Theory OtwayRees_Bad

theory OtwayRees_Bad
imports Public
(*  Title:      HOL/Auth/OtwayRees_Bad.thy
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1996  University of Cambridge
*)


header{*The Otway-Rees Protocol: The Faulty BAN Version*}

theory OtwayRees_Bad imports Public begin

text{*The FAULTY version omitting encryption of Nonce NB, as suggested on 
page 247 of
  Burrows, Abadi and Needham (1988).  A Logic of Authentication.
  Proc. Royal Soc. 426

This file illustrates the consequences of such errors.  We can still prove
impressive-looking properties such as @{text Spy_not_see_encrypted_key}, yet
the protocol is open to a middleperson attack.  Attempting to prove some key
lemmas indicates the possibility of this attack.*}

inductive_set otway :: "event list set"
  where
   Nil: --{*The empty trace*}
        "[] ∈ otway"

 | Fake: --{*The Spy may say anything he can say.  The sender field is correct,
            but agents don't use that information.*}
         "[| evsf ∈ otway;  X ∈ synth (analz (knows Spy evsf)) |]
          ==> Says Spy B X  # evsf ∈ otway"

        
 | Reception: --{*A message that has been sent can be received by the
                  intended recipient.*}
              "[| evsr ∈ otway;  Says A B X ∈set evsr |]
               ==> Gets B X # evsr ∈ otway"

 | OR1:  --{*Alice initiates a protocol run*}
         "[| evs1 ∈ otway;  Nonce NA ∉ used evs1 |]
          ==> Says A B {|Nonce NA, Agent A, Agent B,
                         Crypt (shrK A) {|Nonce NA, Agent A, Agent B|} |}
                 # evs1 ∈ otway"

 | OR2:  --{*Bob's response to Alice's message.
             This variant of the protocol does NOT encrypt NB.*}
         "[| evs2 ∈ otway;  Nonce NB ∉ used evs2;
             Gets B {|Nonce NA, Agent A, Agent B, X|} ∈ set evs2 |]
          ==> Says B Server
                  {|Nonce NA, Agent A, Agent B, X, Nonce NB,
                    Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
                 # evs2 ∈ otway"

 | OR3:  --{*The Server receives Bob's message and checks that the three NAs
           match.  Then he sends a new session key to Bob with a packet for
           forwarding to Alice.*}
         "[| evs3 ∈ otway;  Key KAB ∉ used evs3;
             Gets Server
                  {|Nonce NA, Agent A, Agent B,
                    Crypt (shrK A) {|Nonce NA, Agent A, Agent B|},
                    Nonce NB,
                    Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
               ∈ set evs3 |]
          ==> Says Server B
                  {|Nonce NA,
                    Crypt (shrK A) {|Nonce NA, Key KAB|},
                    Crypt (shrK B) {|Nonce NB, Key KAB|}|}
                 # evs3 ∈ otway"

 | OR4:  --{*Bob receives the Server's (?) message and compares the Nonces with
             those in the message he previously sent the Server.
             Need @{term "B ≠ Server"} because we allow messages to self.*}
         "[| evs4 ∈ otway;  B ≠ Server;
             Says B Server {|Nonce NA, Agent A, Agent B, X', Nonce NB,
                             Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
               ∈ set evs4;
             Gets B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
               ∈ set evs4 |]
          ==> Says B A {|Nonce NA, X|} # evs4 ∈ otway"

 | Oops: --{*This message models possible leaks of session keys.  The nonces
             identify the protocol run.*}
         "[| evso ∈ otway;
             Says Server B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
               ∈ set evso |]
          ==> Notes Spy {|Nonce NA, Nonce NB, Key K|} # evso ∈ otway"


declare Says_imp_knows_Spy [THEN analz.Inj, dest]
declare parts.Body  [dest]
declare analz_into_parts [dest]
declare Fake_parts_insert_in_Un  [dest]

text{*A "possibility property": there are traces that reach the end*}
lemma "[| B ≠ Server; Key K ∉ used [] |]
      ==> ∃NA. ∃evs ∈ otway.
            Says B A {|Nonce NA, Crypt (shrK A) {|Nonce NA, Key K|}|}
              ∈ set evs"
apply (intro exI bexI)
apply (rule_tac [2] otway.Nil
                    [THEN otway.OR1, THEN otway.Reception,
                     THEN otway.OR2, THEN otway.Reception,
                     THEN otway.OR3, THEN otway.Reception, THEN otway.OR4])
apply (possibility, simp add: used_Cons) 
done

