Theory NSP_Bad (Isabelle2005: October 2005)

# Theory NSP_Bad

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theory NSP_Bad
imports Public UNITY_Main
begin

    (*  Title:      HOL/Auth/NSP_Bad
        ID:         $Id: NSP_Bad.thy,v 1.6 2005/06/17 14:13:10 haftmann Exp $
        Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
        Copyright   1996  University of Cambridge

    ML{*add_path "$ISABELLE_HOME/src/HOL/Auth"*}

    Original file is ../Auth/NS_Public_Bad
    *)

    header{*Analyzing the Needham-Schroeder Public-Key Protocol in UNITY*}

    theory NSP_Bad imports Public UNITY_Main begin

    text{*This is the flawed version, vulnerable to Lowe's attack.
    From page 260 of
      Burrows, Abadi and Needham.  A Logic of Authentication.
      Proc. Royal Soc. 426 (1989).
    *}

    types state = "event list"

    constdefs

      (*The spy MAY say anything he CAN say.  We do not expect him to
        invent new nonces here, but he can also use NS1.  Common to
        all similar protocols.*)
      Fake :: "(state*state) set"
        "Fake == {(s,s').
                  ∃B X. s' = Says Spy B X # s
                        & X ∈ synth (analz (spies s))}"

      (*The numeric suffixes on A identify the rule*)

      (*Alice initiates a protocol run, sending a nonce to Bob*)
      NS1 :: "(state*state) set"
        "NS1 == {(s1,s').
                 ∃A1 B NA.
                     s' = Says A1 B (Crypt (pubK B) {|Nonce NA, Agent A1|}) # s1
                   & Nonce NA ∉ used s1}"

      (*Bob responds to Alice's message with a further nonce*)
      NS2 :: "(state*state) set"
        "NS2 == {(s2,s').
                 ∃A' A2 B NA NB.
                     s' = Says B A2 (Crypt (pubK A2) {|Nonce NA, Nonce NB|}) # s2
                   & Says A' B (Crypt (pubK B) {|Nonce NA, Agent A2|}) ∈ set s2
                   & Nonce NB ∉ used s2}"

      (*Alice proves her existence by sending NB back to Bob.*)
      NS3 :: "(state*state) set"
        "NS3 == {(s3,s').
                 ∃A3 B' B NA NB.
                     s' = Says A3 B (Crypt (pubK B) (Nonce NB)) # s3
                   & Says A3  B (Crypt (pubK B) {|Nonce NA, Agent A3|}) ∈ set s3
                   & Says B' A3 (Crypt (pubK A3) {|Nonce NA, Nonce NB|}) ∈ set s3}"

    constdefs
      Nprg :: "state program"
        (*Initial trace is empty*)
        "Nprg == mk_total_program({[]}, {Fake, NS1, NS2, NS3}, UNIV)"

    declare spies_partsEs [elim]
    declare analz_into_parts [dest]
    declare Fake_parts_insert_in_Un  [dest]

    text{*For other theories, e.g. Mutex and Lift, using [iff] slows proofs down.
      Here, it facilitates re-use of the Auth proofs.*}

    declare Fake_def [THEN def_act_simp, iff]
    declare NS1_def [THEN def_act_simp, iff]
    declare NS2_def [THEN def_act_simp, iff]
    declare NS3_def [THEN def_act_simp, iff]

    declare Nprg_def [THEN def_prg_Init, simp]

    text{*A "possibility property": there are traces that reach the end.
      Replace by LEADSTO proof!*}
    lemma "A ≠ B ==>
           ∃NB. ∃s ∈ reachable Nprg. Says A B (Crypt (pubK B) (Nonce NB)) ∈ set s"
    apply (intro exI bexI)
    apply (rule_tac [2] act = "totalize_act NS3" in reachable.Acts)
    apply (rule_tac [3] act = "totalize_act NS2" in reachable.Acts)
    apply (rule_tac [4] act = "totalize_act NS1" in reachable.Acts)
    apply (rule_tac [5] reachable.Init)
    apply (simp_all (no_asm_simp) add: Nprg_def totalize_act_def)
    apply auto
    done

