RPKI CRL number handling: Handling of Resource Public Key Infrastructure (RPKI) Certificate Revocation List (CRL) Number Extensions
In the RPKI, validators must ignore CRL Number values and rely on the CRL named in the issuer's current manifest.
Snijders, J., Maddison, B., and T. Buehler, "Handling of Resource Public Key Infrastructure (RPKI) Certificate Revocation List (CRL) Number Extensions", RFC 9829, DOI 10.17487/RFC9829, July 2025, <https://www.rfc-editor.org/info/rfc9829>.
@misc{rfc9829,
series = {Request for Comments},
number = 9829,
howpublished = {RFC 9829},
publisher = {RFC Editor},
doi = {10.17487/RFC9829},
url = {https://www.rfc-editor.org/info/rfc9829},
author = {J. Snijders and B. Maddison and T. Buehler},
title = {{Handling of Resource Public Key Infrastructure (RPKI) Certificate Revocation List (CRL) Number Extensions}},
pagetotal = 7,
year = 2025,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Snijders
Request for Comments: 9829
Updates: 6487 B. Maddison
Category: Standards Track Workonline
ISSN: 2070-1721 T. Buehler
OpenBSD
July 2025
Handling of Resource Public Key Infrastructure (RPKI) Certificate
Revocation List (CRL) Number Extensions
Abstract
This document revises how the Resource Public Key Infrastructure
(RPKI) handles Certificate Revocation List (CRL) Number extensions.
This document updates RFC 6487.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9829.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
1.2. Related Work
1.3. Changes from RFC 6487
2. Summary
3. Updates to RFC 6487
3.1. Updates to Section 5
3.2. Update to Section 7.2
4. Operational Considerations
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Section 5.2.3 of [RFC5280] describes the value of the Certificate
Revocation List (CRL) Number extension as a monotonically increasing
sequence number, which "allows users to easily determine when a
particular CRL supersedes another CRL". In other words, in Public
Key Infrastructures (PKIs) in which it is possible for Relying
Parties (RPs) to encounter multiple usable CRLs, the CRL Number
extension is a means for an RP to determine which CRLs to rely upon.
In the Resource Public Key Infrastructure (RPKI), a well-formed
manifest fileList contains exactly one entry for its associated CRL,
together with a collision-resistant message digest of that CRL's
contents (see Section 2.2 of [RFC6481] and Section 2 of [RFC9286]).
Additionally, the target of the CRL Distribution Points extension in
an RPKI Resource Certificate is the same CRL object listed on the
issuing Certification Authorities (CAs) current manifest (see
Section 4.8.6 of [RFC6487]). Together, these properties guarantee
that RPKI RPs will always be able to unambiguously identify exactly
one current CRL for each RPKI CA. Thus, in the RPKI, the ordering
functionality provided by CRL Numbers is fully subsumed by
monotonically increasing manifest numbers (Section 4.2.1 of
[RFC9286]), thereby obviating the need for RPKI RPs to process CRL
Number extensions at all.
Therefore, although the CRL Number extension is mandatory in RPKI
CRLs for compliance with the X.509 v2 CRL Profile (Section 5 of
[RFC5280]), any use of this extension by RPKI RPs merely adds
complexity and fragility to RPKI Resource Certificate path
validation. This document mandates that RPKI RPs ignore the CRL
Number extension.
This document updates [RFC6487]. Refer to Section 3 for more
details.
1.1. Requirements Language
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.
1.2. Related Work
The reader is assumed to be familiar with the terms and concepts
described in "Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile" [RFC5280], "A Profile
for Resource Certificate Repository Structure" [RFC6481], and
"Manifests for the Resource Public Key Infrastructure (RPKI)"
[RFC9286].
1.3. Changes from RFC 6487
This section summarizes the significant changes between [RFC6487] and
this document.
* Revision of CRL Number handling.
* Adjustment of step 5 of the Resource Certification Path
Validation.
* Integration of Errata 3205 [Err3205].
