(* Title: HOL/Auth/Yahalom2
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1996 University of Cambridge
Inductive relation "yahalom" for the Yahalom protocol, Variant 2.
This version trades encryption of NB for additional explicitness in YM3.
From page 259 of
Burrows, Abadi and Needham. A Logic of Authentication.
Proc. Royal Soc. 426 (1989)
*)
AddEs spies_partsEs;
AddDs [impOfSubs analz_subset_parts];
AddDs [impOfSubs Fake_parts_insert];
(*A "possibility property": there are traces that reach the end*)
Goal "EX X NB K. EX evs: yahalom. \
\ Says A B {|X, Crypt K (Nonce NB)|} : set evs";
by (REPEAT (resolve_tac [exI,bexI] 1));
by (rtac (yahalom.Nil RS yahalom.YM1 RS yahalom.YM2 RS yahalom.YM3 RS
yahalom.YM4) 2);
by possibility_tac;
result();
(**** Inductive proofs about yahalom ****)
(** For reasoning about the encrypted portion of messages **)
(*Lets us treat YM4 using a similar argument as for the Fake case.*)
Goal "Says S A {|NB, Crypt (shrK A) Y, X|} : set evs ==> \
\ X : analz (spies evs)";
by (blast_tac (claset() addSDs [Says_imp_spies RS analz.Inj]) 1);
qed "YM4_analz_spies";
bind_thm ("YM4_parts_spies",
YM4_analz_spies RS (impOfSubs analz_subset_parts));
(*Relates to both YM4 and Oops*)
Goal "Says S A {|NB, Crypt (shrK A) {|B,K,NA|}, X|} : set evs ==> \
\ K : parts (spies evs)";
by (Blast_tac 1);
qed "YM4_Key_parts_spies";
(*For proving the easier theorems about X ~: parts (spies evs).*)
fun parts_spies_tac i =
forward_tac [YM4_Key_parts_spies] (i+6) THEN
forward_tac [YM4_parts_spies] (i+5) THEN
prove_simple_subgoals_tac i;
(*Induction for regularity theorems. If induction formula has the form
X ~: analz (spies evs) --> ... then it shortens the proof by discarding
needless information about analz (insert X (spies evs)) *)
fun parts_induct_tac i =
etac yahalom.induct i
THEN
REPEAT (FIRSTGOAL analz_mono_contra_tac)
THEN parts_spies_tac i;
(** Theorems of the form X ~: parts (spies evs) imply that NOBODY
sends messages containing X! **)
(*Spy never sees another agent's shared key! (unless it's bad at start)*)
Goal "evs : yahalom ==> (Key (shrK A) : parts (spies evs)) = (A : bad)";
by (parts_induct_tac 1);
by (ALLGOALS Blast_tac);
qed "Spy_see_shrK";
Addsimps [Spy_see_shrK];
Goal "evs : yahalom ==> (Key (shrK A) : analz (spies evs)) = (A : bad)";
by Auto_tac;
qed "Spy_analz_shrK";
Addsimps [Spy_analz_shrK];
AddSDs [Spy_see_shrK RSN (2, rev_iffD1),
Spy_analz_shrK RSN (2, rev_iffD1)];
(*Nobody can have used non-existent keys! Needed to apply analz_insert_Key*)
Goal "evs : yahalom ==> \
\ Key K ~: used evs --> K ~: keysFor (parts (spies evs))";
by (parts_induct_tac 1);
(*YM4: Key K is not fresh!*)
by (Blast_tac 3);
(*YM3*)
by (blast_tac (claset() addss (simpset())) 2);
(*Fake*)
by (blast_tac (claset() addSDs [keysFor_parts_insert]) 1);
qed_spec_mp "new_keys_not_used";
bind_thm ("new_keys_not_analzd",
[analz_subset_parts RS keysFor_mono,
new_keys_not_used] MRS contra_subsetD);
Addsimps [new_keys_not_used, new_keys_not_analzd];
(*Describes the form of K when the Server sends this message. Useful for
Oops as well as main secrecy property.*)
Goal "[| Says Server A {|nb', Crypt (shrK A) {|Agent B, Key K, na|}, X|} \
\ : set evs; \
\ evs : yahalom |] \
\ ==> K ~: range shrK";
by (etac rev_mp 1);
by (etac yahalom.induct 1);
by (ALLGOALS Asm_simp_tac);
qed "Says_Server_message_form";
(*For proofs involving analz.*)
val analz_spies_tac =
dtac YM4_analz_spies 6 THEN
forward_tac [Says_Server_message_form] 7 THEN
assume_tac 7 THEN
REPEAT ((etac conjE ORELSE' hyp_subst_tac) 7);
(****
The following is to prove theorems of the form
Key K : analz (insert (Key KAB) (spies evs)) ==>
Key K : analz (spies evs)
A more general formula must be proved inductively.
