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.
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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.
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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
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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.
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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 }
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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
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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.
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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.
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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
-- 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
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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
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RFC 4334 Supporting Authentication in PPP and WLAN February 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
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