(* Title: HOL/Auth/TLS
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1997 University of Cambridge
Inductive relation "tls" for the baby TLS (Transport Layer Security) protocol.
An RSA cryptosystem is assumed, and X.509v3 certificates are abstracted down
to the trivial form {A, publicKey(A)}privateKey(Server), where Server is a
global signing authority.
A is the client and B is the server, not to be confused with the constant
Server, who is in charge of all public keys.
The model assumes that no fraudulent certificates are present, but it does
assume that some private keys are lost to the spy.
Abstracted from "The TLS Protocol, Version 1.0" by Tim Dierks and Christopher
Allen, Transport Layer Security Working Group, 21 May 1997,
INTERNET-DRAFT draft-ietf-tls-protocol-03.txt
*)
TLS = Public +
consts
(*Client, server write keys. They implicitly include the MAC secrets.*)
clientK, serverK :: "nat*nat*nat => key"
certificate :: "[agent,key] => msg"
defs
certificate_def
"certificate A KA == Crypt (priK Server) {|Agent A, Key KA|}"
rules
(*clientK is collision-free and makes symmetric keys*)
inj_clientK "inj clientK"
isSym_clientK "isSymKey (clientK x)" (*client write keys are symmetric*)
(*serverK is similar*)
inj_serverK "inj serverK"
isSym_serverK "isSymKey (serverK x)" (*server write keys are symmetric*)
(*Clashes with pubK and priK are impossible, but this axiom is needed.*)
clientK_range "range clientK <= Compl (range serverK)"
(*Spy has access to his own key for spoof messages, but Server is secure*)
Spy_in_lost "Spy: lost"
Server_not_lost "Server ~: lost"
consts lost :: agent set (*No need for it to be a variable*)
tls :: event list set
inductive tls
intrs
Nil (*Initial trace is empty*)
"[]: tls"
Fake (*The spy, an active attacker, MAY say anything he CAN say.*)
"[| evs: tls; B ~= Spy;
X: synth (analz (sees lost Spy evs)) |]
==> Says Spy B X # evs : tls"
SpyKeys (*The spy may apply clientK & serverK to nonces he's found*)
"[| evs: tls;
Says Spy B {|Nonce NA, Nonce NB, Nonce M|} : set evs |]
==> Says Spy B {| Key (clientK(NA,NB,M)),
Key (serverK(NA,NB,M)) |} # evs : tls"
ClientHello
(*XA represents CLIENT_VERSION, CIPHER_SUITES and COMPRESSION_METHODS.
It is uninterpreted but will be confirmed in the FINISHED messages.
As an initial simplification, SESSION_ID is identified with NA
and reuse of sessions is not supported.*)
"[| evs: tls; A ~= B; Nonce NA ~: used evs |]
==> Says A B {|Agent A, Nonce NA, Agent XA|} # evs : tls"
ServerHello
(*XB represents CLIENT_VERSION, CIPHER_SUITE and COMPRESSION_METHOD.
Na is returned in its role as SESSION_ID. A CERTIFICATE_REQUEST is
implied and a SERVER CERTIFICATE is always present.*)
"[| evs: tls; A ~= B; Nonce NB ~: used evs;
Says A' B {|Agent A, Nonce NA, Agent XA|} : set evs |]
==> Says B A {|Nonce NA, Nonce NB, Agent XB,
certificate B (pubK B)|}
# evs : tls"
ClientCertKeyEx
(*CLIENT CERTIFICATE and KEY EXCHANGE. M is the pre-master-secret.
Note that A encrypts using the supplied KB, not pubK B.*)
"[| evs: tls; A ~= B; Nonce M ~: used evs;
Says B' A {|Nonce NA, Nonce NB, Agent XB, certificate B KB|}
: set evs |]
==> Says A B {|certificate A (pubK A), Crypt KB (Nonce M)|}
# evs : tls"
CertVerify
(*The optional CERTIFICATE VERIFY message contains the specific
components listed in the security analysis, Appendix F.1.1.2;
it also contains the pre-master-secret. Checking the signature,
which is the only use of A's certificate, assures B of A's presence*)
"[| evs: tls; A ~= B;
Says B' A {|Nonce NA, Nonce NB, Agent XB, certificate B KB|}
: set evs;
Says A B {|certificate A (pubK A), Crypt KB (Nonce M)|}
: set evs |]
==> Says A B (Crypt (priK A)
(Hash{|Nonce NB, certificate B KB, Nonce M|}))
# evs : tls"
(*Finally come the FINISHED messages, confirming XA and XB among
other things. The master-secret is the hash of NA, NB and M.
Either party may sent its message first.*)
ClientFinished
"[| evs: tls; A ~= B;
Says A B {|Agent A, Nonce NA, Agent XA|} : set evs;
Says B' A {|Nonce NA, Nonce NB, Agent XB, certificate B KB|}
: set evs;
Says A B {|certificate A (pubK A), Crypt KB (Nonce M)|}
: set evs |]
==> Says A B (Crypt (clientK(NA,NB,M))
(Hash{|Hash{|Nonce NA, Nonce NB, Nonce M|},
Nonce NA, Agent XA,
certificate A (pubK A),
Nonce NB, Agent XB, Agent B|}))
# evs : tls"
(*Keeping A' and A'' distinct means B cannot even check that the
two messages originate from the same source.*)
ServerFinished
"[| evs: tls; A ~= B;
Says A' B {|Agent A, Nonce NA, Agent XA|} : set evs;
Says B A {|Nonce NA, Nonce NB, Agent XB,
certificate B (pubK B)|}
: set evs;
Says A'' B {|CERTA, Crypt (pubK B) (Nonce M)|} : set evs |]
==> Says B A (Crypt (serverK(NA,NB,M))
(Hash{|Hash{|Nonce NA, Nonce NB, Nonce M|},
Nonce NA, Agent XA, Agent A,
Nonce NB, Agent XB,
certificate B (pubK B)|}))
# evs : tls"
(**Oops message??**)
end