src/HOL/Auth/OtwayRees_Bad.thy
author wenzelm
Mon Aug 31 21:28:08 2015 +0200 (2015-08-31)
changeset 61070 b72a990adfe2
parent 58889 5b7a9633cfa8
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permissions -rw-r--r--
prefer symbols;
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(*  Title:      HOL/Auth/OtwayRees_Bad.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 Otway-Rees Protocol: The Faulty BAN Version*}
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theory OtwayRees_Bad imports Public begin
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text{*The FAULTY version omitting encryption of Nonce NB, as suggested on 
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page 247 of
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  Burrows, Abadi and Needham (1988).  A Logic of Authentication.
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  Proc. Royal Soc. 426
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This file illustrates the consequences of such errors.  We can still prove
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impressive-looking properties such as @{text Spy_not_see_encrypted_key}, yet
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the protocol is open to a middleperson attack.  Attempting to prove some key
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lemmas indicates the possibility of this attack.*}
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inductive_set otway :: "event list set"
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  where
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   Nil: --{*The empty trace*}
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        "[] \<in> otway"
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 | Fake: --{*The Spy may say anything he can say.  The sender field is correct,
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            but agents don't use that information.*}
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         "[| evsf \<in> otway;  X \<in> synth (analz (knows Spy evsf)) |]
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          ==> Says Spy B X  # evsf \<in> otway"
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 | Reception: --{*A message that has been sent can be received by the
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                  intended recipient.*}
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              "[| evsr \<in> otway;  Says A B X \<in>set evsr |]
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               ==> Gets B X # evsr \<in> otway"
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 | OR1:  --{*Alice initiates a protocol run*}
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         "[| evs1 \<in> otway;  Nonce NA \<notin> used evs1 |]
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          ==> Says A B {|Nonce NA, Agent A, Agent B,
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                         Crypt (shrK A) {|Nonce NA, Agent A, Agent B|} |}
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                 # evs1 \<in> otway"
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 | OR2:  --{*Bob's response to Alice's message.
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             This variant of the protocol does NOT encrypt NB.*}
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         "[| evs2 \<in> otway;  Nonce NB \<notin> used evs2;
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             Gets B {|Nonce NA, Agent A, Agent B, X|} \<in> set evs2 |]
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          ==> Says B Server
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                  {|Nonce NA, Agent A, Agent B, X, Nonce NB,
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                    Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
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                 # evs2 \<in> otway"
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 | OR3:  --{*The Server receives Bob's message and checks that the three NAs
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           match.  Then he sends a new session key to Bob with a packet for
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           forwarding to Alice.*}
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         "[| evs3 \<in> otway;  Key KAB \<notin> used evs3;
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             Gets Server
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                  {|Nonce NA, Agent A, Agent B,
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                    Crypt (shrK A) {|Nonce NA, Agent A, Agent B|},
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                    Nonce NB,
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                    Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
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               \<in> set evs3 |]
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          ==> Says Server B
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                  {|Nonce NA,
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                    Crypt (shrK A) {|Nonce NA, Key KAB|},
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                    Crypt (shrK B) {|Nonce NB, Key KAB|}|}
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                 # evs3 \<in> otway"
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 | OR4:  --{*Bob receives the Server's (?) message and compares the Nonces with
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             those in the message he previously sent the Server.
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             Need @{term "B \<noteq> Server"} because we allow messages to self.*}
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         "[| evs4 \<in> otway;  B \<noteq> Server;
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             Says B Server {|Nonce NA, Agent A, Agent B, X', Nonce NB,
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                             Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
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               \<in> set evs4;
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             Gets B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
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               \<in> set evs4 |]
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          ==> Says B A {|Nonce NA, X|} # evs4 \<in> otway"
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 | Oops: --{*This message models possible leaks of session keys.  The nonces
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             identify the protocol run.*}
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         "[| evso \<in> otway;
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             Says Server B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
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               \<in> set evso |]
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          ==> Notes Spy {|Nonce NA, Nonce NB, Key K|} # evso \<in> otway"
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declare Says_imp_knows_Spy [THEN analz.Inj, dest]
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declare parts.Body  [dest]
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declare analz_into_parts [dest]
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declare Fake_parts_insert_in_Un  [dest]
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text{*A "possibility property": there are traces that reach the end*}
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lemma "[| B \<noteq> Server; Key K \<notin> used [] |]
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      ==> \<exists>NA. \<exists>evs \<in> otway.
