src/HOL/Auth/TLS.thy
author paulson
Wed Oct 01 13:41:38 1997 +0200 (1997-10-01)
changeset 3759 3d1ac6b82b28
parent 3757 7524781c5c83
child 4198 c63639beeff1
permissions -rw-r--r--
Fixed ServerResume to check for ServerHello instead of making a new NB
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(*  Title:      HOL/Auth/TLS
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    ID:         $Id$
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1997  University of Cambridge
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Inductive relation "tls" for the TLS (Transport Layer Security) protocol.
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This protocol is essentially the same as SSL 3.0.
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Abstracted from "The TLS Protocol, Version 1.0" by Tim Dierks and Christopher
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Allen, Transport Layer Security Working Group, 21 May 1997,
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INTERNET-DRAFT draft-ietf-tls-protocol-03.txt.  Section numbers below refer
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to that memo.
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An RSA cryptosystem is assumed, and X.509v3 certificates are abstracted down
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to the trivial form {A, publicKey(A)}privateKey(Server), where Server is a
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global signing authority.
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A is the client and B is the server, not to be confused with the constant
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Server, who is in charge of all public keys.
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The model assumes that no fraudulent certificates are present, but it does
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assume that some private keys are to the spy.
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REMARK.  The event "Notes A {|Agent B, Nonce PMS|}" appears in ClientKeyExch,
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CertVerify, ClientFinished to record that A knows M.  It is a note from A to
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herself.  Nobody else can see it.  In ClientKeyExch, the Spy can substitute
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his own certificate for A's, but he cannot replace A's note by one for himself.
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The Note event avoids a weakness in the public-key model.  Each
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agent's state is recorded as the trace of messages.  When the true client (A)
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invents PMS, he encrypts PMS with B's public key before sending it.  The model
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does not distinguish the original occurrence of such a message from a replay.
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In the shared-key model, the ability to encrypt implies the ability to
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decrypt, so the problem does not arise.
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Proofs would be simpler if ClientKeyExch included A's name within
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Crypt KB (Nonce PMS).  As things stand, there is much overlap between proofs
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about that message (which B receives) and the stronger event
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	Notes A {|Agent B, Nonce PMS|}.
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*)
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TLS = Public + 
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consts
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  (*Pseudo-random function of Section 5*)
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  PRF  :: "nat*nat*nat => nat"
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  (*Client, server write keys are generated uniformly by function sessionK
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    to avoid duplicating their properties.  They are indexed by a further
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    natural number, not a bool, to avoid the peculiarities of if-and-only-if.
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    Session keys implicitly include MAC secrets.*)
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  sessionK :: "(nat*nat*nat)*nat => key"
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  certificate      :: "[agent,key] => msg"
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defs
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  certificate_def
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    "certificate A KA == Crypt (priK Server) {|Agent A, Key KA|}"
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syntax
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    clientK, serverK :: "nat*nat*nat => key"
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translations
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  "clientK (nonces)"	== "sessionK(nonces,0)"
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  "serverK (nonces)"	== "sessionK(nonces,1)"
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rules
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  (*the pseudo-random function is collision-free*)
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  inj_PRF       "inj PRF"	
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  (*sessionK is collision-free; also, no clientK clashes with any serverK.*)
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  inj_sessionK  "inj sessionK"	
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  (*sessionK makes symmetric keys*)
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  isSym_sessionK "isSymKey (sessionK nonces)"
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consts    tls :: event list set
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inductive tls
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  intrs 
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    Nil  (*Initial trace is empty*)
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         "[]: tls"
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    Fake (*The spy, an active attacker, MAY say anything he CAN say.*)
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         "[| evs: tls;  B ~= Spy;  
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             X: synth (analz (spies evs)) |]
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          ==> Says Spy B X # evs : tls"
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    SpyKeys (*The spy may apply PRF & sessionK to available nonces*)
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         "[| evsSK: tls;
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	     Says Spy B {|Nonce NA, Nonce NB, Nonce M|} : set evsSK |]
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          ==> Says Spy B {| Nonce (PRF(M,NA,NB)),
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			    Key (sessionK((NA,NB,M),b)) |} # evsSK : tls"
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    ClientHello
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	 (*(7.4.1.2)
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	   PA represents CLIENT_VERSION, CIPHER_SUITES and COMPRESSION_METHODS.
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	   It is uninterpreted but will be confirmed in the FINISHED messages.
