| Internet-Draft | RPKI CRL Number handling for RPs | September 2024 |
| Snijders, et al. | Expires 14 March 2025 | [Page] |
This document clarifies how Resource Public Key Infrastructure (RPKI) Relying Parties (RPs) handle 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 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 PKIs in which it is possible for RPs to encounter multiple usable CRLs, the CRL Number extension is a means for the RP to determine which CRL(s) to rely upon.¶
In the RPKI, a well-formed Manifest FileList contains exactly one entry for its associated CRL, together with a collision-resistant message digest of that CRLs 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 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 MUST ignore the CRL Number extension.¶
This document updates [RFC6487] with clarifications for RP implementers.¶
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] and "A Profile for Resource Certificate Repository Structure" [RFC6481].¶
This section updates [RFC6487].¶
In Section 5, this paragraph is removed.¶
REMOVED¶
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.¶
In Section 5, this paragraph is changed.¶
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, and MUST ignore the CRL Number extension.¶
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 following RP implementations check for the presence of the CRL Number extension and whether it is marked non-critical, and otherwise ignore it.¶
The authors wish to thank Tom Harrison whose observations prompted this internet-draft proposal.¶