src/HOL/Auth/NS_Public_Bad.thy
author paulson
Thu Jan 09 10:22:11 1997 +0100 (1997-01-09)
changeset 2497 47de509bdd55
parent 2451 ce85a2aafc7a
child 2516 4d68fbe6378b
permissions -rw-r--r--
New treatment of nonce creation
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(*  Title:      HOL/Auth/NS_Public_Bad
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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 "ns_public" for the Needham-Schroeder Public-Key protocol.
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Flawed version, vulnerable to Lowe's attack.
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From page 260 of
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  Burrows, Abadi and Needham.  A Logic of Authentication.
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  Proc. Royal Soc. 426 (1989)
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*)
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NS_Public_Bad = Public + 
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consts  lost    :: agent set        (*No need for it to be a variable*)
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	ns_public  :: event list set
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inductive ns_public
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  intrs 
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         (*Initial trace is empty*)
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    Nil  "[]: ns_public"
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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: ns_public;  B ~= Spy;  
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             X: synth (analz (sees lost Spy evs)) |]
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          ==> Says Spy B X  # evs : ns_public"
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         (*Alice initiates a protocol run, sending a nonce to Bob*)
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    NS1  "[| evs: ns_public;  A ~= B;  Nonce NA ~: used evs |]
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          ==> Says A B (Crypt (pubK B) {|Nonce NA, Agent A|})
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                # evs  :  ns_public"
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         (*Bob responds to Alice's message with a further nonce*)
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    NS2  "[| evs: ns_public;  A ~= B;  Nonce NB ~: used evs;
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             Says A' B (Crypt (pubK B) {|Nonce NA, Agent A|})
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               : set_of_list evs |]
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          ==> Says B A (Crypt (pubK A) {|Nonce NA, Nonce NB|})
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                # evs  :  ns_public"
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         (*Alice proves her existence by sending NB back to Bob.*)
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    NS3  "[| evs: ns_public;  A ~= B;
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             Says B' A (Crypt (pubK A) {|Nonce NA, Nonce NB|})
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               : set_of_list evs;
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             Says A  B (Crypt (pubK B) {|Nonce NA, Agent A|})
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               : set_of_list evs |]
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          ==> Says A B (Crypt (pubK B) (Nonce NB)) # evs : ns_public"
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  (**Oops message??**)
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rules
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  (*Spy has access to his own key for spoof messages*)
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  Spy_in_lost "Spy: lost"
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end