doc-src/TutorialI/Protocol/NS_Public.thy

adapted tsig interface;

(*  Title:      HOL/Auth/NS_Public
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1996  University of Cambridge

Inductive relation "ns_public" for the Needham-Schroeder Public-Key protocol.
Version incorporating Lowe's fix (inclusion of B's identity in round 2).
*)

theory NS_Public = Public:

consts  ns_public  :: "event list set"

inductive ns_public
  intros
         (*Initial trace is empty*)
   Nil:  "[] \<in> ns_public"

         (*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: "\<lbrakk>evs \<in> ns_public;  X \<in> synth (analz (knows Spy evs))\<rbrakk>
          \<Longrightarrow> Says Spy B X  # evs \<in> ns_public"

         (*Alice initiates a protocol run, sending a nonce to Bob*)
   NS1:  "\<lbrakk>evs1 \<in> ns_public;  Nonce NA \<notin> used evs1\<rbrakk>
          \<Longrightarrow> Says A B (Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>)
                 # evs1  \<in>  ns_public"

         (*Bob responds to Alice's message with a further nonce*)
   NS2:  "\<lbrakk>evs2 \<in> ns_public;  Nonce NB \<notin> used evs2;
           Says A' B (Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs2\<rbrakk>
          \<Longrightarrow> Says B A (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>)
                # evs2  \<in>  ns_public"

         (*Alice proves her existence by sending NB back to Bob.*)
   NS3:  "\<lbrakk>evs3 \<in> ns_public;
           Says A  B (Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs3;
           Says B' A (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>)
              \<in> set evs3\<rbrakk>
          \<Longrightarrow> Says A B (Crypt (pubK B) (Nonce NB)) # evs3 \<in> ns_public"

declare knows_Spy_partsEs [elim]
declare analz_subset_parts [THEN subsetD, dest]
declare Fake_parts_insert [THEN subsetD, dest]
declare image_eq_UN [simp]  (*accelerates proofs involving nested images*)

(*A "possibility property": there are traces that reach the end*)
lemma "\<exists>NB. \<exists>evs \<in> ns_public. Says A B (Crypt (pubK B) (Nonce NB)) \<in> set evs"
apply (intro exI bexI)
apply (rule_tac [2] ns_public.Nil [THEN ns_public.NS1, THEN ns_public.NS2,
                                   THEN ns_public.NS3])
by possibility


(**** Inductive proofs about ns_public ****)

(** Theorems of the form X \<notin> parts (knows Spy evs) imply that NOBODY
    sends messages containing X! **)

(*Spy never sees another agent's private key! (unless it's bad at start)*)
lemma Spy_see_priK [simp]:
      "evs \<in> ns_public \<Longrightarrow> (Key (priK A) \<in> parts (knows Spy evs)) = (A \<in> bad)"
by (erule ns_public.induct, auto)

lemma Spy_analz_priK [simp]:
      "evs \<in> ns_public \<Longrightarrow> (Key (priK A) \<in> analz (knows Spy evs)) = (A \<in> bad)"
by auto


(*** Authenticity properties obtained from NS2 ***)


(*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:
      "\<lbrakk>Crypt (pubK C) \<lbrace>NA', Nonce NA, Agent D\<rbrace> \<in> parts (knows Spy evs);
        Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace> \<in> parts (knows Spy evs);
        evs \<in> ns_public\<rbrakk>
       \<Longrightarrow> Nonce NA \<in> analz (knows Spy evs)"
apply (erule rev_mp, erule rev_mp)
apply (erule ns_public.induct, simp_all)
apply (blast intro: analz_insertI)+
done

(*Unicity for NS1: nonce NA identifies agents A and B*)
lemma unique_NA:
     "\<lbrakk>Crypt(pubK B)  \<lbrace>Nonce NA, Agent A \<rbrace> \<in> parts(knows Spy evs);
       Crypt(pubK B') \<lbrace>Nonce NA, Agent A'\<rbrace> \<in> parts(knows Spy evs);
       Nonce NA \<notin> analz (knows Spy evs); evs \<in> ns_public\<rbrakk>
      \<Longrightarrow> A=A' \<and> B=B'"
apply (erule rev_mp, erule rev_mp, erule rev_mp)
apply (erule ns_public.induct, simp_all)
(*Fake, NS1*)
apply (blast intro: analz_insertI)+
done


