Network Working Group P. Eronen

Network Working Group P. Eronen
Request for Comments: 4739 Nokia
Category: Experimental J. Korhonen
TeliaSonera
November 2006

Multiple Authentication Exchanges
in the Internet Key Exchange (IKEv2) Protocol

Status of This Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2006).

Abstract

The Internet Key Exchange (IKEv2) protocol supports several
mechanisms for authenticating the parties, including signatures with
public-key certificates, shared secrets, and Extensible
Authentication Protocol (EAP) methods. Currently, each endpoint uses
only one of these mechanisms to authenticate itself. This document
specifies an extension to IKEv2 that allows the use of multiple
authentication exchanges, using either different mechanisms or the
same mechanism. This extension allows, for instance, performing
certificate-based authentication of the client host followed by an
EAP authentication of the user. When backend authentication servers
are used, they can belong to different administrative domains, such
as the network access provider and the service provider.

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Table of Contents

1. Introduction ....................................................3
1.1. Usage Scenarios ............................................4
1.2. Terminology ................................................5
2. Solution ........................................................5
2.1. Solution Overview ..........................................5
2.2. Example 1: Multiple EAP Authentications ....................6
2.3. Example 2: Mixed EAP and Certificate Authentications .......7
2.4. Example 3: Multiple Initiator Certificates .................8
2.5. Example 4: Multiple Responder Certificates .................8
3. Payload Formats .................................................9
3.1. MULTIPLE_AUTH_SUPPORTED Notify Payload .....................9
3.2. ANOTHER_AUTH_FOLLOWS Notify Payload ........................9
4. IANA Considerations .............................................9
5. Security Considerations .........................................9
6. Acknowledgments ................................................10
7. References .....................................................10
7.1. Normative References ......................................10
7.2. Informative References ....................................10

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1. Introduction

IKEv2 [IKEv2] supports several mechanisms for parties involved in the
IKE_SA (IKE security association). These include signatures with
public-key certificates, shared secrets, and Extensible
Authentication Protocol (EAP) methods.

Currently, each endpoint uses only one of these mechanisms to
authenticate itself. However, there are scenarios where making the
authorization decision in IKEv2 (whether to allow access or not)
requires using several of these methods.

For instance, it may be necessary to authenticate both the host
(machine) requesting access, and the user currently using the host.
These two authentications would use two separate sets of credentials
(such as certificates and associated private keys) and might even use
different authentication mechanisms.

To take another example, when an operator is hosting a Virtual
Private Network (VPN) gateway service for a third party, it may be
necessary to authenticate the client to both the operator (for
billing purposes) and the third party's Authentication,
Authorization, and Accounting (AAA) server (for authorizing access to
the third party's internal network).

This document specifies an extension to IKEv2 that allows the use of
multiple authentication exchanges, using either different mechanisms
or the same mechanism. This extension allows, for instance,
performing certificate-based authentication of the client host
followed by an EAP authentication of the user.

Each authentication exchange requiring communication with backend AAA
servers may be directed to different backend AAA servers, located
even in different administrative domains. However, details of the
communication between the IKEv2 gateway and the backend
authentication servers are beyond the scope of this document. In
particular, this document does not specify any changes to existing
AAA protocols, and it does not require the use of any particular AAA
protocol.

In case of several EAP authentications, it is important to notice
that they are not a "sequence" (as described in Section 2.1 of
[EAP]), but separate independent EAP conversations, which are usually
also terminated in different EAP servers. Multiple authentication
methods within a single EAP conversation are still prohibited as
described in Section 2.1 of [EAP]. Using multiple independent EAP
conversations is similar to the separate Network Access Provider
(NAP) and Internet Service Provider (ISP) authentication exchanges

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planned for [PANA]. The discovery of the appropriate EAP server for
each EAP authentication conversation is based on AAA routing.

1.1. Usage Scenarios

Figure 1 shows an example architecture of an operator-hosted VPN
scenario that could benefit from a two-phase authentication within
the IKEv2 exchange. First, the client authenticates towards the
Network Access Provider (NAP) and gets access to the NAP-hosted VPN
gateway. The first-phase authentication involves the backend AAA
server of the NAP. After the first authentication, the client
initiates the second authentication round that also involves the
Third Party's backend AAA server. If both authentications succeed,
the required IPsec tunnels are set up and the client can access
protected networks behind the Third Party.

Client *Network Access Provider*
+---------+ +---------+ +-----+
| | | NAP's | | NAP |
|Protected| IPsec SAs | Tunnel | AAA Protocol | AAA |
|Endpoint |<------------------>|Endpoint |<------------>|Serv/|
| | | | |Proxy|
+---------+ +---------+ +-----+
^ ^
IPsec or / AAA |
Leased Line / Protocol |
/ |
v |
+---------+ *Third Party* v
|3rd Party| +-----+
Protected | Tunnel | | 3rd |
Subnet <----|Endpoint | |Party|
| | | AAA |
+---------+ +-----+

Figure 1: Two-phase authentication used to gain access to
the Third Party network via Network Access Provider. AAA
traffic goes through NAP's AAA server.

