src/HOL/Auth/Guard/P2.thy

modernized header uniformly as section;

(*  Title:      HOL/Auth/Guard/P2.thy
    Author:     Frederic Blanqui, University of Cambridge Computer Laboratory
    Copyright   2002  University of Cambridge

From G. Karjoth, N. Asokan and C. Gulcu
"Protecting the computation results of free-roaming agents"
Mobiles Agents 1998, LNCS 1477.
*)

section{*Protocol P2*}

theory P2 imports Guard_Public List_Msg begin

subsection{*Protocol Definition*}


text{*Like P1 except the definitions of @{text chain}, @{text shop},
  @{text next_shop} and @{text nonce}*}

subsubsection{*offer chaining:
B chains his offer for A with the head offer of L for sending it to C*}

definition chain :: "agent => nat => agent => msg => agent => msg" where
"chain B ofr A L C ==
let m1= sign B (Nonce ofr) in
let m2= Hash {|head L, Agent C|} in
{|Crypt (pubK A) m1, m2|}"

declare Let_def [simp]

lemma chain_inj [iff]: "(chain B ofr A L C = chain B' ofr' A' L' C')
= (B=B' & ofr=ofr' & A=A' & head L = head L' & C=C')"
by (auto simp: chain_def Let_def)

lemma Nonce_in_chain [iff]: "Nonce ofr:parts {chain B ofr A L C}"
by (auto simp: chain_def sign_def)

subsubsection{*agent whose key is used to sign an offer*}

fun shop :: "msg => msg" where
"shop {|Crypt K {|B,ofr,Crypt K' H|},m2|} = Agent (agt K')"

lemma shop_chain [simp]: "shop (chain B ofr A L C) = Agent B"
by (simp add: chain_def sign_def)

subsubsection{*nonce used in an offer*}

fun nonce :: "msg => msg" where
"nonce {|Crypt K {|B,ofr,CryptH|},m2|} = ofr"

lemma nonce_chain [simp]: "nonce (chain B ofr A L C) = Nonce ofr"
by (simp add: chain_def sign_def)

subsubsection{*next shop*}

fun next_shop :: "msg => agent" where
"next_shop {|m1,Hash {|headL,Agent C|}|} = C"

lemma "next_shop (chain B ofr A L C) = C"
by (simp add: chain_def sign_def)

subsubsection{*anchor of the offer list*}

definition anchor :: "agent => nat => agent => msg" where
"anchor A n B == chain A n A (cons nil nil) B"

lemma anchor_inj [iff]:
     "(anchor A n B = anchor A' n' B') = (A=A' & n=n' & B=B')"
by (auto simp: anchor_def)

lemma Nonce_in_anchor [iff]: "Nonce n:parts {anchor A n B}"
by (auto simp: anchor_def)

lemma shop_anchor [simp]: "shop (anchor A n B) = Agent A"
by (simp add: anchor_def)

subsubsection{*request event*}

definition reqm :: "agent => nat => nat => msg => agent => msg" where
"reqm A r n I B == {|Agent A, Number r, cons (Agent A) (cons (Agent B) I),
cons (anchor A n B) nil|}"

lemma reqm_inj [iff]: "(reqm A r n I B = reqm A' r' n' I' B')
= (A=A' & r=r' & n=n' & I=I' & B=B')"
by (auto simp: reqm_def)

lemma Nonce_in_reqm [iff]: "Nonce n:parts {reqm A r n I B}"
by (auto simp: reqm_def)

definition req :: "agent => nat => nat => msg => agent => event" where
"req A r n I B == Says A B (reqm A r n I B)"

lemma req_inj [iff]: "(req A r n I B = req A' r' n' I' B')
= (A=A' & r=r' & n=n' & I=I' & B=B')"
by (auto simp: req_def)

subsubsection{*propose event*}

definition prom :: "agent => nat => agent => nat => msg => msg =>
msg => agent => msg" where
"prom B ofr A r I L J C == {|Agent A, Number r,
app (J, del (Agent B, I)), cons (chain B ofr A L C) L|}"

lemma prom_inj [dest]: "prom B ofr A r I L J C = prom B' ofr' A' r' I' L' J' C'
==> B=B' & ofr=ofr' & A=A' & r=r' & L=L' & C=C'"
by (auto simp: prom_def)

