| Internet-Draft | RPKI CRL Number handling | March 2025 |
| Snijders, et al. | Expires 26 September 2025 | [Page] |
This document revises how the Resource Public Key Infrastructure (RPKI) handles Certificate Revocation List (CRL) Number extensions. This document updates RFC 6487.¶
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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.¶
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.¶
It is assumed that the reader is 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].¶
This document clarifies that, in the RPKI, there is exactly one CRL appropriate and relevant for determining the revocation status of a given resource certificate. It is the unique CRL object that is simultaneously:¶
In particular, a resource certificate cannot be validated without recourse to the current Manifest of the certificate's issuer.¶
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" matches 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.¶
This section updates Section 7.2 of [RFC6487] as follows:¶
OLD¶
- 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¶
- 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.¶
This document has no additional operational considerations compared to Section 9 of [RFC6487].¶
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 determinining the revocation status of a resouce certificate. By way of the hash in the manifest's FileList this provides a cryptographic guarantee on the Certification Authority's intent that this is the most recent CRL and removes possible replay vectors.¶
This document has no IANA actions.¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in RFC 7942. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to RFC 7942, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
The authors wish to thank Tom Harrison whose observations prompted this document, Alberto Leiva, Tim Bruijnzeels, Mohamed Boucadair, and Geoff Huston for valuable feedback.¶