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Internet Engineering Task Force M. Badra
INTERNET DRAFT O. Cherkaoui
UQAM University
I. Hajjeh
Expires: 8, February 2004 A. Serhrouchni
ENST, Paris
August, 10 2004
Pre-Shared-Key key Exchange methods for TLS
<draft-badra-tls-key-exchange-00.txt>
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet
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Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 8, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies new key exchange methods for Transport Layer
Security protocol to support authentication based on pre installed
key and to allow anonymous exchanges, identity protection And
Perfect Forward Secrecy.
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1. Introduction
Transport Layer Security (TLS) [TLS] is an authentication protocol
that establishes a secure channel, as well as mutual authentication,
protected cipher suite negotiation and key exchange between two
entities. TLS handshake uses certificates and PKI for mutual
authentication and key exchange. In many cases, a TLS public-key-
based handshake is unnecessary; especially for closed environments
or for clients pre-configured. This document specifies how to
establish a TLS session using symmetric keys.
Although several Internet Draft authors ([TLSPSK], [TLSSK],
[TSLEXP], etc) propose the pre shared key mechanism, none of them
provides neither anonymous exchanges and identity protection against
eavesdropping nor Perfect Forward Secrecy (PFS). On the other hand,
some approaches like [ISATLS], propose a radical change to the TLS
protocol. Other like [SPTLS], propose Password-based cipher suite
for TLS Handshake scheme.
This document specifies new key exchange methods for TLS for pre
shared key. The advantageous use of the pre shared key regarding the
Public Key Infrastructure (PKI) based certificates is that the pre
shared key reduces the cryptographic operations, the messages load
and the number of round trips.
1.1. Requirements language
The key words "MUST", "SHALL", "SHOULD", and "MAY", in this document
are to be interpreted as described in RFC-2119.
2. Changes to the TLS Handshake protocol
TLS [TLS] defines the client key exchange message that is always
sent by the client. With this message [TLS], the premaster secret is
set, either though direct transmission of the RSA-encrypted secret,
or by the transmission of Diffie-Hellman parameters which will allow
each side to agree upon the same premaster secret. The structure of
this message depends on which key exchange method has been selected.
The actual TLS standard defines two methods using RSA or
Diffie_Hellman algorithms.
The rest of this document describes the changes to the handshake
messages contents when the pre shared key is being used.
2.1. Client Hello
In order to negotiate and to signal to the server that the client
wishes to use a pre_shared_key key exchange method, the client MAY
include an extension of type "psk_key_exchange (9)" in the extended
client hello, such is defined in [TLSEXT]. The "extension_data"
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field of the psk key exchange extension SHALL contain
"PSKKeyExchangeMethod" where:
struct {
PSKMethod psk_methods_list<0..2^16-1>;
} PSKKeyExchangeMethod;
struct {
MethodType method_type;
Select (method_type) {
case rsa_psk : RSAPSK
case diffie_hellman_psk : DHPSK
} method;
} PSKMethod;
enum { rsa_psk(0), diffie_hellmen_psk(1), (255) } MethodType;
Here, "PSKKeyExchangeMethod" provides a list of PSK key exchange
methods that the client supports.
2.3. Server Key Exchange
The format of ServerKeyExchange is as follow:
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case rsa_psk: /*NEW/
ServerRSAParamsPSK params;
Signature signed_params; /*optional/
case diffie_hellman_psk: /*NEW/
ServerDHParamsPSK params;
Signature signed_params;/*optional/
};
} ServerKeyExchange;
rsa_psk and diffie_hellman_psk cases are respectively identical to
rsa and diffie_hellman cases that are definied in [TLS].
Note that because the pre_shared_key SHOULD protect entities against
man-in-the-middle attack (see section 2.4), the server MAY not sign
its Diffie_Hellman parameters and thus the signed_params field MAY
be omitted. For more information, see security considerations
section.
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2.2. Client Key Exchange
This document adds two new key exchange methods to the enumerated
KeyExchangeAlgorithm originally defined in [TLS].
enum {
rsa, diffie_hellman, rsa_psk, diffie_hellman_psk
} KeyExchangeAlgorithm;
Thus, the structure of the client key exchange becomes as follow:
struct {
select (KeyEchangeAlgorithm){
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case rsa_psk: EncryptPreMasterSecretPSK; /*NEW/
case diffie_hellman_psk: ClientDiffieHellmanPublicPSK; /*NEW/
} exchange_key;
} ClientKeyExchange;
2.2.1. rsa_psk encrypted premaster secret message
If rsa_psk is being used for key agreement, the client generates a
30-byte random value, concatenates it with the pre shared key
identity, encrypts the result (premaster secret) using the server
public key and sends it in an encrypted premaster secret message.
