Policy 2.7.1: MRSP Issue #218: Clarify CRL requirements for End Entity Certificates

[Ben Wilson]

This is the last issue that I have marked for discussion in relation to
version 2.7.1 of the Mozilla Root Store Policy
<https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/policy/>.
It is identified and discussed in GitHub Issue #218
<https://github.com/mozilla/pkipolicy/issues/218> for the MRSP.

I will soon update everyone on the status of the other 13 discussion items
already presented, as some of them are in need of revision based on
comments received thus far.

While subsection (b) of section 7.1.2.3 of the Baseline Requirements makes
a cRLDistributionPoint (CDP) in end entity certificates optional, Mozilla
still desires that CRL-based revocation information be available because
CRLite uses CRLs to construct its revocation filters. (Apple also uses
such CRL information in its certificate validation processes and, as I
understand, is making a similar request of CAs with respect to the new
CCADB field, discussed below.)

While all such CRL information is needed, large CRLs are disfavored because
of the time they take to download and process. Thus, CAs shard, partition,
or "scope" their CRLs into smaller chunks. Section 5 of RFC 5280 explains,
"Each CRL has a particular scope. The CRL scope is the set of certificates
that could appear on a given CRL. … A complete CRL lists all unexpired
certificates, within its scope, that have been revoked for one of the
revocation reasons covered by the CRL scope. A *full and complete CRL*
lists all unexpired certificates issued by a CA that have been revoked for
any reason." (Emphasis added.)

There is a new field in the CCADB for CAs to include information needed for
browsers or others to construct a "full and complete CRL", i.e. to gather
information from CAs that don't include the CRL path to their "full and
complete CRL" in end entity certificates they issue. This new CCADB field
is called "Full CRL Issued By This CA" and is located under the heading
"Pertaining to Certificates Issued by this CA." Rather than condition the
requirement that CAs fill in this information in the CCADB only when they
don't include a CDP to a full and complete CRL, I propose that this new
CCADB field be populated in all situations where the CA is enabled for
server certificate issuance. In cases where the CA shards or partitions its
CRL, the CA must provide a JSON-based list of CRLs that when combined are
the equivalent of the full and complete CRL.

Proposed language to add to section 6 of the Mozilla Root Store Policy is
as follows:

*CAs SHOULD place the URL for the associated CRL within the
crlDistributionPoints extension of issued certificates. A CA MAY omit the
crlDistributionPoint extension, if permitted by applicable requirements and
policies, such as the Baseline Requirements. *

*A CA technically capable of issuing server certificates MUST ensure that
the CCADB field "Full CRL Issued By This CA" contains either the URL for
the full and complete CRL or the URL for the JSON file containing all URLs
for CRLs that when combined are the equivalent of the full and complete CRL*
.


I look forward to your comments and suggestions.

Ben

[Ryan Hurst]

I think this text strikes a good balance.

[Corey Bonnell]

Hi Ben,
A few follow-up questions and comments:

1) What are the expectations regarding availability for such CRLs? Do the availability requirements in BR 4.10.2 stand for these CRLs even if such CRL pointers are not encoded in end-entity certificates?
2) What is the expectation for populating the CRLDP in end-entity S/MIME certificates? If no change in policy for S/MIME end-entity certificates is desired, then I think the text should be further qualified with "CAs SHOULD place the URL for the associated CRL within the crlDistributionPoints extension of issued *server* certificates".

Thanks,
Corey

[Rob Stradling]

Hi Ben.

> *A CA technically capable of issuing server certificates MUST ensure that
> the CCADB field "Full CRL Issued By This CA" contains either the URL for
> the full and complete CRL or the URL for the JSON file containing all URLs
> for CRLs that when combined are the equivalent of the full and complete CRL*

As a consumer of this data (crt.sh), I'd much prefer to see "Full CRL Issued By This CA" and "the URL for the JSON file" as 2 separate fields in the CCADB. CAs would then be expected to fill in one field or the other, but not both. Is that possible?

To ensure that these JSON files can be programmatically parsed, I suggest specifying the requirement a bit more strictly. Something like this:
"...or the URL for a file that contains only a JSON Array, whose elements are URLs of DER encoded CRLs that when combined are the equivalent of a full and complete CRL"

> I propose that this new CCADB field be populated in all situations where the CA is enabled for server certificate issuance.

Most Root Certificates are "enabled for server certificate issuance". Obviously CAs shouldn't issue leaf certs directly from roots, but nonetheless the technical capability does exist. However, currently CAs can't edit Root Certificate records in the CCADB, which makes populating these new field(s) "in all situations" rather hard.

Since OneCRL covers revocations of intermediate certs, may I suggest that CAs should only be required to populate these new field(s) in intermediate certificate records (and not in root certificate records)?

