TLS Working Group Y. Nir
Internet-Draft Y. Sheffer
Intended status: Standards Track Check Point
Expires: August 25, 2007 H. Tschofenig
Siemens
February 21, 2007
Protocol Model for TLS with EAP Authentication
draft-nir-tee-pm-00.txt
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Abstract
This document describes an extension to the TLS protocol to allow TLS
clients to authenticate with legacy credentials using the Extensible
Authentication Protocol (EAP).
This work follows the example of IKEv2, where EAP has been added to
the IKEv2 protocol to allow clients to use different credentials such
as passwords, token cards, and shared secrets.
When TLS is used with EAP, additional records are sent after the
ChangeCipherSpec protocol message, effectively creating an extended
handshake before the application layer data can be sent. Each EapMsg
handshake record contains exactly one EAP message. Using EAP for
client authentication allows TLS to be used with various AAA back-end
servers such as RADIUS or Diameter.
TLS with EAP may be used for securing a data connection such as HTTP
or POP3, where the ability of EAP to work with backend servers can
remove that burden from the application layer.
This document is a protocol model, rather than a full protocol
specification.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions Used in This Document . . . . . . . . . . . . 5
2. Operating Environment . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7
3.1. The tee_supported Extension . . . . . . . . . . . . . . . 8
3.2. The InterimAuth Handshake Message . . . . . . . . . . . . 8
3.3. The EapMsg Handshake Message . . . . . . . . . . . . . . . 8
3.4. Calculating the Finished message . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
4.1. InterimAuth vs. Finished . . . . . . . . . . . . . . . . . 10
4.2. Identity Protection . . . . . . . . . . . . . . . . . . . 10
5. Performance Considerations . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
This document describes a new extension to [TLS]. This extension
allows a TLS client to authenticate using [EAP] instead of using a
certificate, or alternatively performing the authentication at the
application level. The extension follows [TLS-EXT]. For the
remainder of this document we will refer to this extension as TEE
(TLS with EAP Extension). The document is a protocol model as
described in [RFC4101].
TEE extends the TLS handshake beyond the regular setup, to allow the
EAP protocol to run between the TLS server (called an "authenticator"
in EAP) and the TLS client (called a "supplicant"). This allows the
TLS architecture to handle client authentication before exposing the
server application software to an unauthenticated client. In doing
this, we follow the approach taken for IKEv2 in [IKEv2]. However,
similar to regular TLS, we protect the user identity by only sending
the client identity after the server has authenticated. In this our
solution defers from that of IKEv2.
Currently used applications use TLS to authenticate the server only.
After that, the application takes over, and presents a login screen
where the user is expected to present their credentials.
This creates several problems. It allows a client to access the
application before authentication, thus creating a potential for
anonymous attacks on non-hardened applications. Additionally, web
pages are not particularly well suited for long shared secrets and
for certain devices such as USB tokens.
TEE allows full mutual authentication to occur for all these
applications within the TLS exchange. The application receives
control only when the user is identified and authenticated. The
authentication can be built into the server infrastructure by
connecting to an AAA server. The client side can be integrated into
client software such as web browsers and mail clients. An EAP
infrastructure is already built-in to some operating systems
providing a user interface for each authentication method within EAP.
We intend TEE to be used for various protocols that use TLS such as
HTTPS, in cases where certificate based authentication is not
practical. This includes web-based mail services, online banking,
premium content websites and mail clients.
Another class of applications that may see benefit from TEE are TLS
based VPN clients used as part of so-called "SSL VPN" products. No
such client protocols have so far been standardized.
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1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Operating Environment
TEE will work between a client application and a server application,
taking care of all encryption and authentication.
Client Server
+-------------------------+ +------------------------+
| |GUI| | Client | |TLS+-+-----+-+TLS| |Server | |
| +-^-+ |Software| +-^-+ | +-+-^-+ |Application | |
| | +--------+ | | | | |Software | |
| | | | | | +------------+ |
| +-v----------------v-+ | | | |
| | EAP | | +---|--------------------+
| | Infrastructure | | |
| +--------------------+ | | +--------+
+-------------------------+ | | AAA |
| | Server |
+----- |
+--------+
The above diagram shows the typical deployment. The client has
software that either includes a UI for some EAP methods, or else is
able to invoke some operating system EAP infrastructure that takes
care of the user interaction. The server is configured with the
address and protocol of the AAA server. Typically the AAA server
communicates using the RADIUS protocol with EAP ([RADIUS] and
[RAD-EAP]), or the Diameter protocol ([Diameter] and [Dia-EAP]).
