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

Network Working Group                                         R. Housley
Request for Comments: 4334                                Vigil Security
Obsoletes: 3770                                                 T. Moore
Category: Standards Track                                      Microsoft
                                                           February 2006


            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 (2006).

Abstract

   This document defines two Extensible Authentication Protocol (EAP)
   extended key usage values and a public key certificate extension to
   carry Wireless LAN (WLAN) System Service identifiers (SSIDs).  This
   document obsoletes RFC 3770.

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, Wireless LANs
   (WLANs) [802.11], and other IEEE 802 networks.

   Automated selection of client certificates for use with PPP and IEEE
   802.1X 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
   supplicant 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.

   This document defines extended key usage values and a WLAN-specific
   certificate extension for use in certificates issued to clients of
   PPP and WLANs.

1.1. Changes since RFC 3770

   This document is primarily same as RFC 3770.  Six significant changes
   are included:

      * This document now uses the same normative reference for ASN.1
        as RFC 3280 [PROFILE].  The intent is to have the same
        dependencies.

      * The discussion of the critical bit in the certificate extension
        in section 2 is aligned with RFC 3280.  Also, the discussion of
        the key usage certificate extension was expanded.

      * RFC 3770 contained a typographical error in the object
        identifier for the Wireless LAN SSID Attribute Certificate
        Attribute.  Section 4 corrects the typographical error.

      * Clarified that the SSID extension may appear in certificates
        that do not include the extended key usage extension.

      * Uses the terms "peer", "EAP Server", and "supplicant" as they
        are defined in [EAP] and [802.1X].  RFC 3770 used "client"
        and "server".

      * The object identifier for the extended key usage certificate
        extension is listed in RFC 3280, and it is no longer
        repeated in this document.

1.2. 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 RFC 2119 [STDWORDS].

1.3. Abstract Syntax Notation

   All X.509 certificate [X.509] extensions are defined using ASN.1
   [X.680,X.690].

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.

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

      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
   by a peer with EAP in the PPP environment.  The inclusion of the
   EAPOL value indicates that the certified public key is appropriate
   for use by a peer with the EAP in the LAN environment.  Inclusion of
   both values indicates that the certified public key is appropriate
   for use by a peer 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.

   Certificate-using applications MAY require the extended key usage
   extension to be present in a certificate, and they MAY require a
   particular KeyPurposeId value to be present (such as id-kp-eapOverPPP
   or id-kp-eapOverLAN) within the extended key usage extension.  If
   multiple KeyPurposeId values are included, the certificate-using
   application need not recognize all of them, as long as the required
   KeyPurposeId value is present.

   If a certificate contains a key usage extension, the KeyUsage bits
   that are needed depends on the EAP method that is employed.

   If a certificate contains both a key usage extension and an 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 extensions, then the certificate-using application MUST NOT
   use the certificate 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.  The list of SSIDs MAY be used to select the correct
   certificate for authentication in a particular WLAN.

   If the extended key usage extension appears in the same certificate
   as the SSID extension, then the extended key usage extension MUST
   indicate that the certified public key is appropriate for use with
   the EAP in the LAN environment by including the id-kp-eapOverLAN
   KeyPurposeId value.

   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 an EAP 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 7 }
   The syntax for the WLAN SSID attribute certificate attribute is
   exactly the same as that for 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 with the WLAN that is currently being accessed.
   The intended use of the SSID extensions is to help a peer determine
   the correct certificate to present when trying to gain access to a
   WLAN.  In most situations, including EAP-TLS, the peer will have the
   opportunity to validate the certificate provided by the EAP server
   before transmitting one of its own certificates to the EAP server.
   While the peer may not be sure that the EAP server has access to the
   corresponding private key until later in the protocol exchange, the
   identity information in the EAP server certificate can be used to
   determine whether or not the peer certificate ought to be provided.
   When the same peer 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 peer certificates ought to be used for the each
   WLAN.

   SSID values are unmanaged; therefore, SSIDs may not be unique.
   Hence, it is possible for peer 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.  If 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).  The OIDs used in this document 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.

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

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

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

   [X.680]     ITU-T Recommendation X.680: Information Technology -
               Abstract Syntax Notation One, 1997.

   [X.690]     ITU-T Recommendation X.660 Information Technology - ASN.1
               encoding rules: Specification of Basic Encoding Rules
               (BER), Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER), 1997.

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

   [PPP]       Simpson, W., "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-extns2005(37) }

   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 7 }

   END

Authors' Addresses

   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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