]> Vigil Security, LLC

housley@vigilsec.com

DigiCert, Inc.

corey.bonnell@digicert.com

AKAYLA, Inc.

joe@akayla.com

Penguin Securities Pte. Ltd.

tomofumi.okubo+ietf@gmail.com

Security Limited Additional Mechanisms for PKIX and SMIME MACsec 802.1AE X.509 SubjectAltName This document defines a new otherName for inclusion in the X.509 Subject Alternative Name (SAN) and Issuer Alternative Name (IAN) extensions to carry an IEEE Media Access Control (MAC) address. The new name form makes it possible to bind a layer‑2 interface identifier to a public key certificate. Additionally, this document defines how constraints on this name form can be encoded and processed in the X.509 Name Constraints extension. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Deployments that use X.509 certificates to identify a device by a Media Access Control (MAC) address need a standard way to encode it in the Subject Alternative Name (SAN) extension defined in . This document defines a new otherName form "MACAddress". The name form carries either a 48‑bit IEEE 802 MAC address (EUI‑48) or a 64‑bit extended identifier (EUI‑64) in an OCTET STRING. Additionally, the name form also can convey constraints on EUI-48 or EUI-64 values when included in the Name Constraints extension defined in . The new name form enables certificate‑based authentication at layer 2 and facilitates secure provisioning in Internet‑of‑Things and automotive networks.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

MACAddress otherName The new name form is identified by the object identifier (OID) id‑on‑MACAddress (TBD1). The name form has variants to convey a EUI-48 as an OCTET STRING comprising of 6 octets, or a EUI-64 as an OCTET STRING comprising of 8 octets. Constraints on EUI-48 and EUI-64 values are conveyed as OCTET STRINGs whose lengths are twice the octet length of the identifiers. The first set of N octets (where N is the length of the address octets) define the bit mask of the constraint, and the second set of N octets defines the bit pattern that the address must match for the asserted bits in the mask. The following sub-sections describe how to encode EUI-48 and EUI-64 values and their corresponding constraints.

Encoding a MACAddress as an alternative name When the name form is included in a Subject Alternative Name or Issuer Alternate Name extension, the syntax consists of exactly six or eight octets. Values are encoded with the most significant octet encoded first ("big-endian" or "left-to-right" encoding). No text representation is permitted in the certificate, as human‑readable forms such as "00‑24‑98‑7B‑19‑02" or "0024.987B.1902" are used only in management interfaces. When a device natively possesses a 48‑bit MAC identifier, the CA MUST encode it using a 6‑octet OCTET STRING as the MACAddress value. When the device’s factory identifier is a 64‑bit EUI‑64 or when no canonical 48‑bit form exists, the CA MUST encode it using an 8‑octet OCTET STRING as the MACAddress value.

Encoding a MACAddress constraint When the name form is included in the Name Constraints extension, the syntax consists of an OCTET STRING that is twice as long as the OCTET STRING representation of the address type being constrained. Within the OCTET STRING, two elements are encoded:

1. The first N octets (where N is 6 for an EUI-48 constraint or 8 for an EUI-64 constraint) encodes the "mask bit pattern" of the constraint. Each bit that is asserted in the mask bit pattern indicates that the bit in the same position in the address is constrained by the second set of N octets.
2. The second set of N octets contains the "value bit pattern". This bit pattern encodes the bits that the masked address must contain to be considered a match.

The bit patterns encoded in both the mask bit pattern and value bit pattern are encoded with the most significant bit encoded first ("big-endian" or "left-to-right" encoding). If a bit is not asserted in the mask bit pattern, then the CA MUST NOT assert the corresponding bit in the value bit pattern. This rule ensures that a distinguished encoding is used for a given mask bit pattern and value bit pattern. When a constraint is included in the permittedSubtrees field of a Name Constraints extension, certificates containing a MACAddress name form of the specific identifier type (EUI-48 or EUI-64) that are issued by the Certification Authority are trusted only when the masked bits (masked according to the "mask bit pattern") of the value are binary equal to the "value bit pattern". When a constraint is included in the excludedSubtrees field of a Name Constraints extension, certificates containing a MACAddress name form of the specific identifier type (EUI-48 or EUI-64) that are issued by the Certification Authority are trusted only when the masked bits (masked according ot the "mask bit pattern") of the value are not binary equal to the pattern.