lemma Nonce_in_prom [iff]: "Nonce ofr:parts {prom B ofr A r I L J C}"
by (auto simp: prom_def)

definition pro :: "agent => nat => agent => nat => msg => msg =>
                  msg => agent => event" where
"pro B ofr A r I L J C == Says B C (prom B ofr A r I L J C)"

lemma pro_inj [dest]: "pro B ofr A r I L J C = pro B' ofr' A' r' I' L' J' C'
==> B=B' & ofr=ofr' & A=A' & r=r' & L=L' & C=C'"
by (auto simp: pro_def dest: prom_inj)

subsubsection{*protocol*}

inductive_set p2 :: "event list set"
where

  Nil: "[]:p2"

| Fake: "[| evsf:p2; X:synth (analz (spies evsf)) |] ==> Says Spy B X # evsf : p2"

| Request: "[| evsr:p2; Nonce n ~:used evsr; I:agl |] ==> req A r n I B # evsr : p2"

| Propose: "[| evsp:p2; Says A' B {|Agent A,Number r,I,cons M L|}:set evsp;
  I:agl; J:agl; isin (Agent C, app (J, del (Agent B, I)));
  Nonce ofr ~:used evsp |] ==> pro B ofr A r I (cons M L) J C # evsp : p2"

subsubsection{*valid offer lists*}

inductive_set
  valid :: "agent => nat => agent => msg set"
  for A :: agent and  n :: nat and B :: agent
where
  Request [intro]: "cons (anchor A n B) nil:valid A n B"

| Propose [intro]: "L:valid A n B
  ==> cons (chain (next_shop (head L)) ofr A L C) L:valid A n B"

subsubsection{*basic properties of valid*}

lemma valid_not_empty: "L:valid A n B ==> EX M L'. L = cons M L'"
by (erule valid.cases, auto)

lemma valid_pos_len: "L:valid A n B ==> 0 < len L"
by (erule valid.induct, auto)

subsubsection{*list of offers*}

fun offers :: "msg => msg"
where
  "offers (cons M L) = cons {|shop M, nonce M|} (offers L)"
| "offers other = nil"


subsection{*Properties of Protocol P2*}

text{*same as @{text P1_Prop} except that publicly verifiable forward
integrity is replaced by forward privacy*}

subsection{*strong forward integrity:
except the last one, no offer can be modified*}

lemma strong_forward_integrity: "ALL L. Suc i < len L
--> L:valid A n B --> repl (L,Suc i,M):valid A n B --> M = ith (L,Suc i)"
apply (induct i)
(* i = 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,xa,l'a|}:valid A n B" for x xa l'a)
apply (ind_cases "{|x,M,l'a|}:valid A n B" for x l'a)
apply (simp add: chain_def)
(* i > 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,repl(l',Suc na,M)|}:valid A n B" for x l' na)
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,l'|}:valid A n B" for x l')
by (drule_tac x=l' in spec, simp, blast)

subsection{*insertion resilience:
except at the beginning, no offer can be inserted*}

lemma chain_isnt_head [simp]: "L:valid A n B ==>
head L ~= chain (next_shop (head L)) ofr A L C"
by (erule valid.induct, auto simp: chain_def sign_def anchor_def)

lemma insertion_resilience: "ALL L. L:valid A n B --> Suc i < len L
--> ins (L,Suc i,M) ~:valid A n B"
apply (induct i)
(* i = 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,l'|}:valid A n B" for x l', simp)
apply (ind_cases "{|x,M,l'|}:valid A n B" for x l', clarsimp)
apply (ind_cases "{|head l',l'|}:valid A n B" for l', simp, simp)
(* i > 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,l'|}:valid A n B" for x l')
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,ins(l',Suc na,M)|}:valid A n B" for x l' na)
apply (frule len_not_empty, clarsimp)
by (drule_tac x=l' in spec, clarsimp)

subsection{*truncation resilience:
only shop i can truncate at offer i*}

lemma truncation_resilience: "ALL L. L:valid A n B --> Suc i < len L
--> cons M (trunc (L,Suc i)):valid A n B --> shop M = shop (ith (L,i))"
apply (induct i)
(* i = 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,l'|}:valid A n B" for x l')
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|M,l'|}:valid A n B" for l')
apply (frule len_not_empty, clarsimp, simp)
(* i > 0 *)
apply clarify
apply (frule len_not_empty, clarsimp)
apply (ind_cases "{|x,l'|}:valid A n B" for x l')
apply (frule len_not_empty, clarsimp)
by (drule_tac x=l' in spec, clarsimp)

