Phil, Presuming that CA Alpha and CA Beta are different organizations, which are not affiliated, then the cross-signed Intermediate CA Gamma certificate does not meet the requirements of 7.1.2.2.3, namely:
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 .... In this hypothetical scenario, the issuer of the cross-signed intermediate certificate would be "organizationName=CA Alpha", but the subject would be "organizationName=CA Beta". While this subject correlation matches the cross-signed "Root CA Beta", that is not considered when determining the extKeyUsage requirements. On Tue, Aug 8, 2023 at 11:00 AM 'Phil Porada' via [email protected] <[email protected]> wrote: > Suppose there are three key-pairs identified by the following names: > > - Root CA Alpha > - Root CA Beta > - Intermediate CA Gamma > > Suppose a fairly traditional hierarchy utilizing those key-pairs: > > - Root CA Alpha has issued a certificate over its own public key. This > self-signed cert must comply with BRs 7.1.2.1 Root CA Certificate Profile. > - Root CA Beta has issued a certificate over its own public key. Same > as above. > - Root CA Beta has issued a basicConstraints CA=true cert over > Intermediate CA Gamma's public key. Intermediate CA Gamma must comply with > BRs 7.1.2.6 TLS Subordinate CA Certificate Profile. > > Suppose that Root CA Alpha also cross-signs Root CA Beta. This cross-sign > would normally be a TLS Subordinate CA Certificate Profile, subject to > BRs 7.1.2.6, except that it also matches the definition of BRs 7.1.2.2 > Cross-Certified Subordinate CA. Specifically it is "a CA Certificate using > the same Subject Name and Subject Public Key Information as one or more > existing CA Certificate(s), whether a Root CA Certificate or Subordinate CA > Certificate." Therefore it has slightly looser requirements, namely in > terms of extKeyUsages. This makes sense, as the whole point of the > Cross-Certified Subordinate CA profile is to allow cross-certs to more > closely match the original cert they're cross-signing.Suppose that Root > CA Alpha also cross-signs Intermediate CA Gamma. Now we arrive at a > problem. This certificate also meets the qualifications of a 7.1.2.2 > Cross-Certified Subordinate CA: it has the same Subject Name and Public Key > as an existing CA Certificate. This means that the EKU extension can be > omitted entirely because it is "unrestricted". But this is very surprising! > The TLS Subordinate CA Certificate it is cross-signing is clearly required > to have EKUs. Why does this cross-sign get to omit them? It feels like this > certificate should be required to abide by the 7.1.2.6 TLS Subordinate CA > Certificate Profile, even though it meets the qualifications of 7.1.2.2. > > If I have confused something, I apologize. Thank you for any guidance here. > > -- > You received this message because you are subscribed to the Google Groups " > [email protected]" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/a/mozilla.org/d/msgid/dev-security-policy/dcb791eb-4754-4389-a0ca-2551c9d55a7an%40mozilla.org > <https://groups.google.com/a/mozilla.org/d/msgid/dev-security-policy/dcb791eb-4754-4389-a0ca-2551c9d55a7an%40mozilla.org?utm_medium=email&utm_source=footer> > . > -- -TZ "When I am working on a problem I never think about beauty. I only think about how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." - Buckminster Fuller (1895-1983) -- You received this message because you are subscribed to the Google Groups "[email protected]" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/a/mozilla.org/d/msgid/dev-security-policy/CAAbOY9JW0iMxdKBQZsBTKprZ40_p1pi5grsiLgbEjV1VrbCikw%40mail.gmail.com.
