IPSEC P. Eronen
Internet-Draft Nokia
Expires: August 9, 2004 H. Tschofenig
Siemens
February 9, 2004
Extension for EAP Authentication in IKEv2
draft-eronen-ipsec-ikev2-eap-auth-00.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
IKEv2 specifies that EAP authentication must be used together with
public key signature based responder authentication. This is
necessary with old EAP methods that provide only unilateral
authentication using e.g. one-time passwords or token cards.
This document specifies how EAP methods that provide mutual
authentication and key agreement can be used to provide extensible
responder authentication for IKEv2 based on other methods than
public-key signatures.
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [7], is an
authentication framework which supports multiple authentication
mechanisms. Today, EAP has been implemented at end hosts and routers
that connect via switched circuits or dial-up lines using PPP [16],
IEEE 802 wired switches [10], and IEEE 802.11 wireless access points
[12].
One of the advantages of the EAP architecture is its flexibility. EAP
is used to select a specific authentication mechanism, typically
after the authenticator requests more information in order to
determine the specific authentication method to be used. Rather than
requiring the authenticator (e.g., wireless LAN access point) to be
updated to support each new authentication method, EAP permits the
use of a backend authentication server which may implement some or
all authentication methods.
IKEv2 [3] is a component of IPsec used for performing mutual
authentication and establishing and maintaining security associations
for IPsec ESP and AH. In addition to supporting authentication using
public key signatures and shared secrets, IKEv2 also supports EAP
authentication.
IKEv2 provides EAP authentication since it was recognized that public
key signatures and shared secrets are not flexible enough to meet the
requirements of many deployment scenarios. By using EAP, IKEv2 can
leverage existing authentication infrastructure and credential
databases, since EAP allows users to choose a method suitable for
existing credentials, and also makes separation of the IKEv2
responder (VPN gateway) from the EAP authentication endpoint (backend
AAA server) easier.
Some older EAP methods are designed for unilateral authentication
only (that is, EAP peer to EAP server). These methods are used in
conjunction with IKEv2 public key based authentication of the
responder to the initiator. It is expected that this approach is
especially useful for "road warrior" VPN gateways that use, for
instance, one-time passwords or token cards to authenticate the
clients.
However, most newer EAP methods, such as those typically used with
IEEE 802.11i wireless LANs, provide mutual authentication and key
agreement. Currently, IKEv2 specifies that also these EAP methods
must be used together with public key signature based responder
authentication.
In some environments, requiring the deployment of PKI for just this
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purpose can be counterproductive. Deploying new infrastructure can be
expensive, and it may weaken security by creating new
vulnerabilities. Mutually authenticating EAP methods alone can
provide a sufficient level of security in many circumstances, and
indeed, IEEE 802.11i uses EAP without any PKI for authenticating the
WLAN access points.
This document specifies how EAP methods that offer mutual
authentication and key agreement can be used to provide responder
authentication in IKEv2 completely based on EAP.
1.1 Terminology
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 [2].
2. Scenarios
In this section we describe two scenarios for extensible
authentication within IKEv2. These scenarios are intended to be
illustrative examples rather than specifying how things should be
done.
Figure 1 shows a configuration where the EAP and the IKEv2 endpoints
are co-located. Authenticating the IKEv2 responder using both EAP and
public-key signatures is redundant. Offering EAP based authentication
has the advantage that multiple different authentication and key
exchange protocols are available with EAP with different security
properties (such as strong password based protocols, protocols
offering user identity confidentiality and many more). As an example
it is possible to use GSS-API support within EAP [5] to support
Kerberos based authentication which effectively replaces the need for
KINK [17].
+------+-----+ +------------+
O | IKEv2 | | IKEv2 |
/|\ | Initiator |<---////////////////////--->| Responder |
/ \ +------------+ IKEv2 +------------+
User | EAP Peer | Exchange | EAP Server |
+------------+ +------------+
Figure 1: EAP and IKEv2 endpoints are co-located
Figure 2 shows a typical corporate network access scenario. The
initiator (client) interacts with the responder (VPN gateway) in the
corporate network. The EAP exchange within IKE runs between the
client and the home AAA server. As a result of a successful EAP
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authentication protocol run, session keys are established and sent
from the AAA server to the VPN gateway, and then used to authenticate
the IKEv2 SA with AUTH payloads.