    subsection{*Inductive Proofs about @{term ns_public}*}

    lemma ns_constrainsI:
         "(!!act s s'. [| act ∈ {Id, Fake, NS1, NS2, NS3};
                          (s,s') ∈ act;  s ∈ A |] ==> s' ∈ A')
          ==> Nprg ∈ A co A'"
    apply (simp add: Nprg_def mk_total_program_def)
    apply (rule constrainsI, auto)
    done

    text{*This ML code does the inductions directly.*}
    ML{*
    val ns_constrainsI = thm "ns_constrainsI";

    fun ns_constrains_tac(cs,ss) i =
       SELECT_GOAL
          (EVERY [REPEAT (etac Always_ConstrainsI 1),
                  REPEAT (resolve_tac [StableI, stableI,
                                       constrains_imp_Constrains] 1),
                  rtac ns_constrainsI 1,
                  full_simp_tac ss 1,
                  REPEAT (FIRSTGOAL (etac disjE)),
                  ALLGOALS (clarify_tac (cs delrules [impI,impCE])),
                  REPEAT (FIRSTGOAL analz_mono_contra_tac),
                  ALLGOALS (asm_simp_tac ss)]) i;

    (*Tactic for proving secrecy theorems*)
    fun ns_induct_tac(cs,ss) =
      (SELECT_GOAL o EVERY)
         [rtac AlwaysI 1,
          force_tac (cs,ss) 1,
          (*"reachable" gets in here*)
          rtac (Always_reachable RS Always_ConstrainsI RS StableI) 1,
          ns_constrains_tac(cs,ss) 1];
    *}

    method_setup ns_induct = {*
        Method.ctxt_args (fn ctxt =>
            Method.METHOD (fn facts =>
                ns_induct_tac (local_clasimpset_of ctxt) 1)) *}
        "for inductive reasoning about the Needham-Schroeder protocol"

    text{*Converts invariants into statements about reachable states*}
    lemmas Always_Collect_reachableD =
         Always_includes_reachable [THEN subsetD, THEN CollectD]

    text{*Spy never sees another agent's private key! (unless it's bad at start)*}
    lemma Spy_see_priK:
         "Nprg ∈ Always {s. (Key (priK A) ∈ parts (spies s)) = (A ∈ bad)}"
    apply ns_induct
    apply blast
    done
    declare Spy_see_priK [THEN Always_Collect_reachableD, simp]

    lemma Spy_analz_priK:
         "Nprg ∈ Always {s. (Key (priK A) ∈ analz (spies s)) = (A ∈ bad)}"
    by (rule Always_reachable [THEN Always_weaken], auto)
    declare Spy_analz_priK [THEN Always_Collect_reachableD, simp]

    subsection{*Authenticity properties obtained from NS2*}

    text{*It is impossible to re-use a nonce in both NS1 and NS2 provided the
         nonce is secret.  (Honest users generate fresh nonces.)*}
    lemma no_nonce_NS1_NS2:
     "Nprg
      ∈ Always {s. Crypt (pubK C) {|NA', Nonce NA|} ∈ parts (spies s) -->
                    Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (spies s) -->
                    Nonce NA ∈ analz (spies s)}"
    apply ns_induct
    apply (blast intro: analz_insertI)+
    done

    text{*Adding it to the claset slows down proofs...*}
    lemmas no_nonce_NS1_NS2_reachable =
           no_nonce_NS1_NS2 [THEN Always_Collect_reachableD, rule_format]

    text{*Unicity for NS1: nonce NA identifies agents A and B*}
    lemma unique_NA_lemma:
         "Nprg
      ∈ Always {s. Nonce NA ∉ analz (spies s) -->
                    Crypt(pubK B) {|Nonce NA, Agent A|} ∈ parts(spies s) -->
                    Crypt(pubK B') {|Nonce NA, Agent A'|} ∈ parts(spies s) -->
                    A=A' & B=B'}"
    apply ns_induct
    apply auto
    txt{*Fake, NS1 are non-trivial*}
    done