The NAP's AAA server can be used to proxy the AAA traffic to the
Third Party's backend AAA server. Alternatively, the AAA traffic
from the NAP's tunnel endpoint could go directly to the Third Party's
backend AAA servers. However, this is more or less an AAA routing
issue.

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1.2. 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 [KEYWORDS].

The terms and abbreviations "authenticator", "backend authentication
server", "EAP server", and "peer" in this document are to be
interpreted as described in [EAP].

When messages containing IKEv2 payloads are described, optional
payloads are shown in brackets (for instance, "[FOO]"), and a plus
sign indicates that a payload can be repeated one or more times (for
instance, "FOO+").

2. Solution

2.1. Solution Overview

The peers announce support for this IKEv2 extension by including a
MULTIPLE_AUTH_SUPPORTED notification in the IKE_SA_INIT response
(responder) and the first IKE_AUTH request (initiator).

If both peers support this extension, either of them can announce
that it wishes to have a second authentication by including an
ANOTHER_AUTH_FOLLOWS notification in any IKE_AUTH message that
contains an AUTH payload. This indicates that the peer sending the
ANOTHER_AUTH_FOLLOWS wishes to authenticate another set of
credentials to the other peer. The next IKE_AUTH message sent by
this peer will contain a second identity payload (IDi or IDr) and
starts another authentication exchange. The IKE_AUTH phase is
considered successful only if all the individual authentication
exchanges complete successfully.

It is assumed that both peers know what credentials they want to
present; there is no negotiation about, for instance, what type of
authentication is to be done. As in IKEv2, EAP-based authentication
is always requested by the initiator (by omitting the AUTH payload).

The AUTH payloads are calculated as specified in [IKEv2] Sections
2.15 and 2.16, where IDi' refers to the latest IDi payload sent by
the initiator, and IDr' refers to the latest IDr payload sent by the
responder. If EAP methods that do not generate shared keys are used,
it is possible that several AUTH payloads with identical contents are
sent. When such EAP methods are used, the purpose of the AUTH
payload is simply to delimit the authentication exchanges, and ensure
that the IKE_SA_INIT request/response messages were not modified.

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2.2. Example 1: Multiple EAP Authentications

This example shows certificate-based authentication of the responder
followed by an EAP authentication exchange (messages 1-10). When the
first EAP exchange is ending (the initiator is sending its AUTH
payload), the initiator announces that it wishes to have a second
authentication exchange by including an ANOTHER_AUTH_FOLLOWS
notification (message 9).

After this, a second authentication exchange begins. The initiator
sends a new IDi payload but no AUTH payload (message 11), indicating
that EAP will be used. After that, another EAP authentication
exchange follows (messages 12-18).

Initiator Responder
----------- -----------
1. HDR, SA, KE, Ni -->
<-- 2. HDR, SA, KE, Nr, [CERTREQ],
N(MULTIPLE_AUTH_SUPPORTED)
3. HDR, SK { IDi, [CERTREQ+], [IDr],
SA, TSi, TSr, N(MULTIPLE_AUTH_SUPPORTED) } -->
<-- 4. HDR, SK { IDr, [CERT+], AUTH,
EAP(Request) }
5. HDR, SK { EAP(Response) } -->
<-- 6. HDR, SK { EAP(Request) }
7. HDR, SK { EAP(Response) } -->
<-- 8. HDR, SK { EAP(Success) }
9. HDR, SK { AUTH,
N(ANOTHER_AUTH_FOLLOWS) } -->
<-- 10. HDR, SK { AUTH }
11. HDR, SK { IDi } -->
<-- 12. HDR, SK { EAP(Request) }
13. HDR, SK { EAP(Response) } -->
<-- 14. HDR, SK { EAP(Request) }
15. HDR, SK { EAP(Response) } -->
<-- 16. HDR, SK { EAP(Success) }
17. HDR, SK { AUTH } -->
<-- 18. HDR, SK { AUTH, SA, TSi, TSr }

Example 1: Certificate-based authentication of the
responder, followed by two EAP authentication exchanges.

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2.3. Example 2: Mixed EAP and Certificate Authentications

Another example is shown below: here both the initiator and the
responder are first authenticated using certificates (or shared
secrets); this is followed by an EAP authentication exchange.

Initiator Responder
----------- -----------
1. HDR, SA, KE, Ni -->
<-- 2. HDR, SA, KE, Nr, [CERTREQ],
N(MULTIPLE_AUTH_SUPPORTED)
3. HDR, SK { IDi, [CERT+], [CERTREQ+], [IDr], AUTH,
SA, TSi, TSr, N(MULTIPLE_AUTH_SUPPORTED),
N(ANOTHER_AUTH_FOLLOWS) } -->
<-- 4. HDR, SK { IDr, [CERT+], AUTH }
5. HDR, SK { IDi } -->
<-- 6. HDR, SK { EAP(Request) }
7. HDR, SK { EAP(Response) } -->
<-- 8. HDR, SK { EAP(Request) }
9. HDR, SK { EAP(Response) } -->
<-- 10. HDR, SK { EAP(Success) }
11. HDR, SK { AUTH } -->
<-- 12. HDR, SK { AUTH, SA, TSi, TSr }

Example 2: Certificate-based (or shared-secret-based)
authentication of the initiator and the responder,
followed by an EAP authentication exchange.