subsection{*declarations for tactics*}

declare knows_Spy_partsEs [elim]
declare Fake_parts_insert [THEN subsetD, dest]
declare initState.simps [simp del]

subsection{*get components of a message*}

lemma get_ML [dest]: "Says A' B {|A,R,I,M,L|}:set evs ==>
M:parts (spies evs) & L:parts (spies evs)"
by blast

subsection{*general properties of p2*}

lemma reqm_neq_prom [iff]:
"reqm A r n I B ~= prom B' ofr A' r' I' (cons M L) J C"
by (auto simp: reqm_def prom_def)

lemma prom_neq_reqm [iff]:
"prom B' ofr A' r' I' (cons M L) J C ~= reqm A r n I B"
by (auto simp: reqm_def prom_def)

lemma req_neq_pro [iff]: "req A r n I B ~= pro B' ofr A' r' I' (cons M L) J C"
by (auto simp: req_def pro_def)

lemma pro_neq_req [iff]: "pro B' ofr A' r' I' (cons M L) J C ~= req A r n I B"
by (auto simp: req_def pro_def)

lemma p2_has_no_Gets: "evs:p2 ==> ALL A X. Gets A X ~:set evs"
by (erule p2.induct, auto simp: req_def pro_def)

lemma p2_is_Gets_correct [iff]: "Gets_correct p2"
by (auto simp: Gets_correct_def dest: p2_has_no_Gets)

lemma p2_is_one_step [iff]: "one_step p2"
by (unfold one_step_def, clarify, ind_cases "ev#evs:p2" for ev evs, auto)

lemma p2_has_only_Says' [rule_format]: "evs:p2 ==>
ev:set evs --> (EX A B X. ev=Says A B X)"
by (erule p2.induct, auto simp: req_def pro_def)

lemma p2_has_only_Says [iff]: "has_only_Says p2"
by (auto simp: has_only_Says_def dest: p2_has_only_Says')

lemma p2_is_regular [iff]: "regular p2"
apply (simp only: regular_def, clarify)
apply (erule_tac p2.induct)
apply (simp_all add: initState.simps knows.simps pro_def prom_def
req_def reqm_def anchor_def chain_def sign_def)
by (auto dest: no_Key_in_agl no_Key_in_appdel parts_trans)

subsection{*private keys are safe*}

lemma priK_parts_Friend_imp_bad [rule_format,dest]:
     "[| evs:p2; Friend B ~= A |]
      ==> (Key (priK A):parts (knows (Friend B) evs)) --> (A:bad)"
apply (erule p2.induct)
apply (simp_all add: initState.simps knows.simps pro_def prom_def
                req_def reqm_def anchor_def chain_def sign_def) 
apply (blast dest: no_Key_in_agl)
apply (auto del: parts_invKey disjE  dest: parts_trans
            simp add: no_Key_in_appdel)
done

lemma priK_analz_Friend_imp_bad [rule_format,dest]:
     "[| evs:p2; Friend B ~= A |]
==> (Key (priK A):analz (knows (Friend B) evs)) --> (A:bad)"
by auto

lemma priK_notin_knows_max_Friend:
     "[| evs:p2; A ~:bad; A ~= Friend C |]
      ==> Key (priK A) ~:analz (knows_max (Friend C) evs)"
apply (rule not_parts_not_analz, simp add: knows_max_def, safe)
apply (drule_tac H="spies' evs" in parts_sub)
apply (rule_tac p=p2 in knows_max'_sub_spies', simp+)
apply (drule_tac H="spies evs" in parts_sub)
by (auto dest: knows'_sub_knows [THEN subsetD] priK_notin_initState_Friend)

subsection{*general guardedness properties*}

lemma agl_guard [intro]: "I:agl ==> I:guard n Ks"
by (erule agl.induct, auto)

lemma Says_to_knows_max'_guard: "[| Says A' C {|A'',r,I,L|}:set evs;
Guard n Ks (knows_max' C evs) |] ==> L:guard n Ks"
by (auto dest: Says_to_knows_max')