(*Secrecy: Spy does not see the nonce sent in msg NS1 if A and B are secure
  The major premise "Says A B ..." makes it a dest-rule, so we use
  (erule rev_mp) rather than rule_format. *)
theorem Spy_not_see_NA:
      "\<lbrakk>Says A B (Crypt(pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs;
        A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk>
       \<Longrightarrow> Nonce NA \<notin> analz (knows Spy evs)"
apply (erule rev_mp)
apply (erule ns_public.induct, simp_all)
apply spy_analz
apply (blast dest: unique_NA intro: no_nonce_NS1_NS2)+
done


(*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_lemma [rule_format]:
   "\<lbrakk>A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk>
    \<Longrightarrow> Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace> \<in> parts (knows Spy evs) \<longrightarrow>
	Says A B (Crypt(pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs \<longrightarrow>
	Says B A (Crypt(pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs"
apply (erule ns_public.induct, simp_all)
(*Fake, NS1*)
apply (blast dest: Spy_not_see_NA)+
done

theorem A_trusts_NS2:
     "\<lbrakk>Says A  B (Crypt(pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs;
       Says B' A (Crypt(pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs;
       A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk>
      \<Longrightarrow> Says B A (Crypt(pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs"
by (blast intro: A_trusts_NS2_lemma)


(*If the encrypted message appears then it originated with Alice in NS1*)
lemma B_trusts_NS1 [rule_format]:
     "evs \<in> ns_public
      \<Longrightarrow> Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace> \<in> parts (knows Spy evs) \<longrightarrow>
	  Nonce NA \<notin> analz (knows Spy evs) \<longrightarrow>
	  Says A B (Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs"
apply (erule ns_public.induct, simp_all)
(*Fake*)
apply (blast intro!: analz_insertI)
done



(*** Authenticity properties obtained from NS2 ***)

(*Unicity for NS2: nonce NB identifies nonce NA and agents A, B
  [unicity of B makes Lowe's fix work]
  [proof closely follows that for unique_NA] *)

lemma unique_NB [dest]:
     "\<lbrakk>Crypt(pubK A)  \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace> \<in> parts(knows Spy evs);
       Crypt(pubK A') \<lbrace>Nonce NA', Nonce NB, Agent B'\<rbrace> \<in> parts(knows Spy evs);
       Nonce NB \<notin> analz (knows Spy evs); evs \<in> ns_public\<rbrakk>
      \<Longrightarrow> A=A' \<and> NA=NA' \<and> B=B'"
apply (erule rev_mp, erule rev_mp, erule rev_mp)
apply (erule ns_public.induct, simp_all)
(*Fake, NS2*)
apply (blast intro: analz_insertI)+
done



text{*
@{thm[display] analz_Crypt_if[no_vars]}
\rulename{analz_Crypt_if}
*}

(*Secrecy: Spy does not see the nonce sent in msg NS2 if A and B are secure*)
theorem Spy_not_see_NB [dest]:
     "\<lbrakk>Says B A (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs;
       A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk>
      \<Longrightarrow> Nonce NB \<notin> analz (knows Spy evs)"
apply (erule rev_mp)
apply (erule ns_public.induct, simp_all)
apply spy_analz
apply (blast intro: no_nonce_NS1_NS2)+
done


(*Authentication for B: if he receives message 3 and has used NB
  in message 2, then A has sent message 3.*)
lemma B_trusts_NS3_lemma [rule_format]:
     "\<lbrakk>A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk> \<Longrightarrow>
      Crypt (pubK B) (Nonce NB) \<in> parts (knows Spy evs) \<longrightarrow>
      Says B A (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs \<longrightarrow>
      Says A B (Crypt (pubK B) (Nonce NB)) \<in> set evs"
by (erule ns_public.induct, auto)

theorem B_trusts_NS3:
     "\<lbrakk>Says B A  (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs;
       Says A' B (Crypt (pubK B) (Nonce NB)) \<in> set evs;
       A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk>
      \<Longrightarrow> Says A B (Crypt (pubK B) (Nonce NB)) \<in> set evs"
by (blast intro: B_trusts_NS3_lemma)

(*** Overall guarantee for B ***)


(*If NS3 has been sent and the nonce NB agrees with the nonce B joined with
  NA, then A initiated the run using NA.*)
theorem B_trusts_protocol:
     "\<lbrakk>A \<notin> bad;  B \<notin> bad;  evs \<in> ns_public\<rbrakk> \<Longrightarrow>
      Crypt (pubK B) (Nonce NB) \<in> parts (knows Spy evs) \<longrightarrow>
      Says B A  (Crypt (pubK A) \<lbrace>Nonce NA, Nonce NB, Agent B\<rbrace>) \<in> set evs \<longrightarrow>
      Says A B (Crypt (pubK B) \<lbrace>Nonce NA, Agent A\<rbrace>) \<in> set evs"
by (erule ns_public.induct, auto)

end