Theory Shared

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

Theory of Shared Keys (common to all symmetric-key protocols)

Shared, long-term keys; initial states of agents

theory Shared
imports Event All_Symmetric

  shrK    :: "agent  key"  (*symmetric keys*)

specification (shrK)
  inj_shrK: "inj shrK"
  ― ‹No two agents have the same long-term key›
   apply (rule exI [of _ "case_agent 0 (λn. n + 2) 1"]) 
   apply (simp add: inj_on_def split: agent.split) 

text‹Server knows all long-term keys; other agents know only their own›

  initState  initState

primrec initState where
  initState_Server:  "initState Server     = Key ` range shrK"
| initState_Friend:  "initState (Friend i) = {Key (shrK (Friend i))}"
| initState_Spy:     "initState Spy        = Key`shrK`bad"


subsection‹Basic properties of shrK›

(*Injectiveness: Agents' long-term keys are distinct.*)
lemmas shrK_injective = inj_shrK [THEN inj_eq]
declare shrK_injective [iff]

lemma invKey_K [simp]: "invKey K = K"
apply (insert isSym_keys)
apply (simp add: symKeys_def) 

lemma analz_Decrypt' [dest]:
     "Crypt K X  analz H;  Key K   analz H  X  analz H"
by auto

text‹Now cancel the dest› attribute given to
 analz.Decrypt› in its declaration.›
declare analz.Decrypt [rule del]

text‹Rewrites should not refer to  terminitState(Friend i) because
  that expression is not in normal form.›

lemma keysFor_parts_initState [simp]: "keysFor (parts (initState C)) = {}"
unfolding keysFor_def
apply (induct_tac "C", auto)

(*Specialized to shared-key model: no @{term invKey}*)
lemma keysFor_parts_insert:
     "K  keysFor (parts (insert X G));  X  synth (analz H)
       K  keysFor (parts (G  H)) | Key K  parts H"
by (metis invKey_K keysFor_parts_insert)

lemma Crypt_imp_keysFor: "Crypt K X  H  K  keysFor H"
by (metis Crypt_imp_invKey_keysFor invKey_K)

subsection‹Function "knows"›

(*Spy sees shared keys of agents!*)
lemma Spy_knows_Spy_bad [intro!]: "A  bad  Key (shrK A)  knows Spy evs"
apply (induct_tac "evs")
apply (simp_all (no_asm_simp) add: imageI knows_Cons split: event.split)

(*For case analysis on whether or not an agent is compromised*)
lemma Crypt_Spy_analz_bad: "Crypt (shrK A) X  analz (knows Spy evs);  A  bad  
       X  analz (knows Spy evs)"
by (metis Spy_knows_Spy_bad analz.Inj analz_Decrypt')

(** Fresh keys never clash with long-term shared keys **)

(*Agents see their own shared keys!*)
lemma shrK_in_initState [iff]: "Key (shrK A)  initState A"
by (induct_tac "A", auto)

lemma shrK_in_used [iff]: "Key (shrK A)  used evs"
by (rule initState_into_used, blast)

(*Used in parts_induct_tac and analz_Fake_tac to distinguish session keys
  from long-term shared keys*)
lemma Key_not_used [simp]: "Key K  used evs  K  range shrK"
by blast

lemma shrK_neq [simp]: "Key K  used evs  shrK B  K"
by blast

lemmas shrK_sym_neq = shrK_neq [THEN not_sym]
declare shrK_sym_neq [simp]

subsection‹Fresh nonces›

lemma Nonce_notin_initState [iff]: "Nonce N  parts (initState B)"
by (induct_tac "B", auto)

lemma Nonce_notin_used_empty [simp]: "Nonce N  used []"
by (simp add: used_Nil)

subsection‹Supply fresh nonces for possibility theorems.›

(*In any trace, there is an upper bound N on the greatest nonce in use.*)
lemma Nonce_supply_lemma: "N. n. N  n  Nonce n  used evs"
apply (induct_tac "evs")
apply (rule_tac x = 0 in exI)
apply (simp_all (no_asm_simp) add: used_Cons split: event.split)
apply (metis le_sup_iff msg_Nonce_supply)

lemma Nonce_supply1: "N. Nonce N  used evs"
by (metis Nonce_supply_lemma order_eq_iff)

lemma Nonce_supply2: "N N'. Nonce N  used evs  Nonce N'  used evs'  N  N'"
apply (cut_tac evs = evs in Nonce_supply_lemma)
apply (cut_tac evs = "evs'" in Nonce_supply_lemma, clarify)
apply (metis Suc_n_not_le_n nat_le_linear)

