Certificate Extensions and Attributes Supporting Authentication in Point-to-Point Protocol (PPP) and Wireless Local Area Networks (WLAN)
obsoleted by
- rfc-4334 — Certificate Extensions and Attributes Supporting Authentication in Point-to-Point Protocol (PPP) and Wireless Local Area Networks (WLAN)
Extracted elements (17)
Certificate extensions are used to identify the intended environment (PPP vs. WLAN) and SSID list to enable automated certificate selection, minimizing the need for user input. This also facilitates administrative separation between certificates used in different environments.
The WLAN SSID extension is always non-critical because certificate relying parties that do not understand it should still be able to process the certificate; the extension is advisory for client-side certificate selection rather than a mandatory access control mechanism.
A WLAN SSID attribute certificate (per RFC 3281) can be used as an alternative to embedding the SSID list directly in the public key certificate extension, allowing SSID association without modifying the underlying PKC.
The extended key usage extension MAY be either critical or non-critical at the certificate issuer's option. When non-critical, it MAY be used to support locating an appropriate certified public key without mandating rejection of the certificate for other uses.
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 no such purpose exists, the certificate MUST NOT be used for any purpose.
If the extended key usage extension is marked as critical, then the certified public key MUST be used only for the purposes indicated by that extension.
In cases where a human user is unavailable, each potential certificate MAY be tried until one succeeds; however, sequentially trying each certificate is RECOMMENDED only when user selection is unavailable or impractical, because doing so discloses the list of SSIDs associated with each certificate.
The CA MUST ensure that correct values for the extended key usage extension and SSID extension are inserted in each issued certificate, as incorrect representation could cause relying parties to reject appropriate certificates or accept ones that ought to be rejected.
When multiple certificates include the EAPOL extended key usage and SSID values collide (same SSID in different certificates), automatic selection will fail and the implementation SHOULD obtain help from the user to choose the correct certificate.
The WLAN SSID attribute certificate attribute (id-aca-wlanSSID, id-aca 6) allows associating a list of SSIDs with a public key certificate when the certificate itself does not include the WLAN SSID extension. The syntax is identical to the certificate extension: SSIDList of SSID OCTET STRINGs.
The WLAN SSID public key certificate extension (id-pe-wlanSSID, id-pe 13) is always non-critical and contains a list of SSIDs encoded as SSIDList ::= SEQUENCE SIZE (1..MAX) OF SSID, where SSID ::= OCTET STRING (SIZE (1..32)).
Two EAP extended key usage OIDs are defined: id-kp-eapOverPPP (id-kp 13) indicating the certified public key is appropriate for EAP in the PPP environment, and id-kp-eapOverLAN (id-kp 14) indicating appropriateness for EAP in the LAN (EAPOL) environment. Inclusion of both values indicates suitability for either environment.
OIDs for id-kp-eapOverPPP (id-kp 13) and id-kp-eapOverLAN (id-kp 14) are assigned from the PKIX id-kp arc delegated by IANA. OIDs for id-pe-wlanSSID (id-pe 13) and id-aca-wlanSSID (id-aca 6) are assigned from PKIX arcs. No further IANA action is required.
Sequentially trying each certificate when automatic selection fails discloses the SSIDs associated with each certificate, which might otherwise not be disclosed. This trial-and-error approach is therefore RECOMMENDED only when user selection is unavailable or impractical.
SSID values are unmanaged and non-unique, so certificates intended for different WLANs may share the same SSID, causing automatic certificate selection to fail. Masking the SSID in AP beacons provides little security because the SSID appears in other WLAN management messages, so placing SSIDs in certificates does not materially worsen disclosure.
When the same client certificate is used to authenticate to multiple WLANs, the list of SSIDs in that certificate is exposed to servers and eavesdroppers on each WLAN. Whenever SSID disclosure is a concern, different client certificates ought to be used for each WLAN.
The SSID field is encoded as an OCTET STRING of SIZE (1..32) bytes. The SSIDList is a SEQUENCE of one or more SSID values, used in both the certificate extension and the attribute certificate attribute.