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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.

Housley & Moore Standards Track [Page 1] 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

Housley & Moore Standards Track [Page 2] 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

Housley & Moore Standards Track [Page 3] 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

Housley & Moore Standards Track [Page 4] 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.

Housley & Moore Standards Track [Page 5] 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.

Housley & Moore Standards Track [Page 6] 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 }

Housley & Moore Standards Track [Page 7] 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: housley@vigilsec.com

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

 EMail: timmoore@microsoft.com

Housley & Moore Standards Track [Page 8] RFC 3770 PPP and WLAN May 2004

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Acknowledgement

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 Internet Society.

Housley & Moore Standards Track [Page 9]