Structure of the premaster secret:
struct {
ProtocolVersion client_version;
opaque random[30];
opaque psk_identity<1..2^16-1>;
opaque pad[16-psk_identity.length];
} PreMasterSecret;
struct { public-key-encrypted PreMasterSecret pre_master_secret;
} EncryptedPreMasterSecretPSK;
For interoperation issues, this document uses the same definition
used in [TLSSRP]. Thus, the psk_identity SHALL be UTF-8 encoded
Unicode, where the psk_identity is the pre shared key identifier.
If the psk_identity is less than 16 bytes in length, the premaster
secret will be padded to obtain 46 bytes. For example, if the
psk_identity length is 13 bytes, then the last three bytes of the
premaster secret will be 0x03 0x03 0x03. This mechanism will allow
the server to extract the psk_identity from the premaster secret.
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2.2.2. diffie_hellman_psk encrypted premaster secret message
Because the client does not use any certificate, its value Yc needs
to be sent. As a result, the case implicit MAY be omitted.
struct {
select (PublicValueEncoding) {
case implicit: struct { };
case explicit: opaque dh_Yc<1..2^16-1>;
} dh_public;
opaque psk_identity<1..2^16-1>;
} ClientDiffieHellmanPublicPSK;
dh_Yc
The client's Diffie-Hellman public value (Yc).
psk_identity
The pre shared key identifier.
The psk_identity helps the client to indicate which key it wants to
use and the server to retrieve the corresponding pre shared key
value, if exists. When using a Diffie-Hellman based key exchange
method, the psk_identity is sent in the clear.
2.4. Computing the master secret
This document uses the same mechanism defined in [TLS] for keys
computation and calculation, except the master secret key. It
generates the master secret by applying the PRF on the premaster
secret XOR pre_shared_key value instead of the premaster secret:
master_secret = PRF(pre_master_secret XOR pre_shared_key,
"master_secret",
ClientHello.random + ServerHello.random)[0..47];
As a result, if the server uses a static private key and if this key
is compromised, the intruder must have the pre_shared_key to decrypt
old sessions.
On the other hand, if either the client or the server calculates an
incorrect premaster_secret XOR pre_shared_key value, the finished
messages will fail to decrypt properly and the other party will
return a bad_record_mac alert. This MAY happen when the server does
not send its certificate and that a man-in-the-middle intercepts the
session exchanges and sends its public key instead of the server
public key.
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2.5. Error Alerts
Three new TLS error alerts are defined by this document (This
section is inspired by [TLSSRP]):
a) "unknown_psk_key_exchange" (integer) - this alert MAY be sent by
a server that does not support any PSK key exchange methods sent
by the client. This alert is always a warning. Upon receiving
this alert, the client MAY send a new hello message on the same
connection using another TLS authentication methods.
b) "unknown_psk_identity" (integer) - this alert MAY be sent by a
server that receives an unknown ticket identity. This alert is
always fatal.
c) "missing_psk_identity" (integer) - this alert MAY be sent by a
server that would like to select an offered PSK key exchange
method, if the MethodType extension is absent from the client's
hello message. This alert is always a warning. Upon receiving
this alert, the client MAY send a new hello message on the same
connection, this time including the MethodType extension.
2.6. Handshake
In order to indicate the support of the shared key type, the client
adds the extension "psk_key_exchange (9)" to its extended hello
message.
When the server receives an extended client hello message, it
replies by its hello that contains the following attributes:
Protocol Version, Random, Session ID, Cipher Suite, and Compression
Method.
If the server is able to agree on a key exchange method using the
pre shared key, it will send its server key exchange message that
contains the selected method. In this case, the Certificate message
MAY be omitted from the response.
If the server does not support any PSK key exchange methods sent by
the client, the server MAY abort the handshake with a
unknown_key_exchange alert.
Now the server will send the server hello done message, indicating
that the hello-message phase of the handshake is completed.
The client send its client key exchange message. The content of this
message depends on the method selected between the client hello and
the server key exchange messages.
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The handshake exchange is given in the following diagram:
ClientHello -------->
(MethodType) ServerHello
Certificate*
ServerKeyExchange
<-------- ServerHelloDone
ClientKeyExchange
ChangeCipherSpec
Finished -------->
ChangeCipherSpec
<-------- Finished
* Indicates an optional message which is not always sent.
