(* Title: HOL/Auth/OtwayRees_Bad
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1996 University of Cambridge
Inductive relation "otway" for the Otway-Rees protocol.
The FAULTY version omitting encryption of Nonce NB, as suggested on page 247 of
Burrows, Abadi and Needham. A Logic of Authentication.
Proc. Royal Soc. 426 (1989)
*)
OtwayRees_Bad = Shared +
consts otway :: event list set
inductive otway
intrs
(*Initial trace is empty*)
Nil "[]: otway"
(*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: otway; B ~= Spy; X: synth (analz (spies evs)) |]
==> Says Spy B X # evs : otway"
(*Alice initiates a protocol run*)
OR1 "[| evs1: otway; A ~= B; B ~= Server; Nonce NA ~: used evs1 |]
==> Says A B {|Nonce NA, Agent A, Agent B,
Crypt (shrK A) {|Nonce NA, Agent A, Agent B|} |}
# evs1 : otway"
(*Bob's response to Alice's message. Bob doesn't know who
the sender is, hence the A' in the sender field.
We modify the published protocol by NOT encrypting NB.*)
OR2 "[| evs2: otway; B ~= Server; Nonce NB ~: used evs2;
Says A' B {|Nonce NA, Agent A, Agent B, X|} : set evs2 |]
==> Says B Server
{|Nonce NA, Agent A, Agent B, X, Nonce NB,
Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
# evs2 : otway"
(*The Server receives Bob's message and checks that the three NAs
match. Then he sends a new session key to Bob with a packet for
forwarding to Alice.*)
OR3 "[| evs3: otway; B ~= Server; Key KAB ~: used evs3;
Says B' Server
{|Nonce NA, Agent A, Agent B,
Crypt (shrK A) {|Nonce NA, Agent A, Agent B|},
Nonce NB,
Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
: set evs3 |]
==> Says Server B
{|Nonce NA,
Crypt (shrK A) {|Nonce NA, Key KAB|},
Crypt (shrK B) {|Nonce NB, Key KAB|}|}
# evs3 : otway"
(*Bob receives the Server's (?) message and compares the Nonces with
those in the message he previously sent the Server.*)
OR4 "[| evs4: otway; A ~= B;
Says B Server {|Nonce NA, Agent A, Agent B, X', Nonce NB, X''|}
: set evs4;
Says S' B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
: set evs4 |]
==> Says B A {|Nonce NA, X|} # evs4 : otway"
(*This message models possible leaks of session keys. The nonces
identify the protocol run.*)
Oops "[| evso: otway; B ~= Spy;
Says Server B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
: set evso |]
==> Says B Spy {|Nonce NA, Nonce NB, Key K|} # evso : otway"
end