src/HOL/Auth/WooLam.thy
author paulson
Fri Jul 04 17:34:55 1997 +0200 (1997-07-04)
changeset 3500 0d8ad2f192d8
parent 3481 256f38c01b98
child 3519 ab0a9fbed4c0
permissions -rw-r--r--
New constant "certificate"--just an abbreviation
     1 (*  Title:      HOL/Auth/WooLam
     2     ID:         $Id$
     3     Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
     4     Copyright   1996  University of Cambridge
     5 
     6 Inductive relation "woolam" for the Woo-Lam protocol.
     7 
     8 Simplified version from page 11 of
     9   Abadi and Needham.  Prudent Engineering Practice for Cryptographic Protocols.
    10   IEEE Trans. S.E. 22(1), 1996, pages 6-15.
    11 
    12 Note: this differs from the Woo-Lam protocol discussed by Lowe in his paper
    13   Some New Attacks upon Security Protocols.
    14   Computer Security Foundations Workshop, 1996.
    15 *)
    16 
    17 WooLam = Shared + 
    18 
    19 consts  lost    :: agent set        (*No need for it to be a variable*)
    20 	woolam  :: event list set
    21 inductive woolam
    22   intrs 
    23          (*Initial trace is empty*)
    24     Nil  "[]: woolam"
    25 
    26          (*The spy MAY say anything he CAN say.  We do not expect him to
    27            invent new nonces here, but he can also use NS1.  Common to
    28            all similar protocols.*)
    29     Fake "[| evs: woolam;  B ~= Spy;  
    30              X: synth (analz (sees lost Spy evs)) |]
    31           ==> Says Spy B X  # evs : woolam"
    32 
    33          (*Alice initiates a protocol run*)
    34     WL1  "[| evs: woolam;  A ~= B |]
    35           ==> Says A B (Agent A) # evs : woolam"
    36 
    37          (*Bob responds to Alice's message with a challenge.*)
    38     WL2  "[| evs: woolam;  A ~= B;  
    39              Says A' B (Agent A) : set evs |]
    40           ==> Says B A (Nonce NB) # evs : woolam"
    41 
    42          (*Alice responds to Bob's challenge by encrypting NB with her key.
    43            B is *not* properly determined -- Alice essentially broadcasts
    44            her reply.*)
    45     WL3  "[| evs: woolam;
    46              Says A  B (Agent A)  : set evs;
    47              Says B' A (Nonce NB) : set evs |]
    48           ==> Says A B (Crypt (shrK A) (Nonce NB)) # evs : woolam"
    49 
    50          (*Bob forwards Alice's response to the Server.  NOTE: usually
    51            the messages are shown in chronological order, for clarity.
    52            But here, exchanging the two events would cause the lemma
    53            WL4_analz_sees_Spy to pick up the wrong assumption!*)
    54     WL4  "[| evs: woolam;  B ~= Server;  
    55              Says A'  B X         : set evs;
    56              Says A'' B (Agent A) : set evs |]
    57           ==> Says B Server {|Agent A, Agent B, X|} # evs : woolam"
    58 
    59          (*Server decrypts Alice's response for Bob.*)
    60     WL5  "[| evs: woolam;  B ~= Server;
    61              Says B' Server {|Agent A, Agent B, Crypt (shrK A) (Nonce NB)|}
    62                : set evs |]
    63           ==> Says Server B (Crypt (shrK B) {|Agent A, Nonce NB|})
    64                  # evs : woolam"
    65 
    66 end