    text{*Unicity for NS1: nonce NA identifies agents A and B*}
    lemma unique_NA:
         "[| Crypt(pubK B)  {|Nonce NA, Agent A|} ∈ parts(spies s);
             Crypt(pubK B') {|Nonce NA, Agent A'|} ∈ parts(spies s);
             Nonce NA ∉ analz (spies s);
             s ∈ reachable Nprg |]
          ==> A=A' & B=B'"
    by (blast dest: unique_NA_lemma [THEN Always_Collect_reachableD])

    text{*Secrecy: Spy does not see the nonce sent in msg NS1 if A and B are secure*}
    lemma Spy_not_see_NA:
         "[| A ∉ bad;  B ∉ bad |]
      ==> Nprg ∈ Always
                  {s. Says A B (Crypt(pubK B) {|Nonce NA, Agent A|}) ∈ set s
                      --> Nonce NA ∉ analz (spies s)}"
    apply ns_induct
    txt{*NS3*}
    prefer 4 apply (blast intro: no_nonce_NS1_NS2_reachable)
    txt{*NS2*}
    prefer 3 apply (blast dest: unique_NA)
    txt{*NS1*}
    prefer 2 apply blast
    txt{*Fake*}
    apply spy_analz
    done

    text{*Authentication for A: if she receives message 2 and has used NA
      to start a run, then B has sent message 2.*}
    lemma A_trusts_NS2:
     "[| A ∉ bad;  B ∉ bad |]
      ==> Nprg ∈ Always
                  {s. Says A B (Crypt(pubK B) {|Nonce NA, Agent A|}) ∈ set s &
                      Crypt(pubK A) {|Nonce NA, Nonce NB|} ∈ parts (knows Spy s)
             --> Says B A (Crypt(pubK A) {|Nonce NA, Nonce NB|}) ∈ set s}"
      (*insert an invariant for use in some of the subgoals*)
    apply (insert Spy_not_see_NA [of A B NA], simp, ns_induct)
    apply (auto dest: unique_NA)
    done

    text{*If the encrypted message appears then it originated with Alice in NS1*}
    lemma B_trusts_NS1:
         "Nprg ∈ Always
                  {s. Nonce NA ∉ analz (spies s) -->
                      Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (spies s)
             --> Says A B (Crypt (pubK B) {|Nonce NA, Agent A|}) ∈ set s}"
    apply ns_induct
    apply blast
    done

    subsection{*Authenticity properties obtained from NS2*}

    text{*Unicity for NS2: nonce NB identifies nonce NA and agent A.
       Proof closely follows that of @{text unique_NA}.*}
    lemma unique_NB_lemma:
     "Nprg
      ∈ Always {s. Nonce NB ∉ analz (spies s)  -->
                    Crypt (pubK A) {|Nonce NA, Nonce NB|} ∈ parts (spies s) -->
                    Crypt(pubK A'){|Nonce NA', Nonce NB|} ∈ parts(spies s) -->
                    A=A' & NA=NA'}"
    apply ns_induct
    apply auto
    txt{*Fake, NS2 are non-trivial*}
    done

    lemma unique_NB:
         "[| Crypt(pubK A) {|Nonce NA, Nonce NB|} ∈ parts(spies s);
             Crypt(pubK A'){|Nonce NA', Nonce NB|} ∈ parts(spies s);
             Nonce NB ∉ analz (spies s);
             s ∈ reachable Nprg |]
          ==> A=A' & NA=NA'"
    apply (blast dest: unique_NB_lemma [THEN Always_Collect_reachableD])
    done

