src/HOL/Auth/OtwayRees_Bad.thy
author paulson
Fri Sep 19 18:27:31 1997 +0200 (1997-09-19)
changeset 3686 4b484805b4c4
parent 3683 aafe719dff14
child 4522 0218c486cf07
permissions -rw-r--r--
First working version with Oops event for session keys
     1 (*  Title:      HOL/Auth/OtwayRees_Bad
     2     ID:         $Id$
     3     Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
     4     Copyright   1996  University of Cambridge
     5 
     6 Inductive relation "otway" for the Otway-Rees protocol.
     7 
     8 The FAULTY version omitting encryption of Nonce NB, as suggested on page 247 of
     9   Burrows, Abadi and Needham.  A Logic of Authentication.
    10   Proc. Royal Soc. 426 (1989)
    11 *)
    12 
    13 OtwayRees_Bad = Shared + 
    14 
    15 consts  otway   :: event list set
    16 
    17 inductive otway
    18   intrs 
    19          (*Initial trace is empty*)
    20     Nil  "[]: otway"
    21 
    22          (*The spy MAY say anything he CAN say.  We do not expect him to
    23            invent new nonces here, but he can also use NS1.  Common to
    24            all similar protocols.*)
    25     Fake "[| evs: otway;  B ~= Spy;  X: synth (analz (spies evs)) |]
    26           ==> Says Spy B X  # evs : otway"
    27 
    28          (*Alice initiates a protocol run*)
    29     OR1  "[| evs1: otway;  A ~= B;  B ~= Server;  Nonce NA ~: used evs1 |]
    30           ==> Says A B {|Nonce NA, Agent A, Agent B, 
    31                          Crypt (shrK A) {|Nonce NA, Agent A, Agent B|} |} 
    32                  # evs1 : otway"
    33 
    34          (*Bob's response to Alice's message.  Bob doesn't know who 
    35 	   the sender is, hence the A' in the sender field.
    36            We modify the published protocol by NOT encrypting NB.*)
    37     OR2  "[| evs2: otway;  B ~= Server;  Nonce NB ~: used evs2;
    38              Says A' B {|Nonce NA, Agent A, Agent B, X|} : set evs2 |]
    39           ==> Says B Server 
    40                   {|Nonce NA, Agent A, Agent B, X, Nonce NB,
    41                     Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
    42                  # evs2 : otway"
    43 
    44          (*The Server receives Bob's message and checks that the three NAs
    45            match.  Then he sends a new session key to Bob with a packet for
    46            forwarding to Alice.*)
    47     OR3  "[| evs3: otway;  B ~= Server;  Key KAB ~: used evs3;
    48              Says B' Server 
    49                   {|Nonce NA, Agent A, Agent B, 
    50                     Crypt (shrK A) {|Nonce NA, Agent A, Agent B|}, 
    51                     Nonce NB, 
    52                     Crypt (shrK B) {|Nonce NA, Agent A, Agent B|}|}
    53                : set evs3 |]
    54           ==> Says Server B 
    55                   {|Nonce NA, 
    56                     Crypt (shrK A) {|Nonce NA, Key KAB|},
    57                     Crypt (shrK B) {|Nonce NB, Key KAB|}|}
    58                  # evs3 : otway"
    59 
    60          (*Bob receives the Server's (?) message and compares the Nonces with
    61 	   those in the message he previously sent the Server.*)
    62     OR4  "[| evs4: otway;  A ~= B;
    63              Says B Server {|Nonce NA, Agent A, Agent B, X', Nonce NB, X''|}
    64                : set evs4;
    65              Says S' B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
    66                : set evs4 |]
    67           ==> Says B A {|Nonce NA, X|} # evs4 : otway"
    68 
    69          (*This message models possible leaks of session keys.  The nonces
    70            identify the protocol run.*)
    71     Oops "[| evso: otway;  B ~= Spy;
    72              Says Server B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
    73                : set evso |]
    74           ==> Says B Spy {|Nonce NA, Nonce NB, Key K|} # evso : otway"
    75 
    76 end