lemma Says_from_knows_max'_guard: "[| Says C A' {|A'',r,I,L|}:set evs;
Guard n Ks (knows_max' C evs) |] ==> L:guard n Ks"
by (auto dest: Says_from_knows_max')

lemma Says_Nonce_not_used_guard: "[| Says A' B {|A'',r,I,L|}:set evs;
Nonce n ~:used evs |] ==> L:guard n Ks"
by (drule not_used_not_parts, auto)

subsection{*guardedness of messages*}

lemma chain_guard [iff]: "chain B ofr A L C:guard n {priK A}"
by (case_tac "ofr=n", auto simp: chain_def sign_def)

lemma chain_guard_Nonce_neq [intro]: "n ~= ofr
==> chain B ofr A' L C:guard n {priK A}"
by (auto simp: chain_def sign_def)

lemma anchor_guard [iff]: "anchor A n' B:guard n {priK A}"
by (case_tac "n'=n", auto simp: anchor_def)

lemma anchor_guard_Nonce_neq [intro]: "n ~= n'
==> anchor A' n' B:guard n {priK A}"
by (auto simp: anchor_def)

lemma reqm_guard [intro]: "I:agl ==> reqm A r n' I B:guard n {priK A}"
by (case_tac "n'=n", auto simp: reqm_def)

lemma reqm_guard_Nonce_neq [intro]: "[| n ~= n'; I:agl |]
==> reqm A' r n' I B:guard n {priK A}"
by (auto simp: reqm_def)

lemma prom_guard [intro]: "[| I:agl; J:agl; L:guard n {priK A} |]
==> prom B ofr A r I L J C:guard n {priK A}"
by (auto simp: prom_def)

lemma prom_guard_Nonce_neq [intro]: "[| n ~= ofr; I:agl; J:agl;
L:guard n {priK A} |] ==> prom B ofr A' r I L J C:guard n {priK A}"
by (auto simp: prom_def)

subsection{*Nonce uniqueness*}

lemma uniq_Nonce_in_chain [dest]: "Nonce k:parts {chain B ofr A L C} ==> k=ofr"
by (auto simp: chain_def sign_def)

lemma uniq_Nonce_in_anchor [dest]: "Nonce k:parts {anchor A n B} ==> k=n"
by (auto simp: anchor_def chain_def sign_def)

lemma uniq_Nonce_in_reqm [dest]: "[| Nonce k:parts {reqm A r n I B};
I:agl |] ==> k=n"
by (auto simp: reqm_def dest: no_Nonce_in_agl)

lemma uniq_Nonce_in_prom [dest]: "[| Nonce k:parts {prom B ofr A r I L J C};
I:agl; J:agl; Nonce k ~:parts {L} |] ==> k=ofr"
by (auto simp: prom_def dest: no_Nonce_in_agl no_Nonce_in_appdel)

subsection{*requests are guarded*}

lemma req_imp_Guard [rule_format]: "[| evs:p2; A ~:bad |] ==>
req A r n I B:set evs --> Guard n {priK A} (spies evs)"
apply (erule p2.induct, simp)
apply (simp add: req_def knows.simps, safe)
apply (erule in_synth_Guard, erule Guard_analz, simp)
by (auto simp: req_def pro_def dest: Says_imp_knows_Spy)

lemma req_imp_Guard_Friend: "[| evs:p2; A ~:bad; req A r n I B:set evs |]
==> Guard n {priK A} (knows_max (Friend C) evs)"
apply (rule Guard_knows_max')
apply (rule_tac H="spies evs" in Guard_mono)
apply (rule req_imp_Guard, simp+)
apply (rule_tac B="spies' evs" in subset_trans)
apply (rule_tac p=p2 in knows_max'_sub_spies', simp+)
by (rule knows'_sub_knows)

