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SC-103: Require EKUs for Cross-Certified Subordinate CAs #668

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SC-103: Require EKUs for Cross-Certified Subordinate CAs #668

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77 changes: 13 additions & 64 deletions docs/BR.md
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Expand Up @@ -2303,81 +2303,30 @@ The `subject` MUST comply with the requirements of [Section 7.1.4](#714-name-for
| `authorityInformationAccess` | SHOULD | N | See [Section 7.1.2.10.3](#712103-ca-certificate-authority-information-access) |
| `nameConstraints` | MAY | \*[^name_constraints] | See [Section 7.1.2.10.8](#712108-ca-certificate-name-constraints) |
| Signed Certificate Timestamp List | MAY | N | See [Section 7.1.2.11.3](#712113-signed-certificate-timestamp-list) |
| Extended Key Usage | MUST | N | See [Section 7.1.2.2.5](#71225-cross-certified-subordinate-ca-extended-key-usage---restricted) |
| Any other extension | NOT RECOMMENDED | - | See [Section 7.1.2.11.5](#712115-other-extensions) |

In addition to the above, `extKeyUsage` extension requirements vary based on the relationship between the Issuer and Subject organizations represented in the Cross-Certificate.

The `extKeyUsage` extension MAY be "unrestricted" as described in the following table if:

- the `organizationName` represented in the Issuer and Subject names of the corresponding certificate are either:
- the same, or
- the `organizationName` represented in the Subject name is an affiliate of the `organizationName` represented in the Issuer name
- the corresponding CA represented by the Subject of the Cross-Certificate is operated by the same organization as the Issuing CA or an Affiliate of the Issuing CA organization.

Table: Cross-Certified Subordinate CA with Unrestricted EKU

| **Extension** | **Presence** | **Critical** | **Description** |
| --- | -- | -- | --- |
| `extKeyUsage` | SHOULD[^eku_ca] | N | See [Section 7.1.2.2.4](#71224-cross-certified-subordinate-ca-extended-key-usage---unrestricted) |

In all other cases, the `extKeyUsage` extension MUST be "restricted" as described in the following table:

Table: Cross-Certified Subordinate CA with Restricted EKU

| **Extension** | **Presence** | **Critical** | **Description** |
| --- | -- | -- | --- |
| `extKeyUsage` | MUST[^eku_ca] | N | See [Section 7.1.2.2.5](#71225-cross-certified-subordinate-ca-extended-key-usage---restricted) |

[^eku_ca]: While [RFC 5280, Section 4.2.1.12](https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.12) notes that this extension will generally only appear within end-entity certificates, these Requirements make use of this extension to further protect relying parties by limiting the scope of CA Certificates, as implemented by a number of Application Software Suppliers.

[^name_constraints]: See [Section 7.1.2.10.8](#712108-ca-certificate-name-constraints) for further requirements, including regarding criticality of this extension.

##### 7.1.2.2.4 Cross-Certified Subordinate CA Extended Key Usage - Unrestricted

Table: Unrestricted Extended Key Usage Purposes (Affiliated Cross-Certified CA)

| **Key Purpose** | **Description** |
| --- | ------- |
| `anyExtendedKeyUsage` | The special extended key usage to indicate there are no restrictions applied. If present, this MUST be the only key usage present. |
| Any other value | CAs MUST NOT include any other key usage with the `anyExtendedKeyUsage` key usage present. |

Alternatively, if the Issuing CA does not use this form, then the Extended Key Usage extension, if present, MUST be encoded as specified in [Section 7.1.2.2.5](#71225-cross-certified-subordinate-ca-extended-key-usage---restricted).
This subsection is deprecated. Cross-Certified Subordinate CA Certificates MUST NOT have an unrestricted Extended Key Usage.

##### 7.1.2.2.5 Cross-Certified Subordinate CA Extended Key Usage - Restricted

Restricted TLS Cross-Certified Subordinate CA Extended Key Usage Purposes (i.e., for restricted Cross-Certified Subordinate CAs issuing TLS certificates directly or transitively).

Table: TLS Cross-Certified Subordinate CA EKU

| **Key Purpose** | **Description** |
| --- | ------- |
| `id-kp-serverAuth` | MUST be present. |
| `id-kp-clientAuth` | MAY be present. |
| `id-kp-emailProtection`| MUST NOT be present. |
| `id-kp-codeSigning` | MUST NOT be present. |
| `id-kp-timeStamping` | MUST NOT be present. |
| `anyExtendedKeyUsage` | MUST NOT be present. |
| Any other value | NOT RECOMMENDED. |

Restricted Non-TLS Cross-Certified Subordinate CA Extended Key Usage Purposes (i.e., for restricted Cross-Certified Subordinate CAs not issuing TLS certificates directly or transitively).

Table: Non-TLS Cross-Certified Subordinate CA EKU

| **Key Purpose** | **Description** |
| --- | ------- |
| `id-kp-serverAuth` | MUST NOT be present. |
| `anyExtendedKeyUsage` | MUST NOT be present. |
| Any other value | MAY be present. |

Each included Extended Key Usage key usage purpose:
While [RFC 5280, Section 4.2.1.12](https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.12) notes that the Extended Key Usage extension will generally only appear within end-entity certificates, these Requirements make use of this extension to further protect relying parties by limiting the scope of CA Certificates, as implemented by a number of Application Software Suppliers.

1. MUST apply in the context of the public Internet (e.g. MUST NOT be for a service that is only valid in a privately managed network), unless:
a. the key usage purpose falls within an OID arc for which the Applicant demonstrates ownership; or,
b. the Applicant can otherwise demonstrate the right to assert the key usage purpose in a public context.
2. MUST NOT include semantics that will mislead the Relying Party about the certificate information verified by the CA, such as including a key usage purpose asserting storage on a smart card, where the CA is not able to verify that the corresponding Private Key is confined to such hardware due to remote issuance.
3. MUST be verified by the Issuing CA (i.e. the Issuing CA MUST verify the Cross-Certified Subordinate CA is authorized to assert the key usage purpose).
The contents of the Extended Key Usage extension MUST be exactly one of the following sets of OIDs, as determined by the intended purpose of the CA:

CAs MUST NOT include additional key usage purposes unless the CA is aware of a reason for including the key usage purpose in the Certificate.
| CA purpose | EKU OID(s) |
| --- | ----- |
| TLS server authentication | Only 1.3.6.1.5.5.7.3.1 |
| TLS client authentication | Only 1.3.6.1.5.5.7.3.2 |
| TLS (generic) | Only 1.3.6.1.5.5.7.3.1 and 1.3.6.1.5.5.7.3.2 |
| S/MIME | Only 1.3.6.1.5.5.7.3.4 |
| S/MIME (generic) | Only 1.3.6.1.5.5.7.3.4 and 1.3.6.1.5.5.7.3.2 |
| Code signing | Only 1.3.6.1.5.5.7.3.3 |
| Time stamping | Only 1.3.6.1.5.5.7.3.8 |

##### 7.1.2.2.6 Cross-Certified Subordinate CA Certificate Certificate Policies

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