As stated in the introduction, we expect TEE to be used in both
browsers and applications. Further uses may be authentication and
key generation for other protocols, and tunneling clients, which so
far have not been standardized.
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3. Protocol Overview
The TEE extension defines the following:
o A new extension type called tee_supported, used to indicate that
the client supports this extension.
o A new message type for the handshake protocol, called InterimAuth,
which is used to sign previous messages.
o A new message type for the handshake protocol, called EapMsg,
which is used to carry a single EAP message.
The diagram below outlines the protocol structure. For illustration
purposes only, we use the [I-D.dpotter-pppext-eap-mschap] EAP method
.
Client Server
------ ------
ClientHello(*) -------->
ServerHello(*)
(Certificate)
ServerKeyExchange
EapMsg(Identity-Request)
<-------- ServerHelloDone
ClientKeyExchange
(CertificateVerify)
ChangeCipherSpec
InterimAuth
EapMsg(Identity-Reply) -------->
ChangeCipherSpec
InterimAuth
EapMsg(MS-CHAP-v2-Request)
<--------
EapMsg(MS-CHAP-v2-Reply) -------->
EapMsg(Success)
<-------- Finished
Finished -------->
(*) The ClientHello and ServerHello include the tee_supported
extension to indicate support for TEE
The client indicates in the first message its support for TEE. The
server sends an EAP identity request in the reply. The client sends
the identity reply after the handshake completion. The EAP request-
response sequence continues until the client is either authenticated
or rejected.
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3.1. The tee_supported Extension
The tee_supported extension is a ClientHello and ServerHello
extension as defined in section 2.3 of [TLS-EXT]. The extension_type
field is TBA by IANA. The extension_data is zero-length.
3.2. The InterimAuth Handshake Message
The InterimAuth message is identical in syntax to the Finished
message described in section 7.4.9 of [TLS]. It is calculated in
exactly the same way.
The semantics, however, are somewhat different. The "Finished"
message indicates that application data may now be sent. The
"InterimAuth" message does not indicate this. Instead, further
handshake messages are needed.
Depending on the EAP method used, the Finished message may be
calculated differently. See Section 3.4 for details.
The HandshakeType value for the InterimAuth handshake message is TBA
by IANA.
3.3. The EapMsg Handshake Message
The EapMsg handshake message carries exactly one EAP message as
defined in [EAP].
The HandshakeType value for the EapMsg handshake message is TBA by
IANA.
The EapMsg message is used to tunnel EAP messages between the
authentication server, which may be the co-located with the TLS
server, or may be a separate AAA server, and the supplicant, which is
co-located with the TLS client. TLS on either side receives the EAP
data from the EAP infrastructure, and treats it as opaque. TLS does
not make any changes to the EAP payload or make any decisions based
on the contents of an EapMsg handshake message.
3.4. Calculating the Finished message
If the EAP method is key-generating, the Finished message is
calculated as follows:
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struct {
opaque verify_data[12];
} Finished;
verify_data
PRF(MSK, finished_label, MD5(handshake_messages) +
SHA-1(handshake_messages)) [0..11];
The finished_label is defined exactly as in section 7.4.9 of [TLS].
The handshake_messages, similar to regular TLS is all of the data
from all messages in this handshake, including any EapMsg and
InterimAuth messages, up to but not including this Finished message.
This is the concatenation of all the Handshake structures, as defined
in section 7.4 of [TLS] and here, exchanged thus far.
The MSK is typically received from the AAA server over the RADIUS or
Diameter protocol.
If the EAP method is not key-generating, then the Finished message is
calculated exactly as described in [TLS]. Such methods however, are
NOT RECOMMENDED. See Section 4.1 for details.
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4. Security Considerations
4.1. InterimAuth vs. Finished
In regular TLS, the Finished message provides two functions: it signs
all previous messages, and it signals that application data can now
be used. In TEE, we sign the previous messages twice.
Some EAP methods, such as EAP-TLS, EAP-IKEv2 and EAP-SIM generate
keys in addition to authenticating clients. Such methods are said to
be resistant to MITM attacks as discussed in [MITM]. Such methods
are called key-generating methods.