subsection{*propositions are guarded*}

lemma pro_imp_Guard [rule_format]: "[| evs:p2; B ~:bad; A ~:bad |] ==>
pro B ofr A r I (cons M L) J C:set evs --> Guard ofr {priK A} (spies evs)"
apply (erule p2.induct) (* +3 subgoals *)
(* Nil *)
apply simp
(* Fake *)
apply (simp add: pro_def, safe) (* +4 subgoals *)
(* 1 *)
apply (erule in_synth_Guard, drule Guard_analz, simp, simp)
(* 2 *)
apply simp
(* 3 *)
apply (simp, simp add: req_def pro_def, blast)
(* 4 *)
apply (simp add: pro_def)
apply (blast dest: prom_inj Says_Nonce_not_used_guard Nonce_not_used_Guard)
(* 5 *)
apply simp
apply safe (* +1 subgoal *)
apply (simp add: pro_def)
apply (blast dest: prom_inj Says_Nonce_not_used_guard)
(* 6 *)
apply (simp add: pro_def)
apply (blast dest: Says_imp_knows_Spy)
(* Request *)
apply (simp add: pro_def)
apply (blast dest: prom_inj Says_Nonce_not_used_guard Nonce_not_used_Guard)
(* Propose *)
apply simp
apply safe (* +1 subgoal *)
(* 1 *)
apply (simp add: pro_def)
apply (blast dest: prom_inj Says_Nonce_not_used_guard)
(* 2 *)
apply (simp add: pro_def)
by (blast dest: Says_imp_knows_Spy)

lemma pro_imp_Guard_Friend: "[| evs:p2; B ~:bad; A ~:bad;
pro B ofr A r I (cons M L) J C:set evs |]
==> Guard ofr {priK A} (knows_max (Friend D) evs)"
apply (rule Guard_knows_max')
apply (rule_tac H="spies evs" in Guard_mono)
apply (rule pro_imp_Guard, simp+)
apply (rule_tac B="spies' evs" in subset_trans)
apply (rule_tac p=p2 in knows_max'_sub_spies', simp+)
by (rule knows'_sub_knows)

subsection{*data confidentiality:
no one other than the originator can decrypt the offers*}

lemma Nonce_req_notin_spies: "[| evs:p2; req A r n I B:set evs; A ~:bad |]
==> Nonce n ~:analz (spies evs)"
by (frule req_imp_Guard, simp+, erule Guard_Nonce_analz, simp+)

lemma Nonce_req_notin_knows_max_Friend: "[| evs:p2; req A r n I B:set evs;
A ~:bad; A ~= Friend C |] ==> Nonce n ~:analz (knows_max (Friend C) evs)"
apply (clarify, frule_tac C=C in req_imp_Guard_Friend, simp+)
apply (simp add: knows_max_def, drule Guard_invKey_keyset, simp+)
by (drule priK_notin_knows_max_Friend, auto simp: knows_max_def)

lemma Nonce_pro_notin_spies: "[| evs:p2; B ~:bad; A ~:bad;
pro B ofr A r I (cons M L) J C:set evs |] ==> Nonce ofr ~:analz (spies evs)"
by (frule pro_imp_Guard, simp+, erule Guard_Nonce_analz, simp+)

lemma Nonce_pro_notin_knows_max_Friend: "[| evs:p2; B ~:bad; A ~:bad;
A ~= Friend D; pro B ofr A r I (cons M L) J C:set evs |]
==> Nonce ofr ~:analz (knows_max (Friend D) evs)"
apply (clarify, frule_tac A=A in pro_imp_Guard_Friend, simp+)
apply (simp add: knows_max_def, drule Guard_invKey_keyset, simp+)
by (drule priK_notin_knows_max_Friend, auto simp: knows_max_def)

subsection{*forward privacy:
only the originator can know the identity of the shops*}

lemma forward_privacy_Spy: "[| evs:p2; B ~:bad; A ~:bad;
pro B ofr A r I (cons M L) J C:set evs |]
==> sign B (Nonce ofr) ~:analz (spies evs)"
by (auto simp:sign_def dest: Nonce_pro_notin_spies)

lemma forward_privacy_Friend: "[| evs:p2; B ~:bad; A ~:bad; A ~= Friend D;
pro B ofr A r I (cons M L) J C:set evs |]
==> sign B (Nonce ofr) ~:analz (knows_max (Friend D) evs)"
by (auto simp:sign_def dest:Nonce_pro_notin_knows_max_Friend )

subsection{*non repudiability: an offer signed by B has been sent by B*}

lemma Crypt_reqm: "[| Crypt (priK A) X:parts {reqm A' r n I B}; I:agl |] ==> A=A'"
by (auto simp: reqm_def anchor_def chain_def sign_def dest: no_Crypt_in_agl)