Relatedly, "A CA technically capable of...that the CCADB field" seems wrong. CCADB "CA Owner" records don't/won't contain the new field(s). Similar language elsewhere in the policy (section 5.3.2) says "All certificates that are capable of being used to..." (rather than "All CAs...").

Technically-constrained intermediate certs don't have to be disclosed to CCADB, but "in all situations where the CA is enabled for server certificate issuance" clearly includes technically-constrained intermediates. How would a CA populate the "Full CRL Issued By This CA" field for a technically-constrained intermediate cert that has (legitimately) not been disclosed to CCADB?

________________________________
From: dev-security-policy <dev-security-...@lists.mozilla.org> on behalf of Ben Wilson via dev-security-policy <dev-secur...@lists.mozilla.org>
Sent: 08 January 2021 01:00
To: mozilla-dev-security-policy <mozilla-dev-s...@lists.mozilla.org>
Subject: Policy 2.7.1: MRSP Issue #218: Clarify CRL requirements for End Entity Certificates

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This is the last issue that I have marked for discussion in relation to
version 2.7.1 of the Mozilla Root Store Policy

<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.mozilla.org%2Fen-US%2Fabout%2Fgovernance%2Fpolicies%2Fsecurity-group%2Fcerts%2Fpolicy%2F&amp;data=04%7C01%7Crob%40sectigo.com%7C65685639e2bf45be5f6f08d8b370cf17%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C637456644391892862%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=shOhNu8IGrT0iSSt2LY4E6LQlsr6y435Vv%2BNezNCh98%3D&amp;reserved=0>.

It is identified and discussed in GitHub Issue #218

<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fmozilla%2Fpkipolicy%2Fissues%2F218&amp;data=04%7C01%7Crob%40sectigo.com%7C65685639e2bf45be5f6f08d8b370cf17%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C637456644391892862%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=Zb0abofrs3IaJzX9nkEnFf6RbyCemLYIi%2B7l4SmUz5U%3D&amp;reserved=0> for the MRSP.

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[Ben Wilson]

All,

Another suggestion came in for clarification that hasn't been raised on the
list yet, so I'll try and explain the scenario here.

Normally, a CA must publish and update its CRLs until the end of the CA
certificate's expiration. However, I think that some CAs partition their
CRLs based on issuance time, e.g., all certificates issued during January
2021. And all of those certificates would expire after the applicable
validity period. I think CAs don't continue to regenerate or reissue those
types of partitioned CRLs which only contain certificates that have
expired. So maybe we need to add an express exception that allows CAs to
omit those obsolete CRLs from the JSON file -- as long as the JSON file
contains the equivalent of a "full and complete" CRL.

Thoughts?

Thanks,
Ben

> ------------------------------
> *From:* dev-security-policy <dev-security-...@lists.mozilla.org>

> on behalf of Ben Wilson via dev-security-policy <
> dev-secur...@lists.mozilla.org>

> *Sent:* 08 January 2021 01:00
> *To:* mozilla-dev-security-policy <
> mozilla-dev-s...@lists.mozilla.org>
> *Subject:* Policy 2.7.1: MRSP Issue #218: Clarify CRL requirements for

[Aaron Gable]

I think that an explicit carve-out for time-scoped CRLs is a very good idea.

In the case that this change to the MRSP is adopted, I suspect that LE
would scope CRLs by notAfter quite tightly, with perhaps one CRL per 24 or
even 6 hours of issuance. We would pick a small interval such that we could
guarantee that each CRL would still be a reasonable size even in the face
of a mass revocation event.

Producing CRLs at that rate, it would be very valuable to be able to
gracefully age CRLs out once there is no possibility for a revocation
status update for any certificate in their scope.

Aaron

> https://lists.mozilla.org/listinfo/dev-security-policy
>

[Ben Wilson]

As placeholder in the Mozilla Root Store Policy, I'm proposing the
following sentence for section 6.1 - "A CA MUST ensure that it populates
the CCADB with the appropriate 'full CRL' in the CCADB revocation
information field pertaining to certificates issued by the CA
<https://www.ccadb.org/cas/fields#revocation-information> for each
intermediate CA technically capable of issuing server certificates." (The
hyperlink isn't active yet until we have the CCADB language and
implementation clarified, per Kathleen's recent email and responses
thereto.) Here it is on GitHub -
https://github.com/BenWilson-Mozilla/pkipolicy/commit/26c1ee4ea8be1a07f86253e38fbf0cc043e12d48.
Caveat - other browsers, such as Apple, will likely have more encompassing
implementation requirements for when to populate these "full CRL" fields.

[Ben Wilson]

All,
We are going to postpone the resolution of this Issue #218 and the addition
of language to address the "Full CRL" until MRSP version 2.8.
Thanks for your input thus far.
Ben