author  paulson 
Thu, 24 Oct 1996 10:38:35 +0200  
changeset 2125  92a08ee6a9cb 
parent 2110  d01151e66cd4 
child 2156  9c361df93bd5 
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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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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1995  13 
Yahalom = Shared + 
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2032  15 
consts yahalom :: "agent set => event list set" 
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inductive "yahalom lost" 

1985
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intrs 
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(*Initial trace is empty*) 
2032  19 
Nil "[]: yahalom lost" 
1985
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2032  21 
(*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.*) 
2032  24 
Fake "[ evs: yahalom lost; B ~= Spy; 
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X: synth (analz (sees lost Spy evs)) ] 

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==> Says Spy B X # evs : yahalom lost" 

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(*Alice initiates a protocol run*) 
2032  29 
YM1 "[ evs: yahalom lost; A ~= B ] 
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==> Says A B {Agent A, Nonce (newN evs)} # evs : yahalom lost" 

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(*Bob's response to Alice's message. Bob doesn't know who 
1995  33 
the sender is, hence the A' in the sender field.*) 
2032  34 
YM2 "[ evs: yahalom lost; B ~= Server; 
1995  35 
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)} 
2032  39 
# evs : yahalom lost" 
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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.*) 
2032  43 
YM3 "[ evs: yahalom lost; A ~= Server; 
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Says B' Server 
1995  45 
{Agent B, Crypt {Agent A, Nonce NA, Nonce NB} (shrK B)} 
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: set_of_list evs ] 
1995  47 
==> 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)} 

2032  51 
# evs : yahalom lost" 
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1995  53 
(*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 lost; A ~= Server; A ~= B; 
1995  56 
Says S A {Crypt {Agent B, Key K, Nonce NA, Nonce NB} (shrK A), 
2125  57 
X} : set_of_list evs; 
1995  58 
Says A B {Agent A, Nonce NA} : set_of_list evs ] 
2032  59 
==> Says A B {X, Crypt (Nonce NB) K} # evs : yahalom lost" 
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2110  61 
(*This message models possible leaks of session keys. The Nonces 
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identify the protocol run.*) 

2125  63 
Oops "[ evs: yahalom lost; A ~= Spy; 
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Says Server A {Crypt {Agent B, Key K, Nonce NA, Nonce NB} 

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(shrK A), 

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X} : set_of_list evs ] 

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==> Says A Spy {Nonce NA, Nonce NB, Key K} # evs : yahalom lost" 
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end 