The protocol used between the VPN gateway and AAA server could be,
for instance, Diameter [7] or RADIUS [4]. See Section 5 for related
security considerations.
+-------------------------------+
| Corporate network |
| |
+-----------+ +--------+ |
| IKEv2 | AAA | Home | |
IKEv2 +////----->+ Responder +<---------->+ AAA | |
Exchange / | (VPN GW) | (RADIUS/ | Server | |
/ +-----------+ Diameter) +--------+ |
/ | carrying EAP |
| | |
| +-------------------------------+
v
+------+-----+
o | IKEv2 |
/|\ | Initiator |
/ \ | VPN client |
User +------------+
Figure 2: Corporate Network Access
3. Solution Approaches
IKEv2 specifies that when the EAP method establishes a shared secret
key, that key is used by both the initiator and responder to generate
an AUTH payload (thus authenticating the IKEv2 SA set up by messages
1 and 2).
When used together with public-key responder authentication, the
responder is in effect authenticated using two different methods: the
public-key signature AUTH payload in message #4, and the EAP-based
AUTH payload later.
In this section we explore some possibilities for relaxing this. The
first three approaches require only small modifications to IKEv2.
One approach is more aggressive and only shown for completeness.
It is TBD which of this approaches should be used.
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3.1 Ignore AUTH payload at the initiator
In the first approach, the initiator simply ignores the AUTH payload
in message #4 (but obviously must check the second AUTH payload
later!). The main advantage of this approach is that no protocol
modifications are required and no signature verification is required.
It is TBD whether the initiator should signal the responder (using a
NOTIFY payload) that it did not in fact verify the first AUTH
payload.
3.2 Unauthenticated PKs in AUTH payload (message 4)
The first solution approach suggests the use of unauthenticated
public keys in the public key signature AUTH payload (for message 4).
That is, the initiator verifies the signature in the AUTH payload,
but does not verify that the public key indeed belongs to the
intended party (using certificates)--since it doesn't have a PKI that
would allow this. This could be used with X.509 certificates (the
initiator ignores all other fields of the certificate except the
public key), or "Raw RSA Key" CERT payloads.
This approach has the advantage that initiators that wish to perform
certificate-based responder authentication (in addition to EAP) may
do so, without requiring the responder to handle these cases
separately.
If using RSA, the overhead of signature verification is quite small
(compared to g^xy calculation).
3.3 Omit AUTH payload (message 4)
With this solution approach the AUTH payload from message 4 is
completely omitted.
If the public key is not authenticated, it seems the AUTH payload in
message 4 serves no useful purpose, since other AUTH payloads, based
on the EAP-generated key, are sent later in both directions.
With this approach, the responder needs to know when it can omit the
payload (EAP-only authentication is sufficient) and when public-key
authentication is also needed. This could be determined by the
responder identity chosen, or alternatively, the initiator could use
a NOTIFY payload (e.g., EAP_ONLY_AUTHENTICATION_SUPPORTED) to signal
the responder that it can leave out the AUTH payload if it wishes. If
the initiator includes this payload in message #3 then the responder
would know that the initiator does not require public-key
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authentication.
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK { IDi, [IDr,], EAP_ONLY_AUTHENTICATION_SUPPORTED,
SAi2, TSi, TSr} -->
<-- HDR, SK { IDr, EAP(Request) }
HDR, SK { EAP(Response) } -->
<-- HDR, SK { EAP(Request) }
HDR, SK { EAP(Response) } -->
<-- HDR, SK { EAP(Success), AUTH }
HDR, SK { AUTH } -->
<-- HDR, SK { SAr2, TSi, TSr }
Note that there is currently discussion about which messages should
contain the AUTH payloads. The current IKEv2 specification says that
they are included "..in the first message each end sends after having
sufficient information to compute the key", but this might need some
clarification. Therefore, the sequence shown above may need revision
after this is settled.