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2.4. Example 3: Multiple Initiator Certificates

This example shows yet another possibility: the initiator has two
different certificates (and associated private keys), and
authenticates both of them to the responder.

Initiator Responder
----------- -----------
1. HDR, SA, KE, Ni -->
<-- 2. HDR, SA, KE, Nr, [CERTREQ],
N(MULTIPLE_AUTH_SUPPORTED)
3. HDR, SK { IDi, [CERT+], [CERTREQ+], [IDr], AUTH,
SA, TSi, TSr, N(MULTIPLE_AUTH_SUPPORTED),
N(ANOTHER_AUTH_FOLLOWS) } -->
<-- 4. HDR, SK { IDr, [CERT+], AUTH }
5. HDR, SK { IDi, [CERT+], AUTH } -->
<-- 6. HDR, SK { SA, TSi, TSr }

Example 3: Two certificate-based authentications of the
initiator, and one certificate-based authentication
of the responder.

2.5. Example 4: Multiple Responder Certificates

This example shows yet another possibility: the responder has two
different certificates (and associated private keys), and
authenticates both of them to the initiator.

Initiator Responder
----------- -----------
1. HDR, SA, KE, Ni -->
<-- 2. HDR, SA, KE, Nr, [CERTREQ],
N(MULTIPLE_AUTH_SUPPORTED)
3. HDR, SK { IDi, [CERT+], [CERTREQ+], [IDr], AUTH,
SA, TSi, TSr, N(MULTIPLE_AUTH_SUPPORTED) } -->
<-- 4. HDR, SK { IDr, [CERT+], AUTH,
N(ANOTHER_AUTH_FOLLOWS) }
5. HDR, SK { } -->
<-- 6. HDR, SK { IDr, [CERT+], AUTH,
SA, TSi, TSr }

Example 4: Two certificate-based authentications of the
responder, and one certificate-based authentication
of the initiator.

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3. Payload Formats

3.1. MULTIPLE_AUTH_SUPPORTED Notify Payload

The MULTIPLE_AUTH_SUPPORTED notification is included in the
IKE_SA_INIT response or the first IKE_AUTH request to indicate that
the peer supports this specification. The Notify Message Type is
MULTIPLE_AUTH_SUPPORTED (16404). The Protocol ID and SPI Size fields
MUST be set to zero, and there is no data associated with this Notify
type.

3.2. ANOTHER_AUTH_FOLLOWS Notify Payload

The ANOTHER_AUTH_FOLLOWS notification payload is included in an
IKE_AUTH message containing an AUTH payload to indicate that the peer
wants to continue with another authentication exchange. The Notify
Message Type is ANOTHER_AUTH_FOLLOWS (16405). The Protocol ID and
SPI Size fields MUST be set to zero, and there is no data associated
with this Notify type.

4. IANA Considerations

This document defines two new IKEv2 notifications,
MULTIPLE_AUTH_SUPPORTED and ANOTHER_AUTH_FOLLOWS, whose values are
allocated from the "IKEv2 Notify Message Types" namespace defined in
[IKEv2].

This document does not define any new namespaces to be managed by
IANA.

5. Security Considerations

Security considerations for IKEv2 are discussed in [IKEv2]. The
reader is encouraged to pay special attention to considerations
relating to the use of EAP methods that do not generate shared keys.
However, the use of multiple authentication exchanges results in at
least one new security consideration.

In normal IKEv2, the responder authenticates the initiator before
revealing its identity (except when EAP is used). When multiple
authentication exchanges are used to authenticate the initiator, the
responder has to reveal its identity before all of the initiator
authentication exchanges have been completed.

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6. Acknowledgments

The authors would like to thank Bernard Aboba, Jari Arkko, Spencer
Dawkins, Lakshminath Dondeti, Henry Haverinen, Russ Housley, Mika
Joutsenvirta, Charlie Kaufman, Tero Kivinen, Yoav Nir, Magnus
Nystrom, Mohan Parthasarathy, and Juha Savolainen for their valuable
comments.

7. References

7.1. Normative References

[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

7.2. Informative References

[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.

[PANA] Yegin, A., Ohba, Y., Penno, R., Tsirtsis, G., and C.
Wang, "Protocol for Carrying Authentication for Network
Access (PANA) Requirements", RFC 4058, May 2005.

Authors' Addresses

Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland

EMail: [email protected]

Jouni Korhonen
TeliaSonera
P.O. Box 970
FIN-00051 Sonera
Finland

EMail: [email protected]

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RFC 4739 Multiple Auth. Exchanges in IKEv2 November 2006

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