Internet Engineering Task Force (IETF) P. Eronen
Request for Comments: 5998 Independent
Updates: 5996 H. Tschofenig
Category: Standards Track Nokia Siemens Networks
ISSN: 2070-1721 Y. Sheffer
Independent
September 2010
An Extension for EAP-Only Authentication in IKEv2
Abstract
IKEv2 specifies that Extensible Authentication Protocol (EAP)
authentication must be used together with responder authentication
based on public key signatures. 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 methods other than public
key signatures.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5998.
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Copyright Notice
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1. Introduction
The Extensible Authentication Protocol (EAP), defined in [RFC3748],
is an authentication framework that 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
[RFC1661], IEEE 802 wired switches [IEEE8021X], and IEEE 802.11
wireless access points [IEEE80211i].
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 that may implement some or all
authentication methods.
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IKEv2 ([RFC4306] and [RFC5996]) is a component of IPsec used for
performing mutual authentication and establishing and maintaining
Security Associations (SAs) for IPsec ESP and Authentication Header
(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
Authentication, Authorization, and Accounting (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 these EAP methods must
also be used together with responder authentication based on public
key signatures.
In order for the public key signature authentication of the gateway
to be effective, a deployment of Public Key Infrastructure (PKI) is
required, which has to include management of trust anchors on all
supplicants. In many environments, this is not realistic, and the
security of the gateway public key is the same as the security of a
self-signed certificate. Mutually authenticating EAP methods alone
can provide a sufficient level of security in many circumstances, and
in fact, in some deployments, IEEE 802.11i uses EAP without any PKI
for authenticating the Wireless Local Area Network (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.
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1.1. Terminology
All notation in this protocol extension is taken from [RFC4306].
Numbered messages refer to the IKEv2 message sequence when using EAP.
Thus:
o Message 1 is the request message of IKE_SA_INIT.
o Message 2 is the response message of IKE_SA_INIT.
o Message 3 is the first request of IKE_AUTH.
o Message 4 is the first response of IKE_AUTH.
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].
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).
+------+-----+ +------------+
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
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client and the home AAA server. As a result of a successful EAP
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 [RFC4072] or RADIUS [RFC3579]. See Section 6
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
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.
If the initiator does not wish to use public-key-based responder
authentication, it includes an EAP_ONLY_AUTHENTICATION notification
payload (16417) in message 3. The Protocol ID and Security Parameter
Index (SPI) size fields are set to zero, and there is no additional
data associated with this notification.
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If the responder supports this notification and chooses to use it, it
omits the public-key-based AUTH payload and CERT payloads from
message 4.
If the responder does not support the EAP_ONLY_AUTHENTICATION
notification or does not wish to use it, it ignores the notification
payload, and includes the AUTH payload in message 4. In this case,
the initiator MUST verify that payload and any associated
certificates, as per [RFC4306].
When receiving message 4, the initiator MUST verify that the proposed
EAP method is allowed by this specification, and MUST abort the
protocol immediately otherwise.
Both the initiator and responder MUST verify that the EAP method
actually used provided mutual authentication and established a shared
secret key. The AUTH payloads sent after EAP Success MUST use the
EAP-generated key, and MUST NOT use SK_pi or SK_pr (see Section 2.15
of [RFC5996]).
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An IKEv2 message exchange with this modification is shown below:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni,
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP)] -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ],
[N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP)]
HDR, SK { IDi, [IDr], SAi2, TSi, TSr,
N(EAP_ONLY_AUTHENTICATION),
[CP(CFG_REQUEST)] } -->
<-- HDR, SK { IDr, EAP(Request) }
HDR, SK { EAP(Response) } -->
<-- HDR, SK { EAP(Request) }
HDR, SK { EAP(Response) } -->
<-- HDR, SK { EAP(Success) }
HDR, SK { AUTH } -->
<-- HDR, SK { AUTH, SAr2, TSi, TSr,
[CP(CFG_REPLY] }
Note: all notation in the above protocol sequence and elsewhere in
this specification is as defined in [RFC4306], and see in particular
Sec. 1.2 of [RFC4306] for payload types.
The NAT detection and Configuration payloads are shown for
informative purposes only; they do not change how EAP authentication
works.
An IKE SA that was set up with this extension can be resumed using
the mechanism described in [RFC5723]. However, session resumption
does not change the authentication method. Therefore, during the
IKE_AUTH exchange of the resumed session, this extension MUST NOT be
sent by the initiator.