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	   NA is CLIENT RANDOM, while SID is SESSION_ID.
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           UNIX TIME is omitted because the protocol doesn't use it.
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           May assume NA ~: range PRF because CLIENT RANDOM is 28 bytes
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	   while MASTER SECRET is 48 byptes*)
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         "[| evsCH: tls;  A ~= B;  Nonce NA ~: used evsCH;  NA ~: range PRF |]
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          ==> Says A B {|Agent A, Nonce NA, Number SID, Number PA|}
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	        # evsCH  :  tls"
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    ServerHello
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         (*7.4.1.3 of the TLS Internet-Draft
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	   PB represents CLIENT_VERSION, CIPHER_SUITE and COMPRESSION_METHOD.
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           SERVER CERTIFICATE (7.4.2) is always present.
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           CERTIFICATE_REQUEST (7.4.4) is implied.*)
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         "[| evsSH: tls;  A ~= B;  Nonce NB ~: used evsSH;  NB ~: range PRF;
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             Says A' B {|Agent A, Nonce NA, Number SID, Number PA|}
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	       : set evsSH |]
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          ==> Says B A {|Nonce NB, Number SID, Number PB|} # evsSH  :  tls"
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    Certificate
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         (*SERVER (7.4.2) or CLIENT (7.4.6) CERTIFICATE.*)
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         "[| evsC: tls;  A ~= B |]
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          ==> Says B A (certificate B (pubK B)) # evsC  :  tls"
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    ClientKeyExch
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         (*CLIENT KEY EXCHANGE (7.4.7).
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           The client, A, chooses PMS, the PREMASTER SECRET.
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           She encrypts PMS using the supplied KB, which ought to be pubK B.
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           We assume PMS ~: range PRF because a clash betweem the PMS
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           and another MASTER SECRET is highly unlikely (even though
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	   both items have the same length, 48 bytes).
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           The Note event records in the trace that she knows PMS
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               (see REMARK at top). *)
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         "[| evsCX: tls;  A ~= B;  Nonce PMS ~: used evsCX;  PMS ~: range PRF;
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             Says B'  A {|Nonce NB, Number SID, Number PB|} : set evsCX;
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             Says B'' A (certificate B KB) : set evsCX |]
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          ==> Says A B (Crypt KB (Nonce PMS))
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	      # Notes A {|Agent B, Nonce PMS|}
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	      # evsCX  :  tls"
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    CertVerify
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	(*The optional Certificate Verify (7.4.8) message contains the
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          specific components listed in the security analysis, F.1.1.2.
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          It adds the pre-master-secret, which is also essential!
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          Checking the signature, which is the only use of A's certificate,
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          assures B of A's presence*)
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         "[| evsCV: tls;  A ~= B;  
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             Says B' A {|Nonce NB, Number SID, Number PB|} : set evsCV;
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	     Notes A {|Agent B, Nonce PMS|} : set evsCV |]
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          ==> Says A B (Crypt (priK A) (Hash{|Nonce NB, Agent B, Nonce PMS|}))
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              # evsCV  :  tls"
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	(*Finally come the FINISHED messages (7.4.8), confirming PA and PB
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          among other things.  The master-secret is PRF(PMS,NA,NB).
