src/HOL/Auth/OtwayRees.thy
author paulson
Thu, 26 Sep 1996 12:50:48 +0200
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(*  Title:      HOL/Auth/OtwayRees
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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 "otway" for the Otway-Rees protocol.
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Version that encrypts Nonce NB
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From page 244 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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OtwayRees = Shared + 
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consts  otway   :: "agent set => event list set"
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inductive "otway lost"
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  intrs 
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         (*Initial trace is empty*)
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    Nil  "[]: otway lost"
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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: otway lost;  B ~= Spy;  
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             X: synth (analz (sees lost Spy evs)) |]
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          ==> Says Spy B X  # evs : otway lost"
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         (*Alice initiates a protocol run*)
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    OR1  "[| evs: otway lost;  A ~= B;  B ~= Server |]
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          ==> Says A B {|Nonce (newN evs), Agent A, Agent B, 
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                         Crypt {|Nonce (newN evs), Agent A, Agent B|} 
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                               (shrK A) |} 
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                 # evs : otway lost"
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         (*Bob's response to Alice's message.  Bob doesn't know who 
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	   the sender is, hence the A' in the sender field.
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           We modify the published protocol by NOT encrypting NB.*)
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    OR2  "[| evs: otway lost;  B ~= Server;
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             Says A' B {|Nonce NA, Agent A, Agent B, X|} : set_of_list evs |]
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          ==> Says B Server 
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                  {|Nonce NA, Agent A, Agent B, X, 
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                    Crypt {|Nonce NA, Nonce (newN evs), 
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                            Agent A, Agent B|} (shrK B)|}
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                 # evs : otway lost"
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         (*The Server receives Bob's message and checks that the three NAs
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           match.  Then he sends a new session key to Bob with a packet for
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           forwarding to Alice.*)
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    OR3  "[| evs: otway lost;  B ~= Server;
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             Says B' Server 
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                  {|Nonce NA, Agent A, Agent B, 
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                    Crypt {|Nonce NA, Agent A, Agent B|} (shrK A), 
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                    Crypt {|Nonce NA, Nonce NB, Agent A, Agent B|} (shrK B)|}
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               : set_of_list evs |]
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          ==> Says Server B 
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                  {|Nonce NA, 
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                    Crypt {|Nonce NA, Key (newK evs)|} (shrK A),
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                    Crypt {|Nonce NB, Key (newK evs)|} (shrK B)|}
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                 # evs : otway lost"
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         (*Bob receives the Server's (?) message and compares the Nonces with
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	   those in the message he previously sent the Server.*)
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    OR4  "[| evs: otway lost;  A ~= B;  B ~= Server;
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             Says S B {|Nonce NA, X, Crypt {|Nonce NB, Key K|} (shrK B)|}
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               : set_of_list evs;
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             Says B Server {|Nonce NA, Agent A, Agent B, X', 
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                             Crypt {|Nonce NA, Nonce NB, Agent A, Agent B|} 
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                                   (shrK B)|}
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               : set_of_list evs |]
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          ==> Says B A {|Nonce NA, X|} # evs : otway lost"
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         (*This message models possible leaks of session keys.  Alice's Nonce
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           identifies the protocol run.*)
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    Reveal "[| evs: otway lost;  A ~= Spy;
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               Says B' A {|Nonce NA, Crypt {|Nonce NA, Key K|} (shrK A)|}
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                 : set_of_list evs;
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               Says A  B {|Nonce NA, Agent A, Agent B, 
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                           Crypt {|Nonce NA, Agent A, Agent B|} (shrK A)|}
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                 : set_of_list evs |]
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            ==> Says A Spy {|Nonce NA, Key K|} # evs : otway lost"
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end