erule yahalom.induct, auto)
paulson@11251
   479
paulson@14207
   480
text{*A vital theorem for B, that nonce NB remains secure from the Spy.*}
paulson@11251
   481
lemma Spy_not_see_NB :
paulson@14207
   482
     "[| Says B Server
wenzelm@32960
   483
                {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
wenzelm@32960
   484
           \<in> set evs;
wenzelm@32960
   485
         (\<forall>k. Notes Spy {|Nonce NA, Nonce NB, k|} \<notin> set evs);
paulson@14207
   486
         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
paulson@11251
   487
      ==> Nonce NB \<notin> analz (knows Spy evs)"
paulson@11251
   488
apply (erule rev_mp, erule rev_mp)
paulson@14207
   489
apply (erule yahalom.induct, force,
paulson@11251
   490
       frule_tac [6] YM4_analz_knows_Spy)
paulson@11251
   491
apply (simp_all add: split_ifs pushes new_keys_not_analzd analz_insert_eq
paulson@11251
   492
                     analz_insert_freshK)
paulson@14207
   493
txt{*Fake*}
paulson@11251
   494
apply spy_analz
paulson@14207
   495
txt{*YM1: NB=NA is impossible anyway, but NA is secret because it is fresh!*}
paulson@11251
   496
apply blast
paulson@14207
   497
txt{*YM2*}
paulson@11251
   498
apply blast
paulson@14207
   499
txt{*Prove YM3 by showing that no NB can also be an NA*}
paulson@11251
   500
apply (blast dest!: no_nonce_YM1_YM2 dest: Gets_imp_Says Says_unique_NB)
paulson@14207
   501
txt{*LEVEL 7: YM4 and Oops remain*}
paulson@11251
   502
apply (clarify, simp add: all_conj_distrib)
paulson@14207
   503
txt{*YM4: key K is visible to Spy, contradicting session key secrecy theorem*}
paulson@14207
   504
txt{*Case analysis on Aa:bad; PROOF FAILED problems
wenzelm@17411
   505
  use @{text Says_unique_NB} to identify message components: @{term "Aa=A"}, @{term "Ba=B"}*}
paulson@14207
   506
apply (blast dest!: Says_unique_NB analz_shrK_Decrypt
paulson@14207
   507
                    parts.Inj [THEN parts.Fst, THEN A_trusts_YM3]
paulson@11251
   508
             dest: Gets_imp_Says Says_imp_spies Says_Server_imp_YM2
paulson@11251
   509
                   Spy_not_see_encrypted_key)
paulson@14207
   510
txt{*Oops case: if the nonce is betrayed now, show that the Oops event is
paulson@14207
   511
  covered by the quantified Oops assumption.*}
paulson@11251
   512
apply (clarify, simp add: all_conj_distrib)
paulson@11251
   513
apply (frule Says_Server_imp_YM2, assumption)
paulson@32367
   514
apply (metis Gets_imp_Says Says_Server_not_range Says_unique_NB no_nonce_YM1_YM2 parts.Snd single_Nonce_secrecy spies_partsEs(1))
paulson@11251
   515
done
paulson@11251
   516
paulson@11251
   517
paulson@14207
   518
text{*B's session key guarantee from YM4.  The two certificates contribute to a
paulson@11251
   519
  single conclusion about the Server's message.  Note that the "Notes Spy"
wenzelm@17411
   520
  assumption must quantify over @{text \<forall>} POSSIBLE keys instead of our particular K.