2. Summary
This document clarifies that, in the RPKI, there is exactly one CRL
that is appropriate and relevant for determining the revocation
status of a given resource certificate. It is the unique CRL object
that is simultaneously:
* the target of the certificate's CRL Distribution Points extension,
and
* listed in the issuing CA's current manifest fileList and has a
matching hash (see Section 4.2.1 of [RFC9286]).
In particular, a resource certificate cannot be validated without
consulting the current manifest of the certificate's issuer.
3. Updates to RFC 6487
3.1. Updates to Section 5
This section updates Section 5 of [RFC6487] as follows:
* First change:
OLD
| Where two or more CRLs are issued by the same CA, the CRL with
| the highest value of the "CRL Number" field supersedes all
| other CRLs issued by this CA.
NEW
| Per Section 5.2.3 of [RFC5280], CAs issue new CRLs using a
| monotonically increasing sequence number in the "CRL Number"
| extension. It is RECOMMENDED that the "CRL Number" match the
| "manifestNumber" of the manifest that will include this CRL
| (see Section 4.2.1 of [RFC9286]).
* Second change:
OLD
| An RPKI CA MUST include the two extensions, Authority Key
| Identifier and CRL Number, in every CRL that it issues. RPs
| MUST be prepared to process CRLs with these extensions. No
| other CRL extensions are allowed.
NEW
| An RPKI CA MUST include exactly two extensions in every CRL
| that it issues: an Authority Key Identifier (AKI) and a CRL
| Number. No other CRL extensions are allowed.
|
| - RPs MUST process the AKI extension.
|
| - RPs MUST ignore the CRL Number extension except for checking
| that it is marked as non-critical and contains a non-
| negative integer less than or equal to 2^159-1.
3.2. Update to Section 7.2
This section updates Section 7.2 of [RFC6487] as follows:
OLD
| 5. The issuer has not revoked the certificate. A revoked
| certificate is identified by the certificate's serial number
| being listed on the issuer's current CRL, as identified by the
| CRLDP of the certificate, the CRL is itself valid, and the
| public key used to verify the signature on the CRL is the same
| public key used to verify the certificate itself.
NEW
| 5. The issuer has not revoked the certificate. A revoked
| certificate is identified by the certificate's serial number
| being listed on the issuer's current CRL, as identified by the
| issuer's current manifest and the CRLDP of the certificate.
| The CRL is itself valid and the public key used to verify the
| signature on the CRL is the same public key used to verify the
| certificate itself.
4. Operational Considerations
This document has no additional operational considerations beyond
those described in Section 9 of [RFC6487].
5. Security Considerations
The Security Considerations of [RFC3779], [RFC5280], and [RFC6487]
apply to Resource Certificates and CRLs.
This document explicates that, in the RPKI, the CRL listed on the
certificate issuer's current manifest is the one relevant and
appropriate for determining the revocation status of a resource
certificate. The hash in the manifest's fileList provides a
cryptographic guarantee on the Certification Authority's intent that
this is the most recent CRL and removes possible replay vectors.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6481] Huston, G., Loomans, R., and G. Michaelson, "A Profile for
Resource Certificate Repository Structure", RFC 6481,
DOI 10.17487/RFC6481, February 2012,
<https://www.rfc-editor.org/info/rfc6481>.
[RFC6487] Huston, G., Michaelson, G., and R. Loomans, "A Profile for
X.509 PKIX Resource Certificates", RFC 6487,
DOI 10.17487/RFC6487, February 2012,
<https://www.rfc-editor.org/info/rfc6487>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9286] Austein, R., Huston, G., Kent, S., and M. Lepinski,
"Manifests for the Resource Public Key Infrastructure
(RPKI)", RFC 9286, DOI 10.17487/RFC9286, June 2022,
<https://www.rfc-editor.org/info/rfc9286>.
7.2. Informative References
[Err3205] RFC Errata, Erratum ID 3205, RFC 6487,
<https://www.rfc-editor.org/errata/eid3205>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<https://www.rfc-editor.org/info/rfc3779>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
Acknowledgements
The authors wish to thank Tom Harrison whose observations prompted
this document, Alberto Leiva, Tim Bruijnzeels, Mohamed Boucadair,
Geoff Huston, and the IESG for their valuable comments and feedback.