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          Either party may sent its message first.*)
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    ClientFinished
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        (*The occurrence of Notes A {|Agent B, Nonce PMS|} stops the 
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          rule's applying when the Spy has satisfied the "Says A B" by
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          repaying messages sent by the true client; in that case, the
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          Spy does not know PMS and could not sent ClientFinished.  One
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          could simply put A~=Spy into the rule, but one should not
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          expect the spy to be well-behaved.*)
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         "[| evsCF: tls;  
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	     Says A  B {|Agent A, Nonce NA, Number SID, Number PA|}
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	       : set evsCF;
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             Says B' A {|Nonce NB, Number SID, Number PB|} : set evsCF;
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             Notes A {|Agent B, Nonce PMS|} : set evsCF;
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	     M = PRF(PMS,NA,NB) |]
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          ==> Says A B (Crypt (clientK(NA,NB,M))
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			(Hash{|Number SID, Nonce M,
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			       Nonce NA, Number PA, Agent A, 
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			       Nonce NB, Number PB, Agent B|}))
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              # evsCF  :  tls"
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    ServerFinished
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	(*Keeping A' and A'' distinct means B cannot even check that the
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          two messages originate from the same source. *)
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         "[| evsSF: tls;
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	     Says A' B  {|Agent A, Nonce NA, Number SID, Number PA|}
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	       : set evsSF;
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	     Says B  A  {|Nonce NB, Number SID, Number PB|} : set evsSF;
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	     Says A'' B (Crypt (pubK B) (Nonce PMS)) : set evsSF;
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	     M = PRF(PMS,NA,NB) |]
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          ==> Says B A (Crypt (serverK(NA,NB,M))
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			(Hash{|Number SID, Nonce M,
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			       Nonce NA, Number PA, Agent A, 
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			       Nonce NB, Number PB, Agent B|}))
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              # evsSF  :  tls"
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    ClientAccepts
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	(*Having transmitted ClientFinished and received an identical
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          message encrypted with serverK, the client stores the parameters
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          needed to resume this session.  The "Notes A ..." premise is
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          used to prove Notes_master_imp_Crypt_PMS.*)
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         "[| evsCA: tls;
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	     Notes A {|Agent B, Nonce PMS|} : set evsCA;
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	     M = PRF(PMS,NA,NB);  
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	     X = Hash{|Number SID, Nonce M,
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	               Nonce NA, Number PA, Agent A, 
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		       Nonce NB, Number PB, Agent B|};
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             Says A  B (Crypt (clientK(NA,NB,M)) X) : set evsCA;
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             Says B' A (Crypt (serverK(NA,NB,M)) X) : set evsCA |]
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          ==> 
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             Notes A {|Number SID, Agent A, Agent B, Nonce M|} # evsCA  :  tls"
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    ServerAccepts
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	(*Having transmitted ServerFinished and received an identical
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          message encrypted with clientK, the server stores the parameters
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          needed to resume this session.  The "Says A'' B ..." premise is
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          used to prove Notes_master_imp_Crypt_PMS.*)
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         "[| evsSA: tls;
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             Says A'' B (Crypt (pubK B) (Nonce PMS)) : set evsSA;
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	     M = PRF(PMS,NA,NB);  
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	     X = Hash{|Number SID, Nonce M,
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	               Nonce NA, Number PA, Agent A, 
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		       Nonce NB, Number PB, Agent B|};
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             Says B  A (Crypt (serverK(NA,NB,M)) X) : set evsSA;
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             Says A' B (Crypt (clientK(NA,NB,M)) X) : set evsSA |]
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          ==> 
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             Notes B {|Number SID, Agent A, Agent B, Nonce M|} # evsSA  :  tls"
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    ClientResume
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         (*If A recalls the SESSION_ID, then she sends a FINISHED message
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           using the new nonces and stored MASTER SECRET.*)
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         "[| evsCR: tls;  
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	     Says A  B {|Agent A, Nonce NA, Number SID, Number PA|}: set evsCR;
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             Says B' A {|Nonce NB, Number SID, Number PB|} : set evsCR;
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             Notes A {|Number SID, Agent A, Agent B, Nonce M|} : set evsCR |]
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          ==> Says A B (Crypt (clientK(NA,NB,M))
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			(Hash{|Number SID, Nonce M,
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			       Nonce NA, Number PA, Agent A, 
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			       Nonce NB, Number PB, Agent B|}))
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              # evsCR  :  tls"
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    ServerResume
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         (*Resumption (7.3):  If B finds the SESSION_ID then he can send
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           a FINISHED message using the recovered MASTER SECRET*)
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         "[| evsSR: tls;
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	     Says A' B {|Agent A, Nonce NA, Number SID, Number PA|}: set evsSR;
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	     Says B  A {|Nonce NB, Number SID, Number PB|} : set evsSR;  
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             Notes B {|Number SID, Agent A, Agent B, Nonce M|} : set evsSR |]
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          ==> Says B A (Crypt (serverK(NA,NB,M))
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			(Hash{|Number SID, Nonce M,
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			       Nonce NA, Number PA, Agent A, 
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			       Nonce NB, Number PB, Agent B|})) # evsSR
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	        :  tls"
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    Oops 
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         (*The most plausible compromise is of an old session key.  Losing
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           the MASTER SECRET or PREMASTER SECRET is more serious but
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           rather unlikely.*)
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         "[| evso: tls;  A ~= Spy;  
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	     Says A B (Crypt (sessionK((NA,NB,M),b)) X) : set evso |]
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          ==> Says A Spy (Key (sessionK((NA,NB,M),b))) # evso  :  tls"
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end