src/HOL/Auth/WooLam.thy
author paulson
Tue, 01 Jul 1997 17:38:49 +0200
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permissions -rw-r--r--
Deleted a redundant A~=B in rules that refer to a previous event
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(*  Title:      HOL/Auth/WooLam
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    ID:         $Id$
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1996  University of Cambridge
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Inductive relation "woolam" for the Woo-Lam protocol.
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Simplified version from page 11 of
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  Abadi and Needham.  Prudent Engineering Practice for Cryptographic Protocols.
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  IEEE Trans. S.E. 22(1), 1996, pages 6-15.
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Note: this differs from the Woo-Lam protocol discussed by Lowe in his paper
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  Some New Attacks upon Security Protocols.
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  Computer Security Foundations Workshop, 1996.
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*)
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WooLam = Shared + 
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consts  lost    :: agent set        (*No need for it to be a variable*)
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	woolam  :: event list set
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inductive woolam
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  intrs 
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         (*Initial trace is empty*)
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    Nil  "[]: woolam"
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         (*The spy MAY say anything he CAN say.  We do not expect him to
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           invent new nonces here, but he can also use NS1.  Common to
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           all similar protocols.*)
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    Fake "[| evs: woolam;  B ~= Spy;  
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             X: synth (analz (sees lost Spy evs)) |]
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          ==> Says Spy B X  # evs : woolam"
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         (*Alice initiates a protocol run*)
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    WL1  "[| evs: woolam;  A ~= B |]
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          ==> Says A B (Agent A) # evs : woolam"
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         (*Bob responds to Alice's message with a challenge.*)
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    WL2  "[| evs: woolam;  A ~= B;  
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             Says A' B (Agent A) : set evs |]
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          ==> Says B A (Nonce NB) # evs : woolam"
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         (*Alice responds to Bob's challenge by encrypting NB with her key.
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           B is *not* properly determined -- Alice essentially broadcasts
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           her reply.*)
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    WL3  "[| evs: woolam;
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             Says A  B (Agent A)  : set evs;
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             Says B' A (Nonce NB) : set evs |]
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          ==> Says A B (Crypt (shrK A) (Nonce NB)) # evs : woolam"
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         (*Bob forwards Alice's response to the Server.  NOTE: usually
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           the messages are shown in chronological order, for clarity.
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           But here, exchanging the two events would cause the lemma
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           WL4_analz_sees_Spy to pick up the wrong assumption!*)
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    WL4  "[| evs: woolam;  B ~= Server;  
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             Says A'  B X         : set evs;
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             Says A'' B (Agent A) : set evs |]
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          ==> Says B Server {|Agent A, Agent B, X|} # evs : woolam"
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         (*Server decrypts Alice's response for Bob.*)
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    WL5  "[| evs: woolam;  B ~= Server;
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             Says B' Server {|Agent A, Agent B, Crypt (shrK A) (Nonce NB)|}
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               : set evs |]
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          ==> Says Server B (Crypt (shrK B) {|Agent A, Nonce NB|})
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                 # evs : woolam"
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end