lemma Gets_imp_Says [dest!]:
     "[| Gets B X ∈ set evs; evs ∈ otway |] ==> ∃A. Says A B X ∈ set evs"
apply (erule rev_mp)
apply (erule otway.induct, auto)
done


subsection{*For reasoning about the encrypted portion of messages *}

lemma OR2_analz_knows_Spy:
     "[| Gets B {|N, Agent A, Agent B, X|} ∈ set evs;  evs ∈ otway |]
      ==> X ∈ analz (knows Spy evs)"
by blast

lemma OR4_analz_knows_Spy:
     "[| Gets B {|N, X, Crypt (shrK B) X'|} ∈ set evs;  evs ∈ otway |]
      ==> X ∈ analz (knows Spy evs)"
by blast

lemma Oops_parts_knows_Spy:
     "Says Server B {|NA, X, Crypt K' {|NB,K|}|} ∈ set evs
      ==> K ∈ parts (knows Spy evs)"
by blast

text{*Forwarding lemma: see comments in OtwayRees.thy*}
lemmas OR2_parts_knows_Spy =
    OR2_analz_knows_Spy [THEN analz_into_parts]


text{*Theorems of the form @{term "X ∉ parts (spies evs)"} imply that
NOBODY sends messages containing X! *}

text{*Spy never sees a good agent's shared key!*}
lemma Spy_see_shrK [simp]:
     "evs ∈ otway ==> (Key (shrK A) ∈ parts (knows Spy evs)) = (A ∈ bad)"
by (erule otway.induct, force,
    drule_tac [4] OR2_parts_knows_Spy, simp_all, blast+)


lemma Spy_analz_shrK [simp]:
     "evs ∈ otway ==> (Key (shrK A) ∈ analz (knows Spy evs)) = (A ∈ bad)"
by auto

lemma Spy_see_shrK_D [dest!]:
     "[|Key (shrK A) ∈ parts (knows Spy evs);  evs ∈ otway|] ==> A ∈ bad"
by (blast dest: Spy_see_shrK)


subsection{*Proofs involving analz *}

text{*Describes the form of K and NA when the Server sends this message.  Also
  for Oops case.*}
lemma Says_Server_message_form:
     "[| Says Server B {|NA, X, Crypt (shrK B) {|NB, Key K|}|} ∈ set evs;
         evs ∈ otway |]
      ==> K ∉ range shrK & (∃i. NA = Nonce i) & (∃j. NB = Nonce j)"
apply (erule rev_mp)
apply (erule otway.induct, simp_all)
done


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

  Key K ∈ analz (insert (Key KAB) (knows Spy evs)) ==>
  Key K ∈ analz (knows Spy evs)

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


text{*Session keys are not used to encrypt other session keys*}

text{*The equality makes the induction hypothesis easier to apply*}
lemma analz_image_freshK [rule_format]:
 "evs ∈ otway ==>
   ∀K KK. KK <= -(range shrK) -->
          (Key K ∈ analz (Key`KK Un (knows Spy evs))) =
          (K ∈ KK | Key K ∈ analz (knows Spy evs))"
apply (erule otway.induct)
apply (frule_tac [8] Says_Server_message_form)
apply (drule_tac [7] OR4_analz_knows_Spy)
apply (drule_tac [5] OR2_analz_knows_Spy, analz_freshK, spy_analz, auto) 
done

lemma analz_insert_freshK:
  "[| evs ∈ otway;  KAB ∉ range shrK |] ==>
      (Key K ∈ analz (insert (Key KAB) (knows Spy evs))) =
      (K = KAB | Key K ∈ analz (knows Spy evs))"
by (simp only: analz_image_freshK analz_image_freshK_simps)


text{*The Key K uniquely identifies the Server's  message. *}
lemma unique_session_keys:
     "[| Says Server B {|NA, X, Crypt (shrK B) {|NB, K|}|}   ∈ set evs;
         Says Server B' {|NA',X',Crypt (shrK B') {|NB',K|}|} ∈ set evs;
         evs ∈ otway |] ==> X=X' & B=B' & NA=NA' & NB=NB'"
apply (erule rev_mp)
apply (erule rev_mp)
apply (erule otway.induct, simp_all)
apply blast+  --{*OR3 and OR4*}
done