Authors' Addresses
Job Snijders
Amsterdam
The Netherlands
Email: job@sobornost.net
Ben Maddison
Workonline
Cape Town
South Africa
Email: benm@workonline.africa
Theo Buehler
OpenBSD
Switzerland
Email: tb@openbsd.org
AI ELI5
On this page
§1Why CRL Numbers Are Redundant in the RPKI
A Certificate Revocation List (CRL) carries a CRL Number extension, a monotonically increasing sequence number defined in RFC 5280. It lets a relying party (RP) decide which of several usable CRLs supersedes another.
In the Resource Public Key Infrastructure (RPKI) that choice never arises. A well-formed manifest fileList contains exactly one entry for its CRL, with a collision-resistant digest of the contents. The CRL Distribution Points extension of a resource certificate points at the same CRL listed on the issuing CA's current manifest. So an RP can always identify exactly one current CRL for each CA, and the ordering job of the CRL Number is covered by the monotonically increasing manifest number (RFC 9286).
The extension is still mandatory in RPKI CRLs, for compliance with the X.509 v2 CRL profile. The RFC says any use of it by RPs only adds complexity and fragility to certification path validation, so it mandates that RPs ignore it. It updates RFC 6487.
If there is already a trusted sign saying 'this is the current list', counting page numbers on the lists is extra work that can go wrong. So checkers skip it.
§2One Relevant CRL per Certificate
The RFC clarifies that for a given resource certificate there is exactly one CRL that is appropriate for checking revocation. It is the unique CRL object that is both the target of the certificate's CRL Distribution Points extension and listed in the issuing CA's current manifest fileList with a matching hash.
One consequence is stated directly: a resource certificate cannot be validated without consulting the current manifest of the certificate's issuer.
To check a pass, you look at the list the office names as current, and only that one. You cannot skip asking the office.
§3Changes to RFC 6487 Section 5
Two passages of RFC 6487 Section 5 are replaced.
First, the old rule that the CRL with the highest CRL Number supersedes all other CRLs from the same CA is gone. The new text says CAs issue new CRLs with a monotonically increasing number as in RFC 5280, and it is RECOMMENDED that the CRL Number match the manifestNumber of the manifest that will include the CRL.
Second, the old text said an RPKI CA MUST include the Authority Key Identifier and CRL Number extensions and that RPs MUST be prepared to process them. The new text says a CA MUST include exactly two extensions in every CRL, the AKI and the CRL Number, and no others are allowed. RPs MUST process the AKI extension. RPs MUST ignore the CRL Number extension except for checking that it is marked non-critical and holds a non-negative integer less than or equal to 2^159-1.
CAs still number their lists, and are encouraged to use the same number as the office's folder number. Checkers only make sure the number looks sane, then ignore it.
§4Changes to Path Validation
Step 5 of the resource certification path validation in Section 7.2 of RFC 6487 is also adjusted. The check that the issuer has not revoked the certificate still means finding the certificate's serial number on the issuer's current CRL. That CRL is now identified by the issuer's current manifest and the certificate's CRLDP, rather than by the CRLDP alone.
The CRL must itself be valid, and the public key that verifies its signature must be the same key that verifies the certificate.
The RFC also lists the integration of Errata 3205 among its changes from RFC 6487.
When checking if a pass was cancelled, you now use the list the office names, not just any list you can find.
§5Operational Considerations
The RFC adds no operational considerations beyond those in Section 9 of RFC 6487.
Day-to-day running of the system does not change, apart from the new rule about ignoring the counter.
§6Security Considerations
The security considerations of RFC 3779, RFC 5280 and RFC 6487 apply to resource certificates and CRLs. This document makes explicit that, in the RPKI, the CRL on the issuer's current manifest is the relevant one for revocation status.
The hash in the manifest's fileList gives a cryptographic guarantee of the CA's intent that this is the most recent CRL, and it removes possible replay vectors. The RFC has no IANA actions.
The office's seal on the current list proves which list is the newest, so nobody can slip in an old list and pretend it is current.