Global change: lost->bad and sees Spy->spies
First change just gives a more sensible name.
Second change eliminates the agent parameter of "sees" to simplify
definitions and theorems
(* Title: HOL/Auth/OtwayRees
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1996 University of Cambridge
Inductive relation "otway" for the Otway-Rees protocol.
Simplified version with minimal encryption but explicit messages
Note that the formalization does not even assume that nonces are fresh.
This is because the protocol does not rely on uniqueness of nonces for
security, only for freshness, and the proof script does not prove freshness
properties.
From page 11 of
Abadi and Needham. Prudent Engineering Practice for Cryptographic Protocols.
IEEE Trans. SE 22 (1), 1996
*)
OtwayRees_AN = 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 |]
==> Says A B {|Agent A, Agent B, Nonce NA|} # evs1 : otway"
(*Bob's response to Alice's message. Bob doesn't know who
the sender is, hence the A' in the sender field.*)
OR2 "[| evs2: otway; B ~= Server;
Says A' B {|Agent A, Agent B, Nonce NA|} : set evs2 |]
==> Says B Server {|Agent A, Agent B, Nonce NA, Nonce NB|}
# evs2 : otway"
(*The Server receives Bob's message. Then he sends a new
session key to Bob with a packet for forwarding to Alice.*)
OR3 "[| evs3: otway; B ~= Server; A ~= B; Key KAB ~: used evs3;
Says B' Server {|Agent A, Agent B, Nonce NA, Nonce NB|}
: set evs3 |]
==> Says Server B
{|Crypt (shrK A) {|Nonce NA, Agent A, Agent B, Key KAB|},
Crypt (shrK B) {|Nonce NB, Agent A, Agent B, 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 {|Agent A, Agent B, Nonce NA, Nonce NB|} : set evs4;
Says S' B {|X, Crypt(shrK B){|Nonce NB,Agent A,Agent B,Key K|}|}
: set evs4 |]
==> Says B A X # evs4 : otway"
(*This message models possible leaks of session keys. The nonces
identify the protocol run. B is not assumed to know shrK A.*)
Oops "[| evso: otway; B ~= Spy;
Says Server B
{|Crypt (shrK A) {|Nonce NA, Agent A, Agent B, Key K|},
Crypt (shrK B) {|Nonce NB, Agent A, Agent B, Key K|}|}
: set evso |]
==> Says B Spy {|Nonce NA, Nonce NB, Key K|} # evso : otway"
end