text{*Crucial secrecy property: Spy does not see the keys sent in msg OR3
    Does not in itself guarantee security: an attack could violate
    the premises, e.g. by having @{term "A=Spy"} *}
lemma secrecy_lemma:
 "[| A ∉ bad;  B ∉ bad;  evs ∈ otway |]
  ==> Says Server B
        {|NA, Crypt (shrK A) {|NA, Key K|},
          Crypt (shrK B) {|NB, Key K|}|} ∈ set evs -->
      Notes Spy {|NA, NB, Key K|} ∉ set evs -->
      Key K ∉ analz (knows Spy evs)"
apply (erule otway.induct, force)
apply (frule_tac [7] Says_Server_message_form)
apply (drule_tac [6] OR4_analz_knows_Spy)
apply (drule_tac [4] OR2_analz_knows_Spy)
apply (simp_all add: analz_insert_eq analz_insert_freshK pushes)
apply spy_analz  --{*Fake*}
apply (blast dest: unique_session_keys)+  --{*OR3, OR4, Oops*}
done


lemma Spy_not_see_encrypted_key:
     "[| Says Server B
          {|NA, Crypt (shrK A) {|NA, Key K|},
                Crypt (shrK B) {|NB, Key K|}|} ∈ set evs;
         Notes Spy {|NA, NB, Key K|} ∉ set evs;
         A ∉ bad;  B ∉ bad;  evs ∈ otway |]
      ==> Key K ∉ analz (knows Spy evs)"
by (blast dest: Says_Server_message_form secrecy_lemma)


subsection{*Attempting to prove stronger properties *}

text{*Only OR1 can have caused such a part of a message to appear. The premise
  @{term "A ≠ B"} prevents OR2's similar-looking cryptogram from being picked 
  up. Original Otway-Rees doesn't need it.*}
lemma Crypt_imp_OR1 [rule_format]:
     "[| A ∉ bad;  A ≠ B;  evs ∈ otway |]
      ==> Crypt (shrK A) {|NA, Agent A, Agent B|} ∈ parts (knows Spy evs) -->
          Says A B {|NA, Agent A, Agent B,
                     Crypt (shrK A) {|NA, Agent A, Agent B|}|}  ∈ set evs"
by (erule otway.induct, force,
    drule_tac [4] OR2_parts_knows_Spy, simp_all, blast+)


text{*Crucial property: If the encrypted message appears, and A has used NA
  to start a run, then it originated with the Server!
  The premise @{term "A ≠ B"} allows use of @{text Crypt_imp_OR1}*}
text{*Only it is FALSE.  Somebody could make a fake message to Server
          substituting some other nonce NA' for NB.*}
lemma "[| A ∉ bad;  A ≠ B;  evs ∈ otway |]
       ==> Crypt (shrK A) {|NA, Key K|} ∈ parts (knows Spy evs) -->
           Says A B {|NA, Agent A, Agent B,
                      Crypt (shrK A) {|NA, Agent A, Agent B|}|}
            ∈ set evs -->
           (∃B NB. Says Server B
                {|NA,
                  Crypt (shrK A) {|NA, Key K|},
                  Crypt (shrK B) {|NB, Key K|}|}  ∈ set evs)"
apply (erule otway.induct, force,
       drule_tac [4] OR2_parts_knows_Spy, simp_all)
apply blast  --{*Fake*}
apply blast  --{*OR1: it cannot be a new Nonce, contradiction.*}
txt{*OR3 and OR4*}
apply (simp_all add: ex_disj_distrib)
 prefer 2 apply (blast intro!: Crypt_imp_OR1)  --{*OR4*}
txt{*OR3*}
apply clarify
(*The hypotheses at this point suggest an attack in which nonce NB is used
  in two different roles:
          Gets Server
           {|Nonce NA, Agent Aa, Agent A,
             Crypt (shrK Aa) {|Nonce NA, Agent Aa, Agent A|}, Nonce NB,
             Crypt (shrK A) {|Nonce NA, Agent Aa, Agent A|}|}
          ∈ set evs3
          Says A B
           {|Nonce NB, Agent A, Agent B,
             Crypt (shrK A) {|Nonce NB, Agent A, Agent B|}|}
          ∈ set evs3;
*)


(*Thus the key property A_can_trust probably fails too.*)
oops

end