Quelle  OtwayRees_Bad.thy

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


section‹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 ‹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 🍋‹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 🍋‹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 ∪ (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 🍋‹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
  🍋‹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 🍋‹A ≠ B› allows use of ‹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