3.4 Use EAP derived session keys for IKEv2
It has been proposed that when using an EAP methods that provides
mutual authentication and key agreement, the IKEv2 Diffie-Hellman
exchange could also be omitted. This would mean that the sessions
keys for IPsec SAs established later would rely only on EAP-provided
keys.
It seems the only benefit of this approach is saving some computation
time (g^xy calculation). However, since this approaches requires
designing a completely new protocol (which would not resemble IKEv2
anymore) we do not believe that it should be considered.
Nevertheless, we include it for completeness.
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3.5 Discussion
Currently it seems that options 1-3 have approximately the same
security properties. The last option has somewhat weaker security,
since it does not provide PFS against e.g. compromise of an AAA proxy
(however, we have not analyzed option 4 in detail).
It seems that the approach where (1) the initiator uses a NOTIFY
payload to signal the responder that it does not require the AUTH
payload, and (2) if the responder includes it anyway, the initiator
ignores it, might be a good choice. However, more discussion is
required before deciding.
4. IANA considerations
A new NOTIFY message type might be required; details are TBD.
5. Security Considerations
Security considerations applicable to all EAP methods are discussed
in [1]. The EAP Key Management Framework [6] deals with issues that
arise when EAP is used as a part of a larger system.
We believe that the security issues associated with all of the
alternatives 1-3 in Section 3 are approximately the same (TBD: will
be clarified in future versions).
5.1 Authentication of IKEv2 SA
It is important to note that the IKEv2 SA is not authenticated by
just running an EAP conversation: the crucial step is the AUTH
payload based on the EAP-generated key. Thus, EAP methods that do not
provide mutual authentication or establish a shared secret key MUST
NOT be used with the modifications presented in this document.
5.2 Authentication with separated IKEv2 responder/EAP server
As described in Section 2, the EAP conversation can terminate either
at the IKEv2 responder or at a backend AAA server.
If the EAP method terminates at the IKEv2 responder then no key
transport via the AAA infrastructure is required. Pre-shared secret
and public key based authentication offered by IKEv2 is then replaced
by a wider range of authentication and key exchange methods.
However, typically EAP will be used with a backend AAA server. See
[6] for a more complete discussion of the related security issues;
here we provide only a short summary.
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When a backend server is used, there are actually two authentication
exchanges: the EAP method between the client and the AAA server, and
another authentication between the AAA server and IKEv2 gateway. The
AAA server authenticates the client using the selected EAP method,
and they establish a session key. The AAA server then sends this key
to the IKEv2 gateway over a connection authenticated using e.g. IPsec
or TLS.
Some EAP methods do not have any concept of pass-through
authenticator (e.g. NAS or IKEv2 gateway) identity, and these two
authentications remain quite independent of each other. That is,
after the client has verified the AUTH payload sent by the IKEv2
gateway, it knows that it is talking to SOME gateway trusted by the
home AAA server, but not which one. The situation is somewhat similar
if a single cryptographic hardware accelerator, containing a single
private key, would be shared between multiple IKEv2 gateways (perhaps
in some kind of cluster configuration). In particular, if one of the
gateways is compromised, it can impersonate any of the other gateways
towards the user (until the compromise is discovered and access
rights revoked).
In some environments it is not desirable to trust the IKEv2 gateways
this much (also known as the "Lying NAS Problem"). EAP methods that
provide what is called "connection binding" or "channel binding"
transport some identity or identities of the gateway (or WLAN access
point/NAS) inside the EAP method. Then the AAA server can check that
it is indeed sending the key to the gateway expected by the client.
In some deployment configurations, AAA proxies may be present between
the IKEv2 gateway and the backend AAA server. These AAA proxies MUST
be trusted for secure operation, and therefore SHOULD be avoided when
possible; see [7] and [6] for more discussion.
5.3 Protection of EAP payloads
Although the EAP payloads are encrypted and integrity protected with
SK_e/SK_a, this does not provide any protection against active
attackers. Until the AUTH payload has been received and verified, a
man-in-the-middle can change the KEi/KEr payloads and eavesdrop or
modify the EAP payloads.