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            Says B A {|Nonce NA, Crypt (shrK A) {|Nonce NA, Key K|}|}
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              \<in> set evs"
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apply (intro exI bexI)
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apply (rule_tac [2] otway.Nil
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                    [THEN otway.OR1, THEN otway.Reception,
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                     THEN otway.OR2, THEN otway.Reception,
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                     THEN otway.OR3, THEN otway.Reception, THEN otway.OR4])
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apply (possibility, simp add: used_Cons) 
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done
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lemma Gets_imp_Says [dest!]:
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     "[| Gets B X \<in> set evs; evs \<in> otway |] ==> \<exists>A. Says A B X \<in> set evs"
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apply (erule rev_mp)
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apply (erule otway.induct, auto)
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done
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subsection{*For reasoning about the encrypted portion of messages *}
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lemma OR2_analz_knows_Spy:
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     "[| Gets B {|N, Agent A, Agent B, X|} \<in> set evs;  evs \<in> otway |]
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      ==> X \<in> analz (knows Spy evs)"
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by blast
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lemma OR4_analz_knows_Spy:
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     "[| Gets B {|N, X, Crypt (shrK B) X'|} \<in> set evs;  evs \<in> otway |]
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      ==> X \<in> analz (knows Spy evs)"
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by blast
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lemma Oops_parts_knows_Spy:
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     "Says Server B {|NA, X, Crypt K' {|NB,K|}|} \<in> set evs
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      ==> K \<in> parts (knows Spy evs)"
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by blast
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text{*Forwarding lemma: see comments in OtwayRees.thy*}
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lemmas OR2_parts_knows_Spy =
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    OR2_analz_knows_Spy [THEN analz_into_parts]
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text{*Theorems of the form @{term "X \<notin> parts (spies evs)"} imply that
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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> otway ==> (Key (shrK A) \<in> parts (knows Spy evs)) = (A \<in> bad)"
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by (erule otway.induct, force,
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    drule_tac [4] OR2_parts_knows_Spy, simp_all, blast+)
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lemma Spy_analz_shrK [simp]:
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     "evs \<in> otway ==> (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> otway|] ==> A \<in> bad"
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by (blast dest: Spy_see_shrK)
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subsection{*Proofs involving analz *}
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text{*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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lemma Says_Server_message_form:
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     "[| Says Server B {|NA, X, Crypt (shrK B) {|NB, Key K|}|} \<in> set evs;
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         evs \<in> otway |]
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      ==> K \<notin> range shrK & (\<exists>i. NA = Nonce i) & (\<exists>j. NB = Nonce j)"
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apply (erule rev_mp)
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apply (erule otway.induct, simp_all)
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done
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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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text{*The equality makes the induction hypothesis easier to apply*}
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lemma analz_image_freshK [rule_format]:
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 "evs \<in> otway ==>
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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 otway.induct)
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apply (frule_tac [8] Says_Server_message_form)
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apply (drule_tac [7] OR4_analz_knows_Spy)
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apply (drule_tac [5] OR2_analz_knows_Spy, analz_freshK, spy_analz, auto) 
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done
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lemma analz_insert_freshK:
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  "[| evs \<in> otway;  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 B {|NA, X, Crypt (shrK B) {|NB, K|}|}   \<in> set evs;
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         Says Server B' {|NA',X',Crypt (shrK B') {|NB',K|}|} \<in> set evs;
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         evs \<in> otway |] ==> X=X' & B=B' & NA=NA' & NB=NB'"
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apply (erule rev_mp)
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apply (erule rev_mp)
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apply (erule otway.induct, simp_all)
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apply blast+  --{*OR3 and OR4*}
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done
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text{*Crucial secrecy property: Spy does not see the keys sent in msg OR3
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    Does not in itself guarantee security: an attack could violate
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    the premises, e.g. by having @{term "A=Spy"} *}