lemma Nonce_supply3: "N N' N''. Nonce N  used evs  Nonce N'  used evs'   
                    Nonce N''  used evs''  N  N'  N'  N''  N  N''"
apply (cut_tac evs = evs in Nonce_supply_lemma)
apply (cut_tac evs = "evs'" in Nonce_supply_lemma)
apply (cut_tac evs = "evs''" in Nonce_supply_lemma, clarify)
apply (rule_tac x = N in exI)
apply (rule_tac x = "Suc (N+Na)" in exI)
apply (rule_tac x = "Suc (Suc (N+Na+Nb))" in exI)
apply (simp (no_asm_simp) add: less_not_refl3 le_add1 le_add2 less_Suc_eq_le)

lemma Nonce_supply: "Nonce (SOME N. Nonce N  used evs)  used evs"
apply (rule Nonce_supply_lemma [THEN exE])
apply (rule someI, blast)

text‹Unlike the corresponding property of nonces, we cannot prove
    termfinite KK  K. K  KK  Key K  used evs.
    We have infinitely many agents and there is nothing to stop their
    long-term keys from exhausting all the natural numbers.  Instead,
    possibility theorems must assume the existence of a few keys.›

subsection‹Specialized Rewriting for Theorems About termanalz and Image›

lemma subset_Compl_range: "A  - (range shrK)  shrK x  A"
by blast

lemma insert_Key_singleton: "insert (Key K) H = Key ` {K}  H"
by blast

lemma insert_Key_image: "insert (Key K) (Key`KK  C) = Key`(insert K KK)  C"
by blast

(** Reverse the normal simplification of "image" to build up (not break down)
    the set of keys.  Use analz_insert_eq with (Un_upper2 RS analz_mono) to
    erase occurrences of forwarded message components (X). **)

lemmas analz_image_freshK_simps =
       simp_thms mem_simps ― ‹these two allow its use with only:›
       image_insert [THEN sym] image_Un [THEN sym] empty_subsetI insert_subset
       analz_insert_eq Un_upper2 [THEN analz_mono, THEN [2] rev_subsetD]
       insert_Key_singleton subset_Compl_range
       Key_not_used insert_Key_image Un_assoc [THEN sym]

(*Lemma for the trivial direction of the if-and-only-if*)
lemma analz_image_freshK_lemma:
     "(Key K  analz (Key`nE  H))  (K  nE | Key K  analz H)    
         (Key K  analz (Key`nE  H)) = (K  nE | Key K  analz H)"
by (blast intro: analz_mono [THEN [2] rev_subsetD])

subsection‹Tactics for possibility theorems›

structure Shared =

(*Omitting used_Says makes the tactic much faster: it leaves expressions
    such as  Nonce ?N ∉ used evs that match Nonce_supply*)
fun possibility_tac ctxt =
    (ALLGOALS (simp_tac (ctxt
          delsimps [@{thm used_Says}, @{thm used_Notes}, @{thm used_Gets}] 
          setSolver safe_solver))
     REPEAT_FIRST (eq_assume_tac ORELSE' 
                   resolve_tac ctxt [refl, conjI, @{thm Nonce_supply}])))

(*For harder protocols (such as Recur) where we have to set up some
  nonces and keys initially*)
fun basic_possibility_tac ctxt =
    (ALLGOALS (asm_simp_tac (ctxt setSolver safe_solver))
     REPEAT_FIRST (resolve_tac ctxt [refl, conjI]))

val analz_image_freshK_ss =
   (context delsimps [image_insert, image_Un]
      delsimps [@{thm imp_disjL}]    (*reduces blow-up*)
      addsimps @{thms analz_image_freshK_simps})


(*Lets blast_tac perform this step without needing the simplifier*)
lemma invKey_shrK_iff [iff]:
     "(Key (invKey K)  X) = (Key K  X)"
by auto

(*Specialized methods*)

method_setup analz_freshK = Scan.succeed (fn ctxt =>
      (EVERY [REPEAT_FIRST (resolve_tac ctxt [allI, ballI, impI]),
          REPEAT_FIRST (resolve_tac ctxt @{thms analz_image_freshK_lemma}),
          ALLGOALS (asm_simp_tac (put_simpset Shared.analz_image_freshK_ss ctxt))])))
    "for proving the Session Key Compromise theorem"

method_setup possibility = Scan.succeed (fn ctxt => SIMPLE_METHOD (Shared.possibility_tac ctxt))
    "for proving possibility theorems"

method_setup basic_possibility = Scan.succeed (fn ctxt => SIMPLE_METHOD (Shared.basic_possibility_tac ctxt))
    "for proving possibility theorems"

lemma knows_subset_knows_Cons: "knows A evs  knows A (e # evs)"
by (cases e) (auto simp: knows_Cons)