****)
(** Session keys are not used to encrypt other session keys **)
Goal "evs : yahalom ==> \
\ ALL K KK. KK <= - (range shrK) --> \
\ (Key K : analz (Key``KK Un (spies evs))) = \
\ (K : KK | Key K : analz (spies evs))";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (REPEAT_FIRST (resolve_tac [allI, impI]));
by (REPEAT_FIRST (rtac analz_image_freshK_lemma));
by (ALLGOALS (asm_simp_tac analz_image_freshK_ss));
(*Fake*)
by (spy_analz_tac 1);
qed_spec_mp "analz_image_freshK";
Goal "[| evs : yahalom; KAB ~: range shrK |] ==> \
\ Key K : analz (insert (Key KAB) (spies evs)) = \
\ (K = KAB | Key K : analz (spies evs))";
by (asm_simp_tac (analz_image_freshK_ss addsimps [analz_image_freshK]) 1);
qed "analz_insert_freshK";
(*** The Key K uniquely identifies the Server's message. **)
Goal "evs : yahalom ==> \
\ EX A' B' na' nb' X'. ALL A B na nb X. \
\ Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, X|} \
\ : set evs --> A=A' & B=B' & na=na' & nb=nb' & X=X'";
by (etac yahalom.induct 1);
by (ALLGOALS (asm_simp_tac (simpset() addsimps [all_conj_distrib])));
by (Clarify_tac 1);
(*Remaining case: YM3*)
by (expand_case_tac "K = ?y" 1);
by (REPEAT (ares_tac [refl,exI,impI,conjI] 2));
(*...we assume X is a recent message and handle this case by contradiction*)
by (blast_tac (claset() delrules [conjI] (*prevent splitup into 4 subgoals*)
addss (simpset() addsimps [parts_insertI])) 1);
val lemma = result();
Goal "[| Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, X|} : set evs; \
\ Says Server A' \
\ {|nb', Crypt (shrK A') {|Agent B', Key K, na'|}, X'|} : set evs; \
\ evs : yahalom |] \
\ ==> A=A' & B=B' & na=na' & nb=nb'";
by (prove_unique_tac lemma 1);
qed "unique_session_keys";
(** Crucial secrecy property: Spy does not see the keys sent in msg YM3 **)
Goal "[| A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, nb|}|} \
\ : set evs --> \
\ Notes Spy {|na, nb, Key K|} ~: set evs --> \
\ Key K ~: analz (spies evs)";
by (etac yahalom.induct 1);
by analz_spies_tac;
by (ALLGOALS
(asm_simp_tac
(simpset() addsimps split_ifs
addsimps [analz_insert_eq, analz_insert_freshK])));
(*Oops*)
by (blast_tac (claset() addDs [unique_session_keys]) 3);
(*YM3*)
by (blast_tac (claset() delrules [impCE]) 2);
(*Fake*)
by (spy_analz_tac 1);
val lemma = result() RS mp RS mp RSN(2,rev_notE);
(*Final version*)
Goal "[| Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, nb|}|} \
\ : set evs; \
\ Notes Spy {|na, nb, Key K|} ~: set evs; \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> Key K ~: analz (spies evs)";
by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1);
by (blast_tac (claset() addSEs [lemma]) 1);
qed "Spy_not_see_encrypted_key";
(** Security Guarantee for A upon receiving YM3 **)
(*If the encrypted message appears then it originated with the Server.
May now apply Spy_not_see_encrypted_key, subject to its conditions.*)
Goal "[| Crypt (shrK A) {|Agent B, Key K, na|} \
\ : parts (spies evs); \
\ A ~: bad; evs : yahalom |] \
\ ==> EX nb. Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, na|}, \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, nb|}|} \
\ : set evs";
by (etac rev_mp 1);
by (parts_induct_tac 1);
by (ALLGOALS Blast_tac);
qed "A_trusts_YM3";
(*The obvious combination of A_trusts_YM3 with Spy_not_see_encrypted_key*)
Goal "[| Crypt (shrK A) {|Agent B, Key K, na|} : parts (spies evs); \
\ ALL nb. Notes Spy {|na, nb, Key K|} ~: set evs; \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> Key K ~: analz (spies evs)";
by (blast_tac (claset() addSDs [A_trusts_YM3, Spy_not_see_encrypted_key]) 1);
qed "A_gets_good_key";
(** Security Guarantee for B upon receiving YM4 **)
(*B knows, by the first part of A's message, that the Server distributed
the key for A and B, and has associated it with NB.*)
Goal "[| Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|} \
\ : parts (spies evs); \
\ B ~: bad; evs : yahalom |] \
\ ==> EX NA. Says Server A \
\ {|Nonce NB, \
\ Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|}|} \
\ : set evs";
by (etac rev_mp 1);
by (parts_induct_tac 1);
by (ALLGOALS Blast_tac);
qed "B_trusts_YM4_shrK";
