Network Working Group R. Housley

Network Working Group R. Housley
Request for Comments: 3770 Vigil Security
Category: Standards Track T. Moore
Microsoft
May 2004

Certificate Extensions and Attributes Supporting
Authentication in Point-to-Point Protocol (PPP)
and Wireless Local Area Networks (WLAN)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This document defines two EAP extended key usage values and a public
key certificate extension to carry Wireless LAN (WLAN) System Service
identifiers (SSIDs).

1. Introduction

Several Extensible Authentication Protocol (EAP) [EAP] authentication
methods employ X.509 public key certificates. For example, EAP-TLS
[EAP-TLS] can be used with PPP [PPP] as well as IEEE 802.1X [802.1X].
PPP is used for dial-up and VPN environments. IEEE 802.1X defines
port-based, network access control, and it is used to provide
authenticated network access for Ethernet, Token Ring, and Wireless
LANs (WLANs) [802.11].

Automated selection of certificates for PPP and IEEE 802.1X clients
is highly desirable. By using certificate extensions to identify the
intended environment for a particular certificate, the need for user
input is minimized. Further, the certificate extensions facilitate
the separation of administrative functions associated with
certificates used for different environments.

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RFC 3770 PPP and WLAN May 2004

IEEE 802.1X can be used for authentication with multiple networks.
For example, the same wireless station might use IEEE 802.1X to
authenticate to a corporate IEEE 802.11 WLAN and a public IEEE 802.11
"hotspot." Each of these IEEE 802.11 WLANs has a different network
name, called Service Set Identifier (SSID). If the network operators
have a roaming agreement, then cross realm authentication allows the
same certificate to be used on both networks. However, if the
networks do not have a roaming agreement, then the IEEE 802.1X client
needs to select a certificate for the current network environment.
Including a list of SSIDs in a certificate extension facilitates
automated selection of an appropriate X.509 public key certificate
without human user input. Alternatively, a companion attribute
certificate could contain the list of SSIDs.

1.1. Conventions Used In This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[STDWORDS].

1.2. Abstract Syntax Notation

All X.509 certificate [X.509] extensions are defined using ASN.1
[X.208, X.209].

2. EAP Extended Key Usage Values

RFC 3280 [PROFILE] specifies the extended key usage X.509 certificate
extension. The extension indicates one or more purposes for which
the certified public key may be used. The extended key usage
extension can be used in conjunction with key usage extension, which
indicates the intended purpose of the certified public key. For
example, the key usage extension might indicate that the certified
public key ought to be used only for validating digital signatures.

The extended key usage extension definition is repeated here for
convenience:

id-ce-extKeyUsage OBJECT IDENTIFIER ::= {id-ce 37}

ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId

KeyPurposeId ::= OBJECT IDENTIFIER

This specification defines two KeyPurposeId values: one for EAP over
PPP, and one for EAP over LAN (EAPOL). Inclusion of the EAP over PPP
value indicates that the certified public key is appropriate for use

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RFC 3770 PPP and WLAN May 2004

with EAP in the PPP environment, and the inclusion of the EAPOL value
indicates that the certified public key is appropriate for use with
the EAP in the LAN environment. Inclusion of both values indicates
that the certified public key is appropriate for use in either of the
environments.

id-kp OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) 3 }

id-kp-eapOverPPP OBJECT IDENTIFIER ::= { id-kp 13 }

id-kp-eapOverLAN OBJECT IDENTIFIER ::= { id-kp 14 }

The extended key usage extension may, at the option of the
certificate issuer, be either critical or non-critical. If the
extension is marked as critical, then the certified public key MUST
be used only for the purposes indicated. However, if the extension
is marked as non-critical, then extended key usage extension MAY be
used to support the location of an appropriate certified public key.

If a certificate contains both a critical key usage extension and a
critical extended key usage extension, then both extensions MUST be
processed independently, and the certificate MUST only be used for a
purpose consistent with both extensions. If there is no purpose
consistent with both critical extensions, then the certificate MUST
NOT be used for any purpose.

3. WLAN SSID Public Key Certificate Extension

The Wireless LAN (WLAN) System Service identifiers (SSIDs) public key
certificate extension is always non-critical. It contains a list of
SSIDs. When more than one certificate includes an extended key usage
extension indicating that the certified public key is appropriate for
use with the EAP in the LAN environment, then the list of SSIDs MAY
be used to select the correct certificate for authentication in a
particular WLAN.

Since SSID values are unmanaged, the same SSID can appear in
different certificates that are intended to be used with different
WLANs. When this occurs, automatic selection of the certificate will
fail, and the implementation SHOULD obtain help from the user to
choose the correct certificate. In cases where a human user is
unavailable, each potential certificate MAY be tried until one
succeeds. However, by maintaining a cache of Access Point (AP) MAC
addresses or authentication server identity with which the
certificate has successfully authenticated, user involvement can be
minimized. RADIUS [RADIUS1, RADIUS2] is usually used as the

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RFC 3770 PPP and WLAN May 2004

authentication service in WLAN deployments. The cache can be used to
avoid future human user interaction or certificate selection by
trial-and-error.

