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Internet Engineering Task Force (IETF)                         P. EronenRequest for Comments: 5998                                   IndependentUpdates:5996                                              H. TschofenigCategory: Standards Track                         Nokia Siemens NetworksISSN: 2070-1721                                               Y. Sheffer                                                             Independent                                                          September 2010An Extension for EAP-Only Authentication in IKEv2Abstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5998.Eronen, et al.               Standards Track                    [Page 1]

RFC 5998               Extension for EAP in IKEv2         September 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.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.Eronen, et al.               Standards Track                    [Page 2]

RFC 5998               Extension for EAP in IKEv2         September 2010   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.Eronen, et al.               Standards Track                    [Page 3]

RFC 5998               Extension for EAP in IKEv2         September 20101.1.  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 theEronen, et al.               Standards Track                    [Page 4]

RFC 5998               Extension for EAP in IKEv2         September 2010   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].  SeeSection 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 Access3.  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.Eronen, et al.               Standards Track                    [Page 5]

RFC 5998               Extension for EAP in IKEv2         September 2010   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 (seeSection 2.15   of [RFC5996]).Eronen, et al.               Standards Track                    [Page 6]

RFC 5998               Extension for EAP in IKEv2         September 2010   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.Eronen, et al.               Standards Track                    [Page 7]

RFC 5998               Extension for EAP in IKEv2         September 20104.  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 (seeSection 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.Eronen, et al.               Standards Track                    [Page 8]

RFC 5998               Extension for EAP in IKEv2         September 20105.  IANA Considerations   This document defines a new IKEv2 Notification Payload type,   EAP_ONLY_AUTHENTICATION, described inSection 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 inSection 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.  TheEronen, et al.               Standards Track                    [Page 9]

RFC 5998               Extension for EAP in IKEv2         September 2010   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; seeSection 2.3.4 of [RFC4072] andSection 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 inSection 4 are designed to avoid this potential   vulnerability.Eronen, et al.               Standards Track                   [Page 10]

RFC 5998               Extension for EAP in IKEv2         September 20106.4.  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, seeSection 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 toSection 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).Eronen, et al.               Standards Track                   [Page 11]

RFC 5998               Extension for EAP in IKEv2         September 20107.  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.  References8.1.  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.Eronen, et al.               Standards Track                   [Page 12]

RFC 5998               Extension for EAP in IKEv2         September 2010   [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.Eronen, et al.               Standards Track                   [Page 13]

RFC 5998               Extension for EAP in IKEv2         September 2010   [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.Eronen, et al.               Standards Track                   [Page 14]

RFC 5998               Extension for EAP in IKEv2         September 2010Appendix 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 inSection 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.Eronen, et al.               Standards Track                   [Page 15]

RFC 5998               Extension for EAP in IKEv2         September 2010   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.comEronen, et al.               Standards Track                   [Page 16]