src/HOL/Auth/Yahalom.thy
author paulson
Fri Sep 19 18:27:31 1997 +0200 (1997-09-19)
changeset 3686 4b484805b4c4
parent 3683 aafe719dff14
child 3961 6a8996fb7d99
permissions -rw-r--r--
First working version with Oops event for session keys
     1 (*  Title:      HOL/Auth/Yahalom
     2     ID:         $Id$
     3     Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
     4     Copyright   1996  University of Cambridge
     5 
     6 Inductive relation "yahalom" for the Yahalom protocol.
     7 
     8 From page 257 of
     9   Burrows, Abadi and Needham.  A Logic of Authentication.
    10   Proc. Royal Soc. 426 (1989)
    11 *)
    12 
    13 Yahalom = Shared + 
    14 
    15 consts  yahalom   :: event list set
    16 inductive "yahalom"
    17   intrs 
    18          (*Initial trace is empty*)
    19     Nil  "[]: yahalom"
    20 
    21          (*The spy MAY say anything he CAN say.  We do not expect him to
    22            invent new nonces here, but he can also use NS1.  Common to
    23            all similar protocols.*)
    24     Fake "[| evs: yahalom;  B ~= Spy;  
    25              X: synth (analz (spies evs)) |]
    26           ==> Says Spy B X  # evs : yahalom"
    27 
    28          (*Alice initiates a protocol run*)
    29     YM1  "[| evs1: yahalom;  A ~= B;  Nonce NA ~: used evs1 |]
    30           ==> Says A B {|Agent A, Nonce NA|} # evs1 : yahalom"
    31 
    32          (*Bob's response to Alice's message.  Bob doesn't know who 
    33 	   the sender is, hence the A' in the sender field.*)
    34     YM2  "[| evs2: yahalom;  B ~= Server;  Nonce NB ~: used evs2;
    35              Says A' B {|Agent A, Nonce NA|} : set evs2 |]
    36           ==> Says B Server 
    37                   {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
    38                 # evs2 : yahalom"
    39 
    40          (*The Server receives Bob's message.  He responds by sending a
    41             new session key to Alice, with a packet for forwarding to Bob.*)
    42     YM3  "[| evs3: yahalom;  A ~= Server;  Key KAB ~: used evs3;
    43              Says B' Server 
    44                   {|Agent B, Crypt (shrK B) {|Agent A, Nonce NA, Nonce NB|}|}
    45                : set evs3 |]
    46           ==> Says Server A
    47                    {|Crypt (shrK A) {|Agent B, Key KAB, Nonce NA, Nonce NB|},
    48                      Crypt (shrK B) {|Agent A, Key KAB|}|}
    49                 # evs3 : yahalom"
    50 
    51          (*Alice receives the Server's (?) message, checks her Nonce, and
    52            uses the new session key to send Bob his Nonce.*)
    53     YM4  "[| evs4: yahalom;  A ~= Server;  
    54              Says S A {|Crypt (shrK A) {|Agent B, Key K, Nonce NA, Nonce NB|},
    55                         X|}  : set evs4;
    56              Says A B {|Agent A, Nonce NA|} : set evs4 |]
    57           ==> Says A B {|X, Crypt K (Nonce NB)|} # evs4 : yahalom"
    58 
    59          (*This message models possible leaks of session keys.  The Nonces
    60            identify the protocol run.  Quoting Server here ensures they are
    61            correct.*)
    62     Oops "[| evso: yahalom;  A ~= Spy;
    63              Says Server A {|Crypt (shrK A)
    64                                    {|Agent B, Key K, Nonce NA, Nonce NB|},
    65                              X|}  : set evso |]
    66           ==> Says A Spy {|Nonce NA, Nonce NB, Key K|} # evso : yahalom"
    67 
    68 
    69 constdefs 
    70   KeyWithNonce :: [key, nat, event list] => bool
    71   "KeyWithNonce K NB evs ==
    72      EX A B na X. 
    73        Says Server A {|Crypt (shrK A) {|Agent B, Key K, na, Nonce NB|}, X|} 
    74          : set evs"
    75 
    76 end