lemma Crypt_prom: "[| Crypt (priK A) X:parts {prom B ofr A' r I L J C};
I:agl; J:agl |] ==> A=B | Crypt (priK A) X:parts {L}"
apply (simp add: prom_def anchor_def chain_def sign_def)
by (blast dest: no_Crypt_in_agl no_Crypt_in_appdel)

lemma Crypt_safeness: "[| evs:p2; A ~:bad |] ==> Crypt (priK A) X:parts (spies evs)
--> (EX B Y. Says A B Y:set evs & Crypt (priK A) X:parts {Y})"
apply (erule p2.induct)
(* Nil *)
apply simp
(* Fake *)
apply clarsimp
apply (drule_tac P="%G. Crypt (priK A) X:G" in parts_insert_substD, simp)
apply (erule disjE)
apply (drule_tac K="priK A" in Crypt_synth, simp+, blast, blast)
(* Request *)
apply (simp add: req_def, clarify)
apply (drule_tac P="%G. Crypt (priK A) X:G" in parts_insert_substD, simp)
apply (erule disjE)
apply (frule Crypt_reqm, simp, clarify)
apply (rule_tac x=B in exI, rule_tac x="reqm A r n I B" in exI, simp, blast)
(* Propose *)
apply (simp add: pro_def, clarify)
apply (drule_tac P="%G. Crypt (priK A) X:G" in parts_insert_substD, simp)
apply (rotate_tac -1, erule disjE)
apply (frule Crypt_prom, simp, simp)
apply (rotate_tac -1, erule disjE)
apply (rule_tac x=C in exI)
apply (rule_tac x="prom B ofr Aa r I (cons M L) J C" in exI, blast)
apply (subgoal_tac "cons M L:parts (spies evsp)")
apply (drule_tac G="{cons M L}" and H="spies evsp" in parts_trans, blast, blast)
apply (drule Says_imp_spies, rotate_tac -1, drule parts.Inj)
apply (drule parts.Snd, drule parts.Snd, drule parts.Snd)
by auto

lemma Crypt_Hash_imp_sign: "[| evs:p2; A ~:bad |] ==>
Crypt (priK A) (Hash X):parts (spies evs)
--> (EX B Y. Says A B Y:set evs & sign A X:parts {Y})"
apply (erule p2.induct)
(* Nil *)
apply simp
(* Fake *)
apply clarsimp
apply (drule_tac P="%G. Crypt (priK A) (Hash X):G" in parts_insert_substD)
apply simp
apply (erule disjE)
apply (drule_tac K="priK A" in Crypt_synth, simp+, blast, blast)
(* Request *)
apply (simp add: req_def, clarify)
apply (drule_tac P="%G. Crypt (priK A) (Hash X):G" in parts_insert_substD)
apply simp
apply (erule disjE)
apply (frule Crypt_reqm, simp+)
apply (rule_tac x=B in exI, rule_tac x="reqm Aa r n I B" in exI)
apply (simp add: reqm_def sign_def anchor_def no_Crypt_in_agl)
apply (simp add: chain_def sign_def, blast)
(* Propose *)
apply (simp add: pro_def, clarify)
apply (drule_tac P="%G. Crypt (priK A) (Hash X):G" in parts_insert_substD)
apply simp
apply (rotate_tac -1, erule disjE)
apply (simp add: prom_def sign_def no_Crypt_in_agl no_Crypt_in_appdel)
apply (simp add: chain_def sign_def)
apply (rotate_tac -1, erule disjE)
apply (rule_tac x=C in exI)
apply (rule_tac x="prom B ofr Aa r I (cons M L) J C" in exI)
apply (simp add: prom_def chain_def sign_def)
apply (erule impE) 
apply (blast dest: get_ML parts_sub) 
apply (blast del: MPair_parts)+
done

lemma sign_safeness: "[| evs:p2; A ~:bad |] ==> sign A X:parts (spies evs)
--> (EX B Y. Says A B Y:set evs & sign A X:parts {Y})"
apply (clarify, simp add: sign_def, frule parts.Snd)
apply (blast dest: Crypt_Hash_imp_sign [unfolded sign_def])
done

end