    text{*NB remains secret PROVIDED Alice never responds with round 3*}
    lemma Spy_not_see_NB:
         "[| A ∉ bad;  B ∉ bad |]
      ==> Nprg ∈ Always
                  {s. Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s &
                      (ALL C. Says A C (Crypt (pubK C) (Nonce NB)) ∉ set s)
                      --> Nonce NB ∉ analz (spies s)}"
    apply ns_induct
    apply (simp_all (no_asm_simp) add: all_conj_distrib)
    txt{*NS3: because NB determines A*}
    prefer 4 apply (blast dest: unique_NB)
    txt{*NS2: by freshness and unicity of NB*}
    prefer 3 apply (blast intro: no_nonce_NS1_NS2_reachable)
    txt{*NS1: by freshness*}
    prefer 2 apply blast
    txt{*Fake*}
    apply spy_analz
    done

    text{*Authentication for B: if he receives message 3 and has used NB
      in message 2, then A has sent message 3--to somebody....*}
    lemma B_trusts_NS3:
         "[| A ∉ bad;  B ∉ bad |]
      ==> Nprg ∈ Always
                  {s. Crypt (pubK B) (Nonce NB) ∈ parts (spies s) &
                      Says B A  (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s
                      --> (∃C. Says A C (Crypt (pubK C) (Nonce NB)) ∈ set s)}"
      (*insert an invariant for use in some of the subgoals*)
    apply (insert Spy_not_see_NB [of A B NA NB], simp, ns_induct)
    apply (simp_all (no_asm_simp) add: ex_disj_distrib)
    apply auto
    txt{*NS3: because NB determines A. This use of @{text unique_NB} is robust.*}
    apply (blast intro: unique_NB [THEN conjunct1])
    done

    text{*Can we strengthen the secrecy theorem?  NO*}
    lemma "[| A ∉ bad;  B ∉ bad |]
      ==> Nprg ∈ Always
                  {s. Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s
                      --> Nonce NB ∉ analz (spies s)}"
    apply ns_induct
    apply auto
    txt{*Fake*}
    apply spy_analz
    txt{*NS2: by freshness and unicity of NB*}
     apply (blast intro: no_nonce_NS1_NS2_reachable)
    txt{*NS3: unicity of NB identifies A and NA, but not B*}
    apply (frule_tac A'=A in Says_imp_spies [THEN parts.Inj, THEN unique_NB])
    apply (erule Says_imp_spies [THEN parts.Inj], auto)
    apply (rename_tac s B' C)
    txt{*This is the attack!
    @{subgoals[display,indent=0,margin=65]}
    *}
    oops

    (*
    THIS IS THE ATTACK!
    [| A ∉ bad; B ∉ bad |]
    ==> Nprg
       ∈ Always
           {s. Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s -->
               Nonce NB ∉ analz (knows Spy s)}
     1. !!s B' C.
           [| A ∉ bad; B ∉ bad; s ∈ reachable Nprg
              Says A C (Crypt (pubK C) {|Nonce NA, Agent A|}) ∈ set s;
              Says B' A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s;
              C ∈ bad; Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s;
              Nonce NB ∉ analz (knows Spy s) |]
           ==> False
    *)

    end

lemma

      A ≠ B ==> ∃NB. ∃s∈reachable Nprg. Says A B (Crypt (pubK B) (Nonce NB)) ∈ set s

### Inductive Proofs about @{term ns_public}

lemma ns_constrainsI:

      (!!act s s'.
          [| act ∈ {Id, Fake, NS1, NS2, NS3}; (s, s') ∈ act; s ∈ A |] ==> s' ∈ A')
      ==> Nprg ∈ A co A'

lemmas Always_Collect_reachableD:

      [| F2 ∈ Always {x. P x}; a ∈ reachable F2 |] ==> P a

lemmas Always_Collect_reachableD:

      [| F2 ∈ Always {x. P x}; a ∈ reachable F2 |] ==> P a

lemma Spy_see_priK:

      Nprg ∈ Always {s. (Key (priEK A) ∈ parts (knows Spy s)) = (A ∈ bad)}

lemma Spy_analz_priK:

      Nprg ∈ Always {s. (Key (priEK A) ∈ analz (knows Spy s)) = (A ∈ bad)}

### Authenticity properties obtained from NS2

lemma no_nonce_NS1_NS2:

