Media Access Control (MAC) Addresses in X.509 Certificates

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 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.

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:

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.

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. In the pseudo-code below, 'mask' is shorthand for the bit string formed from the mask portion of a constraint (e.g., the second set of N octets in the constraint, where N is 6 for an EUI-48 constraint or 8 for an EUI-64 constraint). Similarly, 'value' refers to the bit string formed from the first set of N octets in the constraint.

Matching Rule To determine if a name matches a given constraint, 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: // Returns true if 'name' matches 'constraint' boolean nameMatchesConstraint (name, constraint) { return (2 * length (name) == length (constraint) && ((constraint.value ^ name) & constraint.mask) == 0) ; } Implementations are not required to implement this algorithm, but MUST calculate an identical result to this algorithm for a given set of inputs.

OtherName.MACAddress Path Validation Processing This section describes the Path Validation Processing specific to OtherName.MACAddress constraints. The following is a utility function used to determine whether or not a given constraint is completely contained within another constraint.

Initialization Per sections 6.1.1 (h) and (i) of , we need to specify NameConstraint.MACAddress set values for both the initial-permitted-subtrees and for initial-excluded-subtrees:

Intersection Operation See Section 6.1.4 (g) (1) of . The intersection of the set of OtherName.MACAddress current permitted_subtrees with each certificate in the path is as follows: one of the requested subtrees (from the cert) // pst -> one of the current permitted subtrees foreach ( constraint rst in tempRequestedSubtrees) { foreach ( constraint pst in prevSubtrees) { if (childIncludedInParent (rst, pst) { tempPermitedSubtree += rst; break; } } } permitted_subtrees{} (i) = tempPermittedSubtree; // } end for each cert on path ]]>

Union Operation See Section 6.1.4 (g) (2) of . The union of the set of OtherName.MACAddress current excluded_subtrees with each certificate in the path is as follows: # 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.