author  paulson 
Fri, 13 Sep 1996 13:16:57 +0200  
changeset 1995  c80e58e78d9c 
parent 1985  84cf16192e03 
child 2032  1bbf1bdcaf56 
permissions  rwrr 
1995  1 
(* Title: HOL/Auth/Yahalom 
1985
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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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5 

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Inductive relation "yahalom" for the Yahalom protocol. 
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7 

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From page 257 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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12 

1995  13 
Yahalom = Shared + 
1985
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consts yahalom :: "event list set" 
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inductive yahalom 
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intrs 
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(*Initial trace is empty*) 
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Nil "[]: yahalom" 
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(*The enemy 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: yahalom; B ~= Enemy; X: synth (analz (sees Enemy evs)) ] 
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==> Says Enemy B X # evs : yahalom" 
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(*Alice initiates a protocol run*) 
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YM1 "[ evs: yahalom; A ~= B ] 
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==> Says A B {Agent A, Nonce (newN evs)} # evs : yahalom" 
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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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YM2 "[ evs: yahalom; B ~= Server; 
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Says A' B {Agent A, Nonce NA} : set_of_list evs ] 
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==> Says B Server 
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{Agent B, 
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Crypt {Agent A, Nonce NA, Nonce (newN evs)} (shrK B)} 
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# evs : yahalom" 
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(*The Server receives Bob's message. He responds by sending a 
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new session key to Alice, with a packet for forwarding to Bob.*) 
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YM3 "[ evs: yahalom; A ~= Server; 
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Says B' Server 
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{Agent B, Crypt {Agent A, Nonce NA, Nonce NB} (shrK B)} 
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: set_of_list evs ] 
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==> Says Server A 
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{Crypt {Agent B, Key (newK evs), 

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Nonce NA, Nonce NB} (shrK A), 

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Crypt {Agent A, Key (newK evs)} (shrK B)} 

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# evs : yahalom" 
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(*Alice receives the Server's (?) message, checks her Nonce, and 
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uses the new session key to send Bob his Nonce.*) 

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YM4 "[ evs: yahalom; A ~= B; 
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Says S A {Crypt {Agent B, Key K, Nonce NA, Nonce NB} (shrK A), 
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X} 

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: set_of_list evs; 
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Says A B {Agent A, Nonce NA} : set_of_list evs ] 
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==> Says A B {X, Crypt (Nonce NB) K} # evs : yahalom" 

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end 