      Nprg
      ∈ Always
         {s. Crypt (pubK C) {|NA', Nonce NA|} ∈ parts (knows Spy s) -->
             Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy s) -->
             Nonce NA ∈ analz (knows Spy s)}

lemmas no_nonce_NS1_NS2_reachable:

      [| a ∈ reachable Nprg; Crypt (pubK C) {|NA', Nonce NA|} ∈ parts (knows Spy a);
         Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy a) |]
      ==> Nonce NA ∈ analz (knows Spy a)

lemmas no_nonce_NS1_NS2_reachable:

      [| a ∈ reachable Nprg; Crypt (pubK C) {|NA', Nonce NA|} ∈ parts (knows Spy a);
         Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy a) |]
      ==> Nonce NA ∈ analz (knows Spy a)

lemma unique_NA_lemma:

      Nprg
      ∈ Always
         {s. Nonce NA ∉ analz (knows Spy s) -->
             Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy s) -->
             Crypt (pubK B') {|Nonce NA, Agent A'|} ∈ parts (knows Spy s) -->
             A = A' ∧ B = B'}

lemma unique_NA:

      [| Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy s);
         Crypt (pubK B') {|Nonce NA, Agent A'|} ∈ parts (knows Spy s);
         Nonce NA ∉ analz (knows Spy s); s ∈ reachable Nprg |]
      ==> A = A' ∧ B = B'

lemma Spy_not_see_NA:

      [| A ∉ bad; B ∉ bad |]
      ==> Nprg
          ∈ Always
             {s. Says A B (Crypt (pubK B) {|Nonce NA, Agent A|}) ∈ set s -->
                 Nonce NA ∉ analz (knows Spy s)}

lemma A_trusts_NS2:

      [| A ∉ bad; B ∉ bad |]
      ==> Nprg
          ∈ Always
             {s. Says A B (Crypt (pubK B) {|Nonce NA, Agent A|}) ∈ set s ∧
                 Crypt (pubK A) {|Nonce NA, Nonce NB|} ∈ parts (knows Spy s) -->
                 Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s}

lemma B_trusts_NS1:

      Nprg
      ∈ Always
         {s. Nonce NA ∉ analz (knows Spy s) -->
             Crypt (pubK B) {|Nonce NA, Agent A|} ∈ parts (knows Spy s) -->
             Says A B (Crypt (pubK B) {|Nonce NA, Agent A|}) ∈ set s}

### Authenticity properties obtained from NS2

lemma unique_NB_lemma:

      Nprg
      ∈ Always
         {s. Nonce NB ∉ analz (knows Spy s) -->
             Crypt (pubK A) {|Nonce NA, Nonce NB|} ∈ parts (knows Spy s) -->
             Crypt (pubK A') {|Nonce NA', Nonce NB|} ∈ parts (knows Spy s) -->
             A = A' ∧ NA = NA'}

lemma unique_NB:

      [| Crypt (pubK A) {|Nonce NA, Nonce NB|} ∈ parts (knows Spy s);
         Crypt (pubK A') {|Nonce NA', Nonce NB|} ∈ parts (knows Spy s);
         Nonce NB ∉ analz (knows Spy s); s ∈ reachable Nprg |]
      ==> A = A' ∧ NA = NA'

lemma Spy_not_see_NB:

      [| A ∉ bad; B ∉ bad |]
      ==> Nprg
          ∈ Always
             {s. Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s ∧
                 (∀C. Says A C (Crypt (pubK C) (Nonce NB)) ∉ set s) -->
                 Nonce NB ∉ analz (knows Spy s)}

lemma B_trusts_NS3:

      [| A ∉ bad; B ∉ bad |]
      ==> Nprg
          ∈ Always
             {s. Crypt (pubK B) (Nonce NB) ∈ parts (knows Spy s) ∧
                 Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|}) ∈ set s -->
                 (∃C. Says A C (Crypt (pubK C) (Nonce NB)) ∈ set s)}