src/HOL/Auth/Yahalom_Bad.thy

tuned;

(*  Title:      HOL/Auth/Yahalom
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1996  University of Cambridge

Inductive relation "yahalom" for the Yahalom protocol.

Example of why Oops is necessary.  This protocol can be attacked because it
doesn't keep NB secret, but without Oops it can be "verified" anyway.
*)

Yahalom_Bad = 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;  X: synth (analz (knows Spy evs)) |]
          ==> Says Spy B X  # evs : yahalom"

         (*A message that has been sent can be received by the
           intended recipient.*)
    Reception "[| evsr: yahalom;  Says A B X : set evsr |]
               ==> Gets B X # evsr : yahalom"

         (*Alice initiates a protocol run*)
    YM1  "[| evs1: yahalom;  Nonce NA ~: used evs1 |]
          ==> Says A B {|Agent A, Nonce NA|} # evs1 : yahalom"

         (*Bob's response to Alice's message.*)
    YM2  "[| evs2: yahalom;  Nonce NB ~: used evs2;
             Gets 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 packet for forwarding to Bob.*)
    YM3  "[| evs3: yahalom;  Key KAB ~: used evs3;
             Gets Server 
                  {|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|}
               : set evs3 |]
          ==> Says Server A
                   {|Crypt (shrK A) {|Agent B, Key KAB, Nonce NA, Nonce NB|},
                     Crypt (shrK B) {|Agent A, Key KAB|}|}
                # evs3 : yahalom"

         (*Alice receives the Server's (?) message, checks her Nonce, and
           uses the new session key to send Bob his Nonce.  The premise
           A ~= Server is needed to prove Says_Server_not_range.*)
    YM4  "[| evs4: yahalom;  A ~= Server;
             Gets A {|Crypt(shrK A) {|Agent B, Key K, Nonce NA, Nonce NB|}, 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.*)

end