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.

   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

   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

   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

   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.

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.

   [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 }

   -- 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

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