| Internet-Draft | Media Access Control (MAC) Addresses in | October 2025 |
| Housley, et al. | Expires 1 May 2026 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://CBonnell.github.io/draft-housley-lamps-macaddress-on/draft-housley-lamps-macaddress-on.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-housley-lamps-macaddress-on/.¶
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Source for this draft and an issue tracker can be found at https://github.com/CBonnell/draft-housley-lamps-macaddress-on.¶
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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 [RFC5280]. 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 [RFC5280]. The new name form enables certificate‑based authentication at layer 2 and facilitates secure provisioning in Internet‑of‑Things and automotive networks.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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 pattern of the constraint that the address must match, and the second set of N octets defines the bit mask that defines the set of significant bits in the bit pattern.¶
The following sub-sections describe how to encode EUI-48 and EUI-64 values and their corresponding constraints.¶
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.¶
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:¶
The first set of N octets (where N is 6 for an EUI-48 constraint or 8 for an EUI-64 constraint) contains the "value bit pattern". This bit pattern encodes the bits that the masked address must contain to be considered a match.¶
The second set of N octets 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 first set of N octets.¶
The bit patterns encoded in both the value bit pattern and mask 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 canonical encoding is used for a given mask bit pattern and value bit pattern.¶
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.¶
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.¶
To determine if a constraint matches a given name, the certificate-consuming application performs the following algorithm:¶
If the name is 6 octets (representing an EUI-48 value) and the constraint is 16 octets (representing an EUI-64 constraint), then the name does not match the constraint.¶
If the name is 8 octets (representing an EUI-64 value) and the constraint is 12 octets (representing an EUI-48 constraint), then the name does not match the constraint.¶
Extract the value bit pattern from the upper (big-endian) N octets of the constraint, where N is "6" for EUI-48 identifiers and "8" for EUI-64 identifiers.¶
Extract the mask bit pattern from the lower (big-endian) N octets of the constraint, where N is "6" for EUI-48 identifiers and "8" for EUI-64 identifiers.¶
Perform an exclusive OR (XOR) operation with the value bit string extracted in step 3 and the octets of the name value.¶
Perform a bitwise AND operation with the bit string calculated in step 5 and the mask bit pattern.¶
If the result of step 6 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 algorithm can be alternatively expressed as:¶
2 * length (name) == length (constraint) &&
((constraint.value_bit_pattern ^ name) & constraint.mask_bit_pattern) == 0
¶
Implementations are not required to implement this algorithm, but MUST calculate an identical result to this algorithm for a given set of inputs.¶
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.¶
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 is requested to make the following assignments in the “SMI Security for PKIX Module Identifier” (1.3.6.1.5.5.7.0) registry:¶
+=========+====================================+===============+
| Decimal | Description | References |
+=========+====================================+===============+
| TBD0 | id-mod-mac-address-other-name-2025 | This document |
+---------+------------------------------------+---------------+
¶
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:¶
+=========+=================================+===============+
| Decimal | Description | References |
+=========+=================================+===============+
| TBD1 | id-on-MACAddress | This document |
+---------+---------------------------------+---------------+
¶
This Appendix contains the ASN.1 Module for the MAC Address; it follows the conventions established by [RFC5912].¶
MACAddressOtherName-2025
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-mac-address-other-name-2025(TBD0) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
OTHER-NAME FROM PKIX1Implicit-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-implicit-02(59) }
id-pkix FROM PKIX1Explicit-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-explicit-02(51) } ;
-- id-pkix 8 is the otherName arc
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }
-- OID for this name form
id-on-MACAddress OBJECT IDENTIFIER ::= { id-on TBD1 }
-- Contents of the otherName field
MACAddressOtherNames OTHER-NAME ::= { on-MACAddress, ... }
on-MACAddress OTHER-NAME ::= {
MACAddress IDENTIFIED BY id-on-MACAddress }
MACAddress ::= OCTET STRING (SIZE (6 | 8 | 12 | 16))
END
¶
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:¶
SEQUENCE {
otherName [0] {
OBJECT IDENTIFIER id-on-MACAddress (TBD)
[0] OCTET STRING '0024987B1902'H
}
}
¶
An EUI‑64 example (AC‑DE‑48‑00‑11‑22‑33‑44):¶
[0] OCTET STRING 'ACDE480011223344'H¶
The first octet of a MAC address contains two flag bits. IEEE bit numbering has bit '0' as the least significant bit of the octet.¶
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 addresses 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 addresses the subscriber legitimately controls (registered OUI or CID). Before issuing a certificate that contains a MACAddress or a name constraint based on such a permitted set of addresses, the CA MUST verify that control: for example, by consulting the IEEE registry or reviewing manufacturer documentation.¶
The following constraint definition constrains EUI-48 values to only those are universal and unicast; locally assigned or multicast values will not match the constraint.¶
[0] OCTET STRING '020000000000030000000000'H¶
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 David von Oheimb, John Mattsson, and Michael StJohns for their reviews and suggestions, which greatly improved the quality of this document.¶