paulson@11251
   521
  If this run is broken and the spy substitutes a certificate containing an
paulson@14207
   522
  old key, B has no means of telling.*}
paulson@11251
   523
lemma B_trusts_YM4:
paulson@14207
   524
     "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|},
paulson@14207
   525
                  Crypt K (Nonce NB)|} \<in> set evs;
paulson@14207
   526
         Says B Server
paulson@14207
   527
           {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
paulson@14207
   528
           \<in> set evs;
paulson@14207
   529
         \<forall>k. Notes Spy {|Nonce NA, Nonce NB, k|} \<notin> set evs;
paulson@14207
   530
         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
paulson@14207
   531
       ==> Says Server A
paulson@14207
   532
                   {|Crypt (shrK A) {|Agent B, Key K,
paulson@14207
   533
                             Nonce NA, Nonce NB|},
paulson@14207
   534
                     Crypt (shrK B) {|Agent A, Key K|}|}
paulson@11251
   535
             \<in> set evs"
paulson@14207
   536
by (blast dest: Spy_not_see_NB Says_unique_NB
paulson@11251
   537
                Says_Server_imp_YM2 B_trusts_YM4_newK)
paulson@11251
   538
paulson@11251
   539
paulson@11251
   540
paulson@14207
   541
text{*The obvious combination of @{text B_trusts_YM4} with 
paulson@14207
   542
  @{text Spy_not_see_encrypted_key}*}
paulson@11251
   543
lemma B_gets_good_key:
paulson@11251
   544
     "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|},
paulson@11251
   545
                  Crypt K (Nonce NB)|} \<in> set evs;
paulson@14207
   546
         Says B Server
paulson@14207
   547
           {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
paulson@14207
   548
           \<in> set evs;
paulson@14207
   549
         \<forall>k. Notes Spy {|Nonce NA, Nonce NB, k|} \<notin> set evs;
paulson@14207
   550
         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
paulson@11251
   551
      ==> Key K \<notin> analz (knows Spy evs)"
paulson@32367
   552
  by (metis B_trusts_YM4 Spy_not_see_encrypted_key)
paulson@11251
   553
paulson@11251
   554
paulson@14207
   555
subsection{*Authenticating B to A*}
paulson@11251
   556
paulson@14207
   557
text{*The encryption in message YM2 tells us it cannot be faked.*}
paulson@11251
   558
lemma B_Said_YM2 [rule_format]:
paulson@11251
   559
     "[|Crypt (shrK B) {|Agent A, Nonce NA, nb|} \<in> parts (knows Spy evs);
paulson@11251
   560
        evs \<in> yahalom|]
paulson@11251
   561
      ==> B \<notin> bad -->
paulson@11251
   562
          Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}
paulson@11251
   563
            \<in> set evs"
paulson@14207
   564
apply (erule rev_mp, erule yahalom.induct, force,
paulson@11251
   565
       frule_tac [6] YM4_parts_knows_Spy, simp_all)
paulson@14207
   566
txt{*Fake*}
paulson@11251
   567
apply blast
paulson@11251
   568
done
paulson@11251
   569
paulson@14207
   570
text{*If the server sends YM3 then B sent YM2*}
paulson@11251
   571
lemma YM3_auth_B_to_A_lemma:
paulson@14207
   572
     "[|Says Server A {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, nb|}, X|}
paulson@11251
   573
       \<in> set evs;  evs \<in> yahalom|]
paulson@14207
   574
      ==> B \<notin> bad -->
paulson@11251
   575
          Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}
paulson@11251
   576
            \<in> set evs"
paulson@11251
   577
apply (erule rev_mp, erule yahalom.induct, simp_all)
paulson@14207
   578
txt{*YM3, YM4*}
paulson@11251
   579
apply (blast dest!: B_Said_YM2)+
paulson@11251
   580
done
paulson@11251
   581
paulson@14207
   582