Generation and Validation Rules A certificate MAY include one or more MACAddress otherName values if and only if the subject device owns (or is expected to own) the corresponding MAC address for the certificate lifetime. MAC addresses SHOULD NOT appear in more than one valid certificate issued by the same Certification Authority (CA) at the same time, unless different layer‑2 interfaces share a public key. A Relying party that matches a presented MAC address to a certificate SHALL perform a byte‑for‑byte comparison of the OCTET STRING contents. Canonicalization, case folding, or removal of delimiter characters MUST NOT be performed. Wildcards are not supported. Self‑signed certificates that carry a MACAddress otherName SHOULD include the address of one of the device’s physical ports.

Name Constraints Processing The MACAddress otherName follows the general rules for otherName constraints in RFC 5280, Section 4.2.1.10. A name constraints extension MAY impose permittedSubtrees and excludedSubtrees on id‑on‑MACAddress. A constraint that is represented as an OCTET STRING of exactly 12 octets is relevant only to macAddress values that are encoded using 6 octets; such a constraint is ignored for macAddress values that are encoded using 8 octets. Likewise, a constraint that is represented as an OCTET STRING of exactly 16 octets is relevant only to macAddress values that are encoded using 8 octets; such a constraint is ignored for macAddress values that are encoded using 6 octets. To determine if a constraint matches a given name value, the certificate-consuming application performs the following steps:

1. Extract the mask bit pattern from the upper N octets of the constraint value, where N is "6" for EUI-48 identifiers and "8" for EUI-64 identifiers.
2. Extract the value bit pattern from the lower N octets of the constraint value, where N is "6" for EUI-48 identifiers and "8" for EUI-64 identifiers.
3. Perform an exclusive OR (XOR) operation with the value bit string extracted in step 2 and the octets of the name value.
4. Perform a bitwise AND operation with the bit string calculated in step 3 and the mask bit pattern.
5. If the result of step 4 is a bit string consisting of entirely zeros, then the name matches the constraint. Conversely, if the result of the operation is a bit string with at least one bit asserted, then the name does not match the constraint.

The first octet of a MAC address contains two flag bits.

• I/G bit (bit 0) – 0 = unicast, 1 = multicast.  Multicast prefixes are never OUIs.
• U/L bit (bit 1) – 0 = universal (IEEE‑assigned), 1 = local.

These flags let the implementations exclude multicast and local prefixes but still cannot prove that a 24‑bit value is an IEEE‑registered OUI. 36‑bit CIDs share the same first 24 bits and enterprises MAY deploy pseudo‑OUIs. CAs MUST include only prefixes the subscriber legitimately controls (registered OUI or CID).  Before issuing a certificate that contains a MACAddress or a name constraint based on such a prefix, the CA MUST verify that control—for example, by consulting the IEEE registry or reviewing manufacturer documentation.

Security Considerations The binding of a MAC address to a certificate is only as strong as the CA’s validation process. CAs MUST verify that the subscriber legitimately controls or owns the asserted MAC address. Some systems dynamically assign or share MAC addresses. Such practices can undermine the uniqueness and accountability that this name form aims to provide. Unlike IP addresses, MAC addresses are not typically routed across layer 3 boundaries. Relying parties in different broadcast domains SHOULD NOT assume uniqueness beyond their local network.

Privacy Considerations A MAC address can uniquely identify a physical device and by extension, its user. Certificates that embed unchanging MAC addresses facilitate long‑term device tracking. Deployments that use the MACAddress name SHOULD consider rotating addresses, using temporary certificates, or employing MAC Address Randomization where feasible.

IANA Considerations IANA is requested to make the following assignments in the “SMI Security for PKIX Module Identifier” (1.3.6.1.5.5.7.0) registry: IANA is requested to make the following assignment in the “SMI Security for PKIX Other Name Forms” (1.3.6.1.5.5.7.8) registry:

ASN.1 Module This Appendix contains the ASN.1 Module for the MAC Address; it follows the conventions established by .

MAC Address otherName Examples

EUI-48 identifier The following is a human‑readable summary of the Subject Alternative Name extension from a certificate containing a single MACAddress otherName with value 00‑24‑98‑7B‑19‑02:

EUI-64 identifier An EUI‑64 example (AC‑DE‑48‑00‑11‑22‑33‑44):

EUI-48 constraint for universal addresses The following constraint definition constrains EUI-48 values to only those are universal; locally assigned values will not match the constraint.

Normative References This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. Institute of Electrical and Electronics Engineers ITU-T ITU-T In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments We thank the participants on the LAMPS Working Group mailing list for their insightful feedback and comments. In particular, the authors extend sincere appreciation to John Mattsson and Michael St. Johns for their reviews and suggestions, which greatly improved the quality of this document.