To realize the benefit of such methods, we need to verify the key
that was generated within the EAP method. This is referred to as the
MSK in EAP. In TEE, the InterimAuth message signs all previous
messages with the master_secret, just like the Finished message in
regular TLS. The Finished message signs all previous messages using
the MSK if such exists. If not, then the messages are signed with
the master_secret as in regular TLS.
The need for signing twice arises from the fact that we need to use
both the master_secret and the MSK. It was possible to use just one
Finished record and blend the MSK into the master_secret. However,
this would needlessly complicate the protocol and make security
analysis more difficult. Instead, we have decided to follow the
example of IKEv2, where two AUTH payloads are exchanged.
It should be noted that using non-key-generating methods may expose
the client to a MITM attack if the same MITM method is used in some
other situation, in which the EAP is done outside of a protected
tunnel with an authenticated server. Unless it can be determined
that the EAP method is never used in such a situation, non-key-
generating methods SHOULD NOT be used.
4.2. Identity Protection
Unlike [TLS-PSK], TEE provides identity protection for the client.
The client's identity is hidden from a passive eavesdropper using TLS
encryption, and it is not sent to the server until after the server's
identity has been authenticated by verifying the certificate.
Active attacks are thwarted by the server authentication using a
certificate or by using a suitable EAP method.
We could save one round-trip by having the client send its identity
within the Client Hello message. This is similar to TLS-PSK.
However, we believe that identity protection is a worthy enough goal,
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so as to justify the extra round-trip.
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5. Performance Considerations
Regular TLS adds two round-trips to a TCP connection. However,
because of the stream nature of TCP, the client does not really need
to wait for the server's Finished message, and can begin sending
application data immediately after its own Finished message. In
practice, many clients do so, and TLS only adds one round-trip of
delay.
TEE adds as many round-trips as the EAP method requires. For
example, EAP-MD5 requires 1 round-trip, while EAP-SIM requires 2
round-trips. Additionally, the client MUST wait for the EAP-Success
message before sending its own Finished message, so we need at least
3 round-trips for the entire handshake. The best a client can do is
two round-trips plus however many round-trips the EAP method
requires.
It should be noted, though, that these extra round-trips save
processing time at the application level. Two extra round-trips take
a lot less time than presenting a log-in web page and processing the
user's input.
It should also be noted, that TEE reverses the order of the Finished
messages. In regular TLS the client sends the Finished message
first. In TEE it is the server that sends the Finished message
first. This should not affect performance, and it is clear that the
client may send application data immediately after the Finished
message.
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6. IANA Considerations
IANA is asked to assign an extension type value from the
"ExtensionType Values" registry for the tee_supported extension.
IANA is asked to assign two handshake message types from the "TLS
HandshakeType Registry", one for "EapMsg" and one for "InterimAuth".
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7. Acknowledgments
The TLS Innel Application Extension work ([TLS/IA]) has inspired the
authors to create this simplified work. TLS/IA provides a somewhat
different approach to integrating non-certificate credentials into
the TLS protocol, in addition to several other features available
from the RADIUS namespace.
The authors would also like to thanks the various contributors to
[IKEv2] whose work inspired this one.
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8. References
8.1. Normative References
[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
8.2. Informative References
[Dia-EAP] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[Diameter]
Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[I-D.dpotter-pppext-eap-mschap]
Potter, D. and J. Zamick, "PPP EAP MS-CHAP-V2
Authentication Protocol",
draft-dpotter-pppext-eap-mschap-01 (work in progress),
January 2002.
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[MITM] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunneled Authentication Protocols", October 2002.
[RAD-EAP] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RADIUS] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
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[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005.
[TLS-PSK] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
[TLS/IA] Funk, P., Blake-Wilson, S., Smith, H., Tschofenig, N., and
T. Hardjono, "TLS Inner Application Extension (TLS/IA)",
draft-funk-tls-inner-application-extension-03 (work in
progress), June 2006.
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Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
3A Jabotinsky St.
Ramat Gan 52520
Israel
Email: ynir@checkpoint.com
Yaron Sheffer
Check Point Software Technologies Ltd.
3A Jabotinsky St.
Ramat Gan 52520
Israel
Email: yaronf at checkpoint dot com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
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