text{*If A receives YM3 then B has used nonce NA (and therefore is alive)*}
paulson@11251
   583
lemma YM3_auth_B_to_A:
paulson@14207
   584
     "[| Gets A {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, nb|}, X|}
paulson@14207
   585
           \<in> set evs;
paulson@14207
   586
         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
paulson@14207
   587
      ==> Says B Server {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, nb|}|}
paulson@11251
   588
       \<in> set evs"
paulson@32367
   589
  by (metis A_trusts_YM3 Gets_imp_analz_Spy YM3_auth_B_to_A_lemma analz.Fst
paulson@32367
   590
         not_parts_not_analz)
paulson@11251
   591
paulson@11251
   592
paulson@14207
   593
subsection{*Authenticating A to B using the certificate 
paulson@14207
   594
  @{term "Crypt K (Nonce NB)"}*}
paulson@11251
   595
paulson@14207
   596
text{*Assuming the session key is secure, if both certificates are present then
paulson@11251
   597
  A has said NB.  We can't be sure about the rest of A's message, but only
paulson@14207
   598
  NB matters for freshness.*}
paulson@11251
   599
lemma A_Said_YM3_lemma [rule_format]:
paulson@11251
   600
     "evs \<in> yahalom
paulson@11251
   601
      ==> Key K \<notin> analz (knows Spy evs) -->
paulson@11251
   602
          Crypt K (Nonce NB) \<in> parts (knows Spy evs) -->
paulson@11251
   603
          Crypt (shrK B) {|Agent A, Key K|} \<in> parts (knows Spy evs) -->
paulson@11251
   604
          B \<notin> bad -->
paulson@11251
   605
          (\<exists>X. Says A B {|X, Crypt K (Nonce NB)|} \<in> set evs)"
paulson@14207
   606
apply (erule yahalom.induct, force,
paulson@11251
   607
       frule_tac [6] YM4_parts_knows_Spy)
paulson@11251
   608
apply (analz_mono_contra, simp_all)
paulson@14207
   609
txt{*Fake*}
paulson@11251
   610
apply blast
paulson@14207
   611
txt{*YM3: by @{text new_keys_not_used}, the message
paulson@14207
   612
   @{term "Crypt K (Nonce NB)"} could not exist*}
paulson@11251
   613
apply (force dest!: Crypt_imp_keysFor)
paulson@14207
   614
txt{*YM4: was @{term "Crypt K (Nonce NB)"} the very last message?
paulson@14207
   615
    If not, use the induction hypothesis*}
paulson@11251
   616
apply (simp add: ex_disj_distrib)
paulson@14207
   617
txt{*yes: apply unicity of session keys*}
paulson@11251
   618
apply (blast dest!: Gets_imp_Says A_trusts_YM3 B_trusts_YM4_shrK
paulson@14207
   619
                    Crypt_Spy_analz_bad
paulson@11251
   620
             dest: Says_imp_knows_Spy [THEN parts.Inj] unique_session_keys)
paulson@11251
   621
done
paulson@11251
   622
paulson@14207
   623
text{*If B receives YM4 then A has used nonce NB (and therefore is alive).
paulson@11251
   624
  Moreover, A associates K with NB (thus is talking about the same run).
paulson@14207
   625
  Other premises guarantee secrecy of K.*}
paulson@11251
   626
lemma YM4_imp_A_Said_YM3 [rule_format]:
paulson@11251
   627
     "[| Gets B {|Crypt (shrK B) {|Agent A, Key K|},
paulson@11251
   628
                  Crypt K (Nonce NB)|} \<in> set evs;
paulson@11251
   629
         Says B Server
paulson@11251
   630
           {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
paulson@11251
   631
           \<in> set evs;
paulson@11251
   632
         (\<forall>NA k. Notes Spy {|Nonce NA, Nonce NB, k|} \<notin> set evs);
paulson@11251
   633
         A \<notin> bad;  B \<notin> bad;  evs \<in> yahalom |]
paulson@11251
   634
      ==> \<exists>X. Says A B {|X, Crypt K (Nonce NB)|} \<in> set evs"
paulson@32367
   635
by (metis A_Said_YM3_lemma B_gets_good_key Gets_imp_analz_Spy YM4_parts_knows_Spy analz.Fst not_parts_not_analz)
paulson@1985
   636
end