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4. Safe EAP Methods
EAP methods to be used with this extension MUST have the following
properties:
1. The method provides mutual authentication of the peers.
2. The method is key-generating.
3. The method is resistant to dictionary attacks.
The authors believe that the following EAP methods are secure when
used with the current extension. The list is not inclusive, and
there are likely other safe methods that have not been listed here.
+-------------------------------+-------------------+---------------+
| Method Name | Allows Channel | Reference |
| | Binding? | |
+-------------------------------+-------------------+---------------+
| EAP-SIM | No | [RFC4186] |
| EAP-AKA | Yes | [RFC4187] |
| EAP-AKA' | Yes | [RFC5448] |
| EAP-GPSK | Yes | [RFC5433] |
| EAP-pwd | No | [RFC5931] |
| EAP-EKE | Yes | [EMU-EAP-EKE] |
| EAP-PAX | Yes | [RFC4746] |
| EAP-SAKE | No | [RFC4763] |
| EAP-SRP | No | [EAP-SRP] |
| EAP-POTP (mutual | Yes | [RFC4793] |
| authentication variant) | | |
| EAP-TLS | No | [RFC5216] |
| EAP-FAST | No | [RFC4851] |
| EAP-TTLS | No | [RFC5281] |
+-------------------------------+-------------------+---------------+
The "Allows channel binding?" column denotes protocols where
protected identity information may be sent between the EAP endpoints.
This third, optional property of the method provides protection
against certain types of attacks (see Section 6.2 for an
explanation), and therefore in some scenarios, methods that allow for
channel binding are to be preferred. It is noted that at the time of
writing, even when such capabilities are provided, they are not fully
specified in an interoperable manner. In particular, no RFC
specifies what identities should be sent under the protection of the
channel binding mechanism, or what policy is to be used to correlate
identities at the different layers.
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5. IANA Considerations
This document defines a new IKEv2 Notification Payload type,
EAP_ONLY_AUTHENTICATION, described in Section 3. This payload has
been assigned the type number 16417 from the "Status Types" range.
6. Security Considerations
Security considerations applicable to all EAP methods are discussed
in [RFC3748]. The EAP Key Management Framework [RFC5247] deals with
issues that arise when EAP is used as a part of a larger system.
6.1. 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.
6.2. 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 is terminated 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
[RFC5247] for a more complete discussion of the related security
issues; here we provide only a short summary.
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 Transport Layer Security (TLS).
Some EAP methods do not have any concept of pass-through
authenticator (e.g., Network Access Server (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
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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. A potential solution is described in [EAP-SERVICE], see also
[EMU-AAAPAY].
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 Section 2.3.4 of [RFC4072] and Section 4.3.7 of
[RFC3579] for more discussion.
6.3. 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 wireless LANs, 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.
The rules in Section 4 are designed to avoid this potential
vulnerability.
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6.4. Identities and Authenticated Identities
When using this protocol, each of the peers sends two identity
values:
1. An identity contained in the IKE ID payload.
2. An identity transferred within the specific EAP method's
messages.
(IKEv2 omits the EAP Identity request/response pair, see Section 3.16
of [RFC5996].) The first identity value can be used by the recipient
to route AAA messages and/or to select authentication and EAP types.
But it is only the second identity that is directly authenticated by
the EAP method. The reader is referred to Section 2.16 of [RFC5996]
regarding the need to base IPsec policy decisions on the
authenticated identity. In the context of the extension described
here, this guidance on IPsec policy applies both to the
authentication of the client by the gateway and vice versa.
6.5. 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's 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 responder is
required then EAP methods that offer this functionality have to be
used (see [RFC3748], Section 7.3).
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7. Acknowledgments
This document borrows some text from [RFC3748], [RFC4306], and
[RFC4072]. We would also like to thank Hugo Krawczyk for interesting
discussions about this topic, Dan Harkins, and David Harrington for
their comments.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption",
RFC 5723, January 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
8.2. Informative References
[EAP-SERVICE] Arkko, J. and P. Eronen, "Authenticated Service
Information for the Extensible Authentication Protocol
(EAP)", Work in Progress, October 2005.
[EAP-SRP] Carlson, J., Aboba, B., and H. Haverinen, "EAP SRP-
SHA1 Authentication Protocol", Work in Progress,
July 2001.
