src/HOL/Auth/Yahalom2.thy
author paulson
Mon Jun 09 10:21:38 1997 +0200 (1997-06-09)
changeset 3432 04412cfe6861
parent 2516 4d68fbe6378b
child 3445 96fcfbfa4fb5
permissions -rw-r--r--
Strengthened and streamlined the Yahalom proofs
     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, Variant 2.
     7 
     8 This version trades encryption of NB for additional explicitness in YM3.
     9 Also in YM3, care is taken to make the two certificates distinct.
    10 
    11 From page 259 of
    12   Burrows, Abadi and Needham.  A Logic of Authentication.
    13   Proc. Royal Soc. 426 (1989)
    14 *)
    15 
    16 Yahalom2 = Shared + 
    17 
    18 consts  yahalom   :: agent set => event list set
    19 inductive "yahalom lost"
    20   intrs 
    21          (*Initial trace is empty*)
    22     Nil  "[]: yahalom lost"
    23 
    24          (*The spy MAY say anything he CAN say.  We do not expect him to
    25            invent new nonces here, but he can also use NS1.  Common to
    26            all similar protocols.*)
    27     Fake "[| evs: yahalom lost;  B ~= Spy;  
    28              X: synth (analz (sees lost Spy evs)) |]
    29           ==> Says Spy B X  # evs : yahalom lost"
    30 
    31          (*Alice initiates a protocol run*)
    32     YM1  "[| evs: yahalom lost;  A ~= B;  Nonce NA ~: used evs |]
    33           ==> Says A B {|Agent A, Nonce NA|} # evs : yahalom lost"
    34 
    35          (*Bob's response to Alice's message.  Bob doesn't know who 
    36 	   the sender is, hence the A' in the sender field.*)
    37     YM2  "[| evs: yahalom lost;  B ~= Server;  Nonce NB ~: used evs;
    38              Says A' B {|Agent A, Nonce NA|} : set_of_list evs |]
    39           ==> Says B Server 
    40                   {|Agent B, Nonce NB, Crypt (shrK B) {|Agent A, Nonce NA|}|}
    41                 # evs : yahalom lost"
    42 
    43          (*The Server receives Bob's message.  He responds by sending a
    44            new session key to Alice, with a packet for forwarding to Bob.
    45            !! Fields are reversed in the 2nd packet to prevent attacks!! *)
    46     YM3  "[| evs: yahalom lost;  A ~= B;  A ~= Server;  Key KAB ~: used evs;
    47              Says B' Server {|Agent B, Nonce NB,
    48 			      Crypt (shrK B) {|Agent A, Nonce NA|}|}
    49                : set_of_list evs |]
    50           ==> Says Server A
    51                {|Nonce NB, 
    52                  Crypt (shrK A) {|Agent B, Key KAB, Nonce NA|},
    53                  Crypt (shrK B) {|Nonce NB, Key KAB, Agent A|}|}
    54                  # evs : yahalom lost"
    55 
    56          (*Alice receives the Server's (?) message, checks her Nonce, and
    57            uses the new session key to send Bob his Nonce.*)
    58     YM4  "[| evs: yahalom lost;  A ~= Server;  A ~= B;  
    59              Says S A {|Nonce NB, Crypt (shrK A) {|Agent B, Key K, Nonce NA|},
    60                         X|}
    61                : set_of_list evs;
    62              Says A B {|Agent A, Nonce NA|} : set_of_list evs |]
    63           ==> Says A B {|X, Crypt K (Nonce NB)|} # evs : yahalom lost"
    64 
    65          (*This message models possible leaks of session keys.  The nonces
    66            identify the protocol run.  Quoting Server here ensures they are
    67            correct. *)
    68     Oops "[| evs: yahalom lost;  A ~= Spy;
    69              Says Server A {|Nonce NB, 
    70                              Crypt (shrK A) {|Agent B, Key K, Nonce NA|},
    71                              X|}  : set_of_list evs |]
    72           ==> Says A Spy {|Nonce NA, Nonce NB, Key K|} # evs : yahalom lost"
    73 
    74 end