In IEEE 802.11i WLANs, the EAP payloads are neither encrypted nor
integrity protected (by the link layer), so EAP methods are typically
designed to take that into account.
In particular, EAP methods that are vulnerable to dictionary attacks
when used in WLANs are still vulnerable (to active attackers) when
run inside IKEv2.
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5.4 User identity confidentiality
IKEv2 provides confidentiality for the initiator identity against
passive eavesdroppers, but not against active attackers. The
initiator announces its identity first (in message #3), before the
responder has been authenticated. The usage of EAP in IKEv2 does not
change this situation, since the ID payload in message #3 is used
instead of the EAP Identity Request/Response exchange. This is
somewhat unfortunate since when EAP is used with public-key
authentication of the responder, it would be possible to provide
active user identity confidentiality for the initiator.
IKEv2 protects the responder identity even against active attacks.
This property cannot be provided when using EAP. If public key
responder authentication is used in addition to EAP, the responder
reveals its identity before authenticating the initiator. If only EAP
is used (as proposed in this document), the situation depends on the
EAP method used (in some EAP methods, the server reveals its identity
first).
Hence, if active user identity confidentiality for the initiator is
required then EAP methods that offer this functionality have to be
used (see [1], Section 7.3).
6. Acknowledgments
This document borrows some text from [1], [3], and [7].
Normative References
[1] Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
draft-ietf-eap-rfc2284bis-07 (work in progress), December 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[3] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-12 (work in progress), January 2004.
Informative References
[4] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
In User Service) Support For Extensible Authentication Protocol
(EAP)", RFC 3579, September 2003.
[5] Aboba, B. and D. Simon, "EAP GSS Authentication Protocol",
draft-aboba-pppext-eapgss-12 (work in progress), April 2002.
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[6] Aboba, B., Simon, D., Arkko, J. and H. Levkowetz, "EAP Key
Management Framework", draft-ietf-eap-keying-01 (work in
progress), October 2003.
[7] Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application",
draft-ietf-aaa-eap-03 (work in progress), October 2003.
[8] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H. and A. Yegin,
"Protocol for Carrying Authentication for Network Access
(PANA)", draft-ietf-pana-pana-02 (work in progress), October
2003.
[9] Funk, P. and S. Blake-Wilson, "EAP Tunneled TLS Authentication
Protocol (EAP-TTLS)", draft-ietf-pppext-eap-ttls-03 (work in
progress), August 2003.
[10] Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access Control",
IEEE Standard 802.1X-2001, 2001.
[11] Institute of Electrical and Electronics Engineers, "Information
technology - Telecommunications and information exchange
between systems - Local and metropolitan area networks -
Specific Requirements Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications", IEEE
Standard 802.11-1999, 1999.
[12] Institute of Electrical and Electronics Engineers, "Draft
Amendment to STANDARD FOR Telecommunications and Information
Exchange between Systems - LAN/MAN Specific Requirements - Part
11: Wireless Medium Access Control (MAC) and physical layer
(PHY) specifications: Medium Access Control (MAC) Security
Enhancements", IEEE Draft 802.11i/D7.0, 2003.
[13] Josefsson, S., Palekar, A., Simon, D. and G. Zorn, "Protected
EAP Protocol (PEAP)", draft-josefsson-pppext-eap-tls-eap-07
(work in progress), October 2003.
[14] Puthenkulam, J., "The Compound Authentication Binding Problem",
draft-puthenkulam-eap-binding-04 (work in progress), October
2003.
[15] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[16] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
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1661, July 1994.
[17] Thomas, M. and J. Vilhuber, "Kerberized Internet Negotiation of
Keys (KINK)", draft-ietf-kink-kink-05 (work in progress),
January 2003.
[18] Tschofenig, H., Kroeselberg, D. and Y. Ohba, "EAP IKEv2 Method
(EAP-IKEv2)", draft-tschofenig-eap-ikev2-02 (work in progress),
October 2003.
Authors' Addresses
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
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