[EMU-AAAPAY] Clancy, C., Lior, A., Zorn, G., and K. Hoeper, "EAP
Method Support for Transporting AAA Payloads", Work
in Progress, May 2010.
[EMU-EAP-EKE] Sheffer, Y., Zorn, G., Tschofenig, H., and S. Fluhrer,
"An EAP Authentication Method Based on the EKE
Protocol", Work in Progress, August 2010.
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[IEEE80211i] Institute of Electrical and Electronics Engineers,
"IEEE Standard for Information technology -
Telecommunications and information exchange between
systems - Local and metropolitan area networks -
Specific requirements - Part 11: Wireless Medium
Access Control (MAC) and Physical Layer (PHY)
specifications: Amendment 6: Medium Access Control
(MAC) Security Enhancements", IEEE Standard 802.11i-
2004, July 2004.
[IEEE8021X] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X-2001,
2001.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote
Authentication Dial In User Service) Support For
Extensible Authentication Protocol (EAP)", RFC 3579,
September 2003.
[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter
Extensible Authentication Protocol (EAP) Application",
RFC 4072, August 2005.
[RFC4186] Haverinen, H. and J. Salowey, "Extensible
Authentication Protocol Method for Global System for
Mobile Communications (GSM) Subscriber Identity
Modules (EAP-SIM)", RFC 4186, January 2006.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication
Protocol Method for 3rd Generation Authentication and
Key Agreement (EAP-AKA)", RFC 4187, January 2006.
[RFC4746] Clancy, T. and W. Arbaugh, "Extensible Authentication
Protocol (EAP) Password Authenticated Exchange",
RFC 4746, November 2006.
[RFC4763] Vanderveen, M. and H. Soliman, "Extensible
Authentication Protocol Method for Shared-secret
Authentication and Key Establishment (EAP-SAKE)",
RFC 4763, November 2006.
[RFC4793] Nystroem, M., "The EAP Protected One-Time Password
Protocol (EAP-POTP)", RFC 4793, February 2007.
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[RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
"The Flexible Authentication via Secure Tunneling
Extensible Authentication Protocol Method (EAP-FAST)",
RFC 4851, May 2007.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, March 2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management
Framework", RFC 5247, August 2008.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible
Authentication Protocol Tunneled Transport Layer
Security Authenticated Protocol Version 0 (EAP-
TTLSv0)", RFC 5281, August 2008.
[RFC5433] Clancy, T. and H. Tschofenig, "Extensible
Authentication Protocol - Generalized Pre-Shared Key
(EAP-GPSK) Method", RFC 5433, February 2009.
[RFC5448] Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
Extensible Authentication Protocol Method for 3rd
Generation Authentication and Key Agreement (EAP-
AKA')", RFC 5448, May 2009.
[RFC5931] Harkins, D. and G. Zorn, "Extensible Authentication
Protocol (EAP) Authentication Using Only A Password",
RFC 5931, August 2010.
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Appendix A. Alternative Approaches
In this section, we list alternatives that have been considered
during the work on this document. We concluded that the solution
presented in Section 3 seems to fit better into IKEv2.
A.1. Ignore AUTH Payload at the Initiator
With this 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. A
significant disadvantage is that the EAP method to be used cannot be
selected to take this behavior into account.
The initiator could signal to the responder (using a notification
payload) that it did not verify the first AUTH payload.
A.2. Unauthenticated Public Keys in AUTH Payload (Message 4)
Another 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. A disadvantage here, again, is that the EAP method
selection cannot take into account the incomplete validation of the
responder's certificate.
If using RSA, the overhead of signature verification is quite small,
compared to the g^xy calculation required by the Diffie-Hellman
exchange.
A.3. Using EAP Derived Session Keys for IKEv2
It has been proposed that when using an EAP method that provides
mutual authentication and key agreement, the IKEv2 Diffie-Hellman
exchange could also be omitted. This would mean that the session
keys for IPsec SAs established later would rely only on EAP-provided
keys.
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It seems the only benefit of this approach is saving some computation
time (g^xy calculation). This approach 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.
Authors' Addresses
Pasi Eronen
Independent
EMail: pe@iki.fi
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
EMail: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Yaron Sheffer
Independent
EMail: yaronf.ietf@gmail.com
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RFC, FYI, BCP