The WLAN SSID extension is identified by id-pe-wlanSSID.

id-pe OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) 1 }

id-pe-wlanSSID OBJECT IDENTIFIER ::= { id-pe 13 }

The syntax for the WLAN SSID extension is:

SSIDList ::= SEQUENCE SIZE (1..MAX) OF SSID

SSID ::= OCTET STRING (SIZE (1..32))

4. WLAN SSID Attribute Certificate Attribute

When the public key certificate does not include the WLAN SSID
certificate extension, then an attribute certificate [ACPROFILE] can
be used to associate a list of SSIDs with the public key certificate.
The WLAN SSIDs attribute certificate attribute contains a list of
SSIDs, and the list of SSIDs MAY be used to select the correct
certificate for authentication in a particular WLAN environment.

The WLAN SSID attribute certificate attribute is identified by
id-aca-wlanSSID.

id-aca OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) 10 }

id-aca-wlanSSID OBJECT IDENTIFIER ::= { id-aca 6 }

The syntax for the WLAN SSID attribute certificate attribute is
exactly the same as the WLAN SSID extension:

SSIDList ::= SEQUENCE SIZE (1..MAX) OF SSID

SSID ::= OCTET STRING (SIZE (1..32))

5. Security Considerations

The procedures and practices employed by the certification authority
(CA) MUST ensure that the correct values for the extended key usage
extension and SSID extension are inserted in each certificate that is
issued. Relying parties may accept or reject a particular
certificate for an intended use based on the information provided in

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RFC 3770 PPP and WLAN May 2004

these extensions. Incorrect representation of the information in
either extension could cause the relying party to reject an otherwise
appropriate certificate or accept a certificate that ought to be
rejected.

If multiple SSIDs are included in a certificate, then information can
be obtained from a certificate about the SSIDs associated with
several WLANs, not the WLAN that is currently being accessed. The
intended use of the SSID extensions is to help a client determine the
correct certificate to present when trying to gain access to a WLAN.
In most situations, including EAP-TLS, the client will have the
opportunity to validate the certificate provided by the server before
transmitting one of its own certificates to the server. While the
client may not be sure that the server has access to the
corresponding private key until later in the protocol exchange, the
identity information in the server certificate can be used to
determine whether or not the client certificate ought to be provided.
When the same client certificate is used to authenticate to multiple
WLANs, the list of SSIDs is available from servers associated with
each WLAN. Of course, the list of SSIDs is also made available to
any eavesdroppers on the WLAN. Whenever this SSID disclosure is a
concern, different client certificates ought to be used for the each
WLAN.

SSID values are unmanaged; therefore SSIDs may not be unique. Hence,
it is possible for client certificates that are intended to be used
with different WLANs to contain the same SSID. In this case,
automatic selection of the certificate will fail, and the
implementation SHOULD obtain help from the user to choose the correct
certificate. In cases where a human user is unavailable, each
potential certificate MAY be tried until one succeeds, disclosing the
list of SSIDs associated with each certificate, which might otherwise
not be disclosed. Therefore, it is RECOMMENDED that sequentially
trying each certificate only be employed when user selection is
unavailable or impractical.

In practice, disclosure of the SSID is of little concern. Some WLAN
security experts recommend that the SSID be masked in the beacon sent
out by Access Points (APs). The intent is to make it harder for an
attacker to find the correct AP to target. However, other WLAN
management messages include the SSID, so this practice only forces
the attacker to eavesdrop on the WLAN management messages instead of
the beacon. Therefore, placing the SSID in the certificate does not
make matters worse.

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RFC 3770 PPP and WLAN May 2004

6. IANA Considerations

Certificate extensions and extended key usage values are identified
by object identifiers (OIDs). Some of the OIDs used in this document
are copied from X.509 [X.509]. Other OIDs were assigned from an arc
delegated by the IANA. No further action by the IANA is necessary
for this document or any anticipated updates.

7. References

7.1. Normative References

[ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281, April
2002.

[PROFILE] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.

[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[X.208] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.

[X.209] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1),
1988.

[X.509] ITU-T. Recommendation X.509: The Directory -
Authentication Framework, 2000.

7.2. Informative References

[802.11] IEEE Std 802.11, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", 1999.

[802.1X] IEEE Std 802.1X, "Port-based Network Access Control",
2001.

[EAP] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.

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RFC 3770 PPP and WLAN May 2004

[EAP-TLS] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.

[PPP] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.

[RADIUS1] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.

[RADIUS2] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese,
"IEEE 802.1X Remote Authentication Dial In User Service
(RADIUS) Usage Guidelines", RFC 3580, September 2003.

8. ASN.1 Module

WLANCertExtn
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-wlan-extns(24) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

-- OID Arcs

id-pe OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) 1 }

id-kp OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) 3 }

id-aca OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) 10 }

-- Extended Key Usage Values

id-kp-eapOverPPP OBJECT IDENTIFIER ::= { id-kp 13 }

id-kp-eapOverLAN OBJECT IDENTIFIER ::= { id-kp 14 }

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RFC 3770 PPP and WLAN May 2004

-- Wireless LAN SSID Extension

id-pe-wlanSSID OBJECT IDENTIFIER ::= { id-pe 13 }

SSIDList ::= SEQUENCE SIZE (1..MAX) OF SSID

SSID ::= OCTET STRING (SIZE (1..32))

-- Wireless LAN SSID Attribute Certificate Attribute
-- Uses same syntax as the certificate extension: SSIDList

id-aca-wlanSSID OBJECT IDENTIFIER ::= { id-aca 6 }

END

9. Author's Address

Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA

EMail: [email protected]

Tim Moore
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA

EMail: [email protected]

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RFC 3770 PPP and WLAN May 2004

10. Full Copyright Statement

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Acknowledgement

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Internet Society.

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