(*With this protocol variant, we don't need the 2nd part of YM4 at all:
Nonce NB is available in the first part.*)
(*What can B deduce from receipt of YM4? Stronger and simpler than Yahalom
because we do not have to show that NB is secret. *)
Goal "[| Says A' B {|Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|}, \
\ X|} \
\ : set evs; \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> EX NA. Says Server A \
\ {|Nonce NB, \
\ Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|}|} \
\ : set evs";
by (blast_tac (claset() addSDs [B_trusts_YM4_shrK]) 1);
qed "B_trusts_YM4";
(*The obvious combination of B_trusts_YM4 with Spy_not_see_encrypted_key*)
Goal "[| Says A' B {|Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|}, \
\ X|} \
\ : set evs; \
\ ALL na. Notes Spy {|na, Nonce NB, Key K|} ~: set evs; \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> Key K ~: analz (spies evs)";
by (blast_tac (claset() addSDs [B_trusts_YM4, Spy_not_see_encrypted_key]) 1);
qed "B_gets_good_key";
(*** Authenticating B to A ***)
(*The encryption in message YM2 tells us it cannot be faked.*)
Goal "[| Crypt (shrK B) {|Agent A, Nonce NA|} : parts (spies evs); \
\ B ~: bad; evs : yahalom \
\ |] ==> EX NB. Says B Server {|Agent B, Nonce NB, \
\ Crypt (shrK B) {|Agent A, Nonce NA|}|} \
\ : set evs";
by (etac rev_mp 1);
by (etac rev_mp 1);
by (parts_induct_tac 1);
by (ALLGOALS Blast_tac);
qed "B_Said_YM2";
(*If the server sends YM3 then B sent YM2, perhaps with a different NB*)
Goal "[| Says Server A \
\ {|nb, Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, X|} \
\ : set evs; \
\ B ~: bad; evs : yahalom \
\ |] ==> EX nb'. Says B Server {|Agent B, nb', \
\ Crypt (shrK B) {|Agent A, Nonce NA|}|} \
\ : set evs";
by (etac rev_mp 1);
by (etac rev_mp 1);
by (etac yahalom.induct 1);
by (ALLGOALS Asm_simp_tac);
(*YM3*)
by (blast_tac (claset() addSDs [B_Said_YM2]) 3);
(*Fake, YM2*)
by (ALLGOALS Blast_tac);
val lemma = result();
(*If A receives YM3 then B has used nonce NA (and therefore is alive)*)
Goal "[| Says S A {|nb, Crypt (shrK A) {|Agent B, Key K, Nonce NA|}, X|} \
\ : set evs; \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\==> EX nb'. Says B Server \
\ {|Agent B, nb', Crypt (shrK B) {|Agent A, Nonce NA|}|} \
\ : set evs";
by (blast_tac (claset() addSDs [A_trusts_YM3, lemma]) 1);
qed "YM3_auth_B_to_A";
(*** Authenticating A to B using the certificate Crypt K (Nonce NB) ***)
(*Assuming the session key is secure, if both certificates are present then
A has said NB. We can't be sure about the rest of A's message, but only
NB matters for freshness. Note that Key K ~: analz (spies evs) must be
the FIRST antecedent of the induction formula.*)
Goal "evs : yahalom \
\ ==> Key K ~: analz (spies evs) --> \
\ Crypt K (Nonce NB) : parts (spies evs) --> \
\ Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|} \
\ : parts (spies evs) --> \
\ B ~: bad --> \
\ (EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs)";
by (parts_induct_tac 1);
(*Fake*)
by (Blast_tac 1);
(*YM3: by new_keys_not_used we note that Crypt K (Nonce NB) could not exist*)
by (fast_tac (claset() addSDs [Crypt_imp_keysFor] addss (simpset())) 1);
(*YM4: was Crypt K (Nonce NB) the very last message? If not, use ind. hyp.*)
by (asm_simp_tac (simpset() addsimps [ex_disj_distrib]) 1);
(*yes: apply unicity of session keys*)
by (not_bad_tac "Aa" 1);
by (blast_tac (claset() addSDs [A_trusts_YM3, B_trusts_YM4_shrK]
addDs [unique_session_keys]) 1);
qed_spec_mp "Auth_A_to_B_lemma";
(*If B receives YM4 then A has used nonce NB (and therefore is alive).
Moreover, A associates K with NB (thus is talking about the same run).
Other premises guarantee secrecy of K.*)
Goal "[| Says A' B {|Crypt (shrK B) {|Agent A, Agent B, Key K, Nonce NB|}, \
\ Crypt K (Nonce NB)|} : set evs; \
\ (ALL NA. Notes Spy {|Nonce NA, Nonce NB, Key K|} ~: set evs); \
\ A ~: bad; B ~: bad; evs : yahalom |] \
\ ==> EX X. Says A B {|X, Crypt K (Nonce NB)|} : set evs";
by (subgoal_tac "Key K ~: analz (spies evs)" 1);
by (blast_tac (claset() addIs [Auth_A_to_B_lemma]) 1);
by (blast_tac (claset() addDs [Spy_not_see_encrypted_key,
B_trusts_YM4_shrK]) 1);
qed_spec_mp "YM4_imp_A_Said_YM3";