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lemma secrecy_lemma:
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 "[| A \<notin> bad;  B \<notin> bad;  evs \<in> otway |]
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  ==> Says Server B
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        {|NA, Crypt (shrK A) {|NA, Key K|},
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          Crypt (shrK B) {|NB, Key K|}|} \<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 otway.induct, force)
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apply (frule_tac [7] Says_Server_message_form)
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apply (drule_tac [6] OR4_analz_knows_Spy)
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apply (drule_tac [4] OR2_analz_knows_Spy)
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apply (simp_all add: analz_insert_eq analz_insert_freshK pushes)
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apply spy_analz  --{*Fake*}
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apply (blast dest: unique_session_keys)+  --{*OR3, OR4, Oops*}
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done
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lemma Spy_not_see_encrypted_key:
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     "[| Says Server B
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          {|NA, Crypt (shrK A) {|NA, Key K|},
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                Crypt (shrK B) {|NB, Key K|}|} \<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> otway |]
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      ==> Key K \<notin> analz (knows Spy evs)"
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by (blast dest: Says_Server_message_form secrecy_lemma)
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subsection{*Attempting to prove stronger properties *}
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text{*Only OR1 can have caused such a part of a message to appear. The premise
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  @{term "A \<noteq> B"} prevents OR2's similar-looking cryptogram from being picked 
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  up. Original Otway-Rees doesn't need it.*}
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lemma Crypt_imp_OR1 [rule_format]:
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     "[| A \<notin> bad;  A \<noteq> B;  evs \<in> otway |]
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      ==> Crypt (shrK A) {|NA, Agent A, Agent B|} \<in> parts (knows Spy evs) -->
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          Says A B {|NA, Agent A, Agent B,
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                     Crypt (shrK A) {|NA, Agent A, Agent B|}|}  \<in> set evs"
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by (erule otway.induct, force,
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    drule_tac [4] OR2_parts_knows_Spy, simp_all, blast+)
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text{*Crucial property: If the encrypted message appears, and A has used NA
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  to start a run, then it originated with the Server!
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  The premise @{term "A \<noteq> B"} allows use of @{text Crypt_imp_OR1}*}
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text{*Only it is FALSE.  Somebody could make a fake message to Server
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          substituting some other nonce NA' for NB.*}
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lemma "[| A \<notin> bad;  A \<noteq> B;  evs \<in> otway |]
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       ==> Crypt (shrK A) {|NA, Key K|} \<in> parts (knows Spy evs) -->
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           Says A B {|NA, Agent A, Agent B,
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                      Crypt (shrK A) {|NA, Agent A, Agent B|}|}
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            \<in> set evs -->
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           (\<exists>B NB. Says Server B
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                {|NA,
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                  Crypt (shrK A) {|NA, Key K|},
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                  Crypt (shrK B) {|NB, Key K|}|}  \<in> set evs)"
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apply (erule otway.induct, force,
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       drule_tac [4] OR2_parts_knows_Spy, simp_all)
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apply blast  --{*Fake*}
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apply blast  --{*OR1: it cannot be a new Nonce, contradiction.*}
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txt{*OR3 and OR4*}
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apply (simp_all add: ex_disj_distrib)
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 prefer 2 apply (blast intro!: Crypt_imp_OR1)  --{*OR4*}
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txt{*OR3*}
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apply clarify
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(*The hypotheses at this point suggest an attack in which nonce NB is used
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  in two different roles:
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          Gets Server
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           {|Nonce NA, Agent Aa, Agent A,
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             Crypt (shrK Aa) {|Nonce NA, Agent Aa, Agent A|}, Nonce NB,
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             Crypt (shrK A) {|Nonce NA, Agent Aa, Agent A|}|}
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          \<in> set evs3
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          Says A B
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           {|Nonce NB, Agent A, Agent B,
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             Crypt (shrK A) {|Nonce NB, Agent A, Agent B|}|}
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          \<in> set evs3;
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*)
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(*Thus the key property A_can_trust probably fails too.*)
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oops
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end