RFC 4334

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

📅 published February 2006, from ietf

Certificate Extensions and Attributes Supporting Authentication in PPP and WLANs 🎀

okay bestie, this one is a Standards Track RFC, so it's an official spec, not just a vibe. 💅 It's RFC 4334, written by R. Housley (Vigil Security) and T. Moore (Microsoft) in February 2006 under the Network Working Group. It obsoletes RFC 3770, so the old version is out and this one is the new main character. 👑

The title is Certificate Extensions and Attributes Supporting Authentication in Point-to-Point Protocol (PPP) and Wireless Local Area Networks (WLAN). ✨

Status of This Memo

This doc specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🦋 For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited, so share it with everyone 💜

Copyright Notice

Abstract ✨

okay so this doc defines two Extensible Authentication Protocol (EAP) extended key usage values, plus a public key certificate extension that carries Wireless LAN (WLAN) System Service identifiers (SSIDs). 🔑 It also obsoletes RFC 3770, so that's the tea. 🫖

1. Introduction 🧚

okay picture this: lots of Extensible Authentication Protocol (EAP) [EAP] authentication methods use X.509 public key certificates. 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's used to provide authenticated network access for Ethernet, Token Ring, Wireless LANs (WLANs) [802.11], and other IEEE 802 networks. It's giving secure network access everywhere 🔥

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. And the certificate extensions also help separate the administrative functions tied to certificates used for different environments. Iconic organizing energy 🗂️✨

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 lets the same certificate be used on both networks. But if the networks don't 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 makes automated selection of an appropriate X.509 public key certificate possible without human user input 🎯 Alternatively, a companion attribute certificate could hold 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

okay this doc is primarily the 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

okay so all X.509 certificate [X.509] extensions are defined using ASN.1 [X.680,X.690]. 🧠💕

2. EAP Extended Key Usage Values 🌈

okay so RFC 3280 [PROFILE] specifies the extended key usage X.509 certificate extension. It indicates one or more purposes the certified public key may be used for. The extended key usage extension can work alongside the key usage extension, which shows the intended purpose of the certified public key 🔑

Here's the extended key usage extension syntax, repeated for convenience 🎀

   ExtKeyUsageSyntax  ::=  SEQUENCE SIZE (1..MAX) OF KeyPurposeId

   KeyPurposeId  ::=  OBJECT IDENTIFIER

This spec defines two KeyPurposeId values: one for EAP over PPP, and one for EAP over LAN (EAPOL) 👯‍♀️ Including the EAP over PPP value means the certified public key is appropriate for use by a peer with EAP in the PPP environment. Including the EAPOL value means the certified public key is appropriate for use by a peer with the EAP in the LAN environment. Including both values means 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 depend 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. 🚫 this part is serious, bestie 😭

3. WLAN SSID Public Key Certificate Extension 🦋

okay so 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 shows up 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 meant to be used with different WLANs. When this happens, 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 keeping a cache of Access Point (AP) MAC addresses or an EAP server identity that the certificate has successfully authenticated with, 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 }

Here's the syntax for the WLAN SSID extension 🧾

   SSIDList  ::=  SEQUENCE SIZE (1..MAX) OF SSID

   SSID  ::=  OCTET STRING (SIZE (1..32))

4. WLAN SSID Attribute Certificate Attribute 💜

okay so when the public key certificate doesn't include the WLAN SSID certificate extension, 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))

more coming in part 2, bestie 🌈💖

5. Security Considerations 🔐✨

okay bestie, this section is SERIOUS but also kinda iconic, so let's go 💅

The CA (certification authority) has to make sure the right values for the extended key usage extension and the SSID extension get put into every certificate it issues 🫶 This is a MUST. A relying party might accept or reject a certificate for a particular use based on what those extensions say. If the info is wrong, the relying party could reject a perfectly fine certificate, or accept one that should have been rejected 😭

If a certificate has multiple SSIDs in it, the certificate can tell you about the SSIDs of several WLANs, not just the one you're trying to get into 🧚 The whole point of the SSID extensions is to help a peer pick the right certificate to show when it's trying to get onto a WLAN 🎀 In most cases, including EAP-TLS, the peer gets to check the EAP server's certificate before it sends one of its own to the EAP server 👑 The peer might not be sure the EAP server has the matching private key until later in the exchange, but the identity info in the EAP server's certificate can still help it decide whether its own certificate should be sent at all 💜 If the same peer certificate is used for several WLANs, the SSID list is available from the servers for each WLAN. And yes, the SSID list is also visible to any eavesdroppers on the WLAN 🙈 So if SSID disclosure is a worry, use a different peer certificate for each WLAN, periodt 💅

SSID values aren't managed, so they might not be unique 🌸 That means two peer certificates meant for different WLANs could contain the same SSID. Then automatic selection fails, and the implementation SHOULD ask the user to pick the right certificate 🦋 If there's no human user around, each candidate certificate MAY be tried until one works. The catch is that this discloses the SSID list of each certificate, which might otherwise stay hidden 😬 So it is RECOMMENDED that trying each certificate in turn is only used when user selection is unavailable or impractical ✨

Honestly, SSID disclosure isn't a huge deal in practice 🙄 Some WLAN security folks suggest masking the SSID in the beacons sent out by Access Points (APs), so it's harder for an attacker to find the right AP to target 💀 But other WLAN management messages include the SSID too, so masking it just means the attacker has to eavesdrop on those management messages instead of the beacon. So putting the SSID in the certificate doesn't make things worse 🍓

6. IANA Considerations 🧚

Certificate extensions and extended key usage values are identified by object identifiers (OIDs) 🔢 The OIDs in this document came from an arc that IANA delegated. No further action by IANA is needed 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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