3. Security considerations
The server MUST stock the shared key in a secure and protected
manner in order to prevent attackers from retrieving its value.
During the handshake phase, the server MAY send its certificate. The
certificate's use protects entities against man-in-the-middle
attack.
If the server certificate is omitted, the client and the server
authenticate each other via the finished messages. In fact, the
finished value is computed using the master_secret calculated during
the establishment session and the pre shared key. Thus, if the
client is intercepted by a bogus server, this later will be
detectable by the client during the finished phase. As a result, no
third party can calculate the same finished value without having the
correct pre_shared_key. Instead, the third party MAY discover the
pre shared key identity sent in the client key exchange message.
When using a Diffie-Hellman based PSK key exchange method, the
client sends its psk_identity in the clear. In order to avoid this
issue, the client could first open a conventional anonymous and then
renegotiate a PSK key exchange method with the handshake protected
by the first connection. Another solution MAY be done using the
pseudonym management.
3.1. Key management with non-human support
In the case where the client does not enter his credentials manually
during the session establishment and that he does not need to
remember them, then he can stock them on a secure token (e.g.
smartcard) or in a local file. In this case, the server and the
client MAY update the pre shared key value after each session has
been formed. In this case, the both MAY add a seed to their
credentials entries. By this method, the client's support and the
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server calculate the seed and update the pre shared key as following
(in the session i):
seed(0) is a random on 16 bytes.
seed(i) = P_MD5(seed(i-1) XOR psk_identity,
"seed" +
ClientHello.random + ServerHello.random)[0..16];
psk(i) = PRF(psk(i-1) XOR premaster secret(i), "pre shared key",
ServerHello.random + ClientHello.random)[0..48];
With this mechanism, the psk_identity remains unchanged. However,
when the client connect to the server, it sends the seed (seed(i-1)
for session i) instead of the psk_identity. The rest of the protocol
is unchangeable. This SHALL ensure, among other, PFS and anonymity.
4. IANA Considerations
To be specified.
5. Acknowledgment
This document has been inspired by [TLS], [TLSSRP] and [TLSPSK].
Thus, it reused extracts of these documents.
6. References
6.1. Normative References
[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwwod, D., Mikkelsen, J.
and Wright, T., "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[TLS] Dierks, T., and Allen, C., "The TLS Protocol Version 1.0",
RFC 2246, November 1998.
[ISATLS] Hajjeh, I., and Serhrouchni, A., "ISAKMP Handshake for
SSL/TLS", IEEE GLOBECOM'03, Vol. 3, San Francisco, USA,
1-5 December 2003, Pages: 1481-1485.
[SPTLS] Steiner, Michael, et. al., "Secure Password-Based Cipher
Suite for TLS", ACM Transaction on Information and System
Security, Vol. 4, No. 2, May 2001, Pages 134-157.
6.2. Informative References
[TLSSRP] Taylor, D., Wu, T., Mavroyanopoulos, N., and Perrin,
T., "Using SRP for TLS Authentication",
draft-ietf-tls-srp-07.txt, Internet Draft (work in
progress), June 2004.
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[TLSPSK] Eronen, P., and Tschofenig, H., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
draft-eronen-tls-psk-00.txt, Internet Draft (work in
progress), August 2004.
[TLSSK] Gutmann, P.,"Use of Shared Keys in the TLS Protocol",
draft-ietf-tls-sharedkeys-02.txt, Internet Draft
(expired), October 2003.
[TSLEXP] Badra, M., Serhrouchni, A., and Urien, P., "TLS Express",
draft-badra-tls-express-00.txt, Internet Draft (work in
progress), June 2004.
6. Author's Addresses
Mohamad Badra
ENST Telecom
46 rue Barrault
75634 Paris Phone: NA
France Email: Mohamad.Badra@enst.fr
Omar Cherkaoui
UQAM University
Montreal (Quebec) Phone: NA
Canada Email: cherkaoui.omar@uqam.ca
Ibrahim Hajjeh
ENST Telecom
46 rue Barrault
75634 Paris Phone: NA
France Email: Ibrahim.Hajjeh@enst.fr
Ahmed Serhrouchni
ENST Telecom
46 rue Barrault
75634 Paris Phone: NA
France Email: Ahmed.Serhrouchni@enst.fr
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Badra, et. al. Expires - February 2005 [Page 10]
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