(* 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.
Also in YM3, care is taken to make the two certificates distinct.
From page 259 of
Burrows, Abadi and Needham. A Logic of Authentication.
Proc. Royal Soc. 426 (1989)
*)
Yahalom2 = Shared +
consts yahalom :: event list set
inductive "yahalom"
intrs
(*Initial trace is empty*)
Nil "[]: yahalom"
(*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 "[| evs: yahalom; B ~= Spy;
X: synth (analz (spies evs)) |]
==> Says Spy B X # evs : yahalom"
(*Alice initiates a protocol run*)
YM1 "[| evs1: yahalom; A ~= B; Nonce NA ~: used evs1 |]
==> Says A B {|Agent A, Nonce NA|} # evs1 : yahalom"
(*Bob's response to Alice's message. Bob doesn't know who
the sender is, hence the A' in the sender field.*)
YM2 "[| evs2: yahalom; B ~= Server; Nonce NB ~: used evs2;
Says A' B {|Agent A, Nonce NA|} : set evs2 |]
==> Says B Server
{|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|}
# evs2 : yahalom"
(*The Server receives Bob's message. He responds by sending a
new session key to Alice, with a certificate for forwarding to Bob.
Fields are reversed in the 2nd certificate to prevent attacks!! *)
YM3 "[| evs3: yahalom; A ~= B; A ~= Server; Key KAB ~: used evs3;
Says B' Server {|Agent B, Nonce NB,
Crypt (shrK B) {|Agent A, Nonce NA|}|}
: set evs3 |]
==> Says Server A
{|Nonce NB,
Crypt (shrK A) {|Agent B, Key KAB, Nonce NA|},
Crypt (shrK B) {|Nonce NB, Key KAB, Agent A|}|}
# evs3 : yahalom"
(*Alice receives the Server's (?) message, checks her Nonce, and
uses the new session key to send Bob his Nonce.*)
YM4 "[| evs4: yahalom; A ~= Server;
Says S A {|Nonce NB, Crypt (shrK A) {|Agent B, Key K, Nonce NA|},
X|} : set evs4;
Says A B {|Agent A, Nonce NA|} : set evs4 |]
==> Says A B {|X, Crypt K (Nonce NB)|} # evs4 : yahalom"
(*This message models possible leaks of session keys. The nonces
identify the protocol run. Quoting Server here ensures they are
correct. *)
Oops "[| evso: yahalom; A ~= Spy;
Says Server A {|Nonce NB,
Crypt (shrK A) {|Agent B, Key K, Nonce NA|},
X|} : set evso |]
==> Says A Spy {|Nonce NA, Nonce NB, Key K|} # evso : yahalom"
end