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rfc:rfc4334

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.

Housley & Moore Standards Track [Page 1] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

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.

Housley & Moore Standards Track [Page 2] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

  • 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

Housley & Moore Standards Track [Page 3] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

 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.

Housley & Moore Standards Track [Page 4] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

 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 }

Housley & Moore Standards Track [Page 5] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

 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

Housley & Moore Standards Track [Page 6] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

 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.

Housley & Moore Standards Track [Page 7] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

 [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.

Housley & Moore Standards Track [Page 8] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

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
  1. - 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 }
  1. - Extended Key Usage Values
 id-kp-eapOverPPP  OBJECT IDENTIFIER  ::=  { id-kp 13 }
 id-kp-eapOverLAN  OBJECT IDENTIFIER  ::=  { id-kp 14 }
  1. - 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))
  1. - Wireless LAN SSID Attribute Certificate Attribute
  2. - Uses same syntax as the certificate extension: SSIDList
 id-aca-wlanSSID  OBJECT IDENTIFIER ::= { id-aca 7 }
 END

Housley & Moore Standards Track [Page 9] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

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

Housley & Moore Standards Track [Page 10] RFC 4334 Supporting Authentication in PPP and WLAN February 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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 Administrative Support Activity (IASA).

Housley & Moore Standards Track [Page 11]

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