src/HOL/Auth/OtwayRees_AN.thy

+(*  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
+
+From page 11 of
+  Abadi and Needham.  Prudent Engineering Practice for Cryptographic Protocols.
+  IEEE Trans. SE 22 (1), 1996
+*)
+
+OtwayRees_AN = Shared + 
+
+consts  otway   :: "agent set => event list set"
+inductive "otway lost"
+  intrs 
+         (*Initial trace is empty*)
+    Nil  "[]: otway lost"
+
+         (*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 lost;  B ~= Spy;  
+             X: synth (analz (sees lost Spy evs)) |]
+          ==> Says Spy B X  # evs : otway lost"
+
+         (*Alice initiates a protocol run*)
+    OR1  "[| evs: otway lost;  A ~= B;  B ~= Server |]
+          ==> Says A B {|Agent A, Agent B, Nonce (newN evs)|}
+                 # evs : otway lost"
+
+         (*Bob's response to Alice's message.  Bob doesn't know who 
+	   the sender is, hence the A' in the sender field.*)
+    OR2  "[| evs: otway lost;  B ~= Server;
+             Says A' B {|Agent A, Agent B, Nonce NA|} : set_of_list evs |]
+          ==> Says B Server {|Agent A, Agent B, Nonce NA, Nonce (newN evs)|}
+                 # evs : otway lost"
+
+         (*The Server receives Bob's message.  Then he sends a new
+           session key to Bob with a packet for forwarding to Alice.*)
+    OR3  "[| evs: otway lost;  B ~= Server;  A ~= B;
+             Says B' Server {|Agent A, Agent B, Nonce NA, Nonce NB|}
+               : set_of_list evs |]
+          ==> Says Server B 
+               {|Crypt {|Nonce NA, Agent A, Agent B, Key(newK evs)|} (shrK A),
+                 Crypt {|Nonce NB, Agent A, Agent B, Key(newK evs)|} (shrK B)|}
+              # evs : otway lost"
+
+         (*Bob receives the Server's (?) message and compares the Nonces with
+	   those in the message he previously sent the Server.*)
+    OR4  "[| evs: otway lost;  A ~= B;  B ~= Server;
+             Says S B {|X, 
+                        Crypt {|Nonce NB, Agent A, Agent B, Key K|} (shrK B)|}
+               : set_of_list evs;
+             Says B Server {|Agent A, Agent B, Nonce NA, Nonce NB|}
+               : set_of_list evs |]
+          ==> Says B A X # evs : otway lost"
+
+         (*This message models possible leaks of session keys.  Alice's Nonce
+           identifies the protocol run.*)
+    Revl "[| evs: otway lost;  A ~= Spy;
+             Says B' A (Crypt {|Nonce NA, Agent A, Agent B, Key K|} (shrK A))
+               : set_of_list evs;
+             Says A  B {|Agent A, Agent B, Nonce NA|} : set_of_list evs |]
+          ==> Says A Spy {|Nonce NA, Key K|} # evs : otway lost"
+
+end