Relying Party Handling of Resource Public Key Infrastructure (RPKI) Certificate Revocation List (CRL) Number Extensions

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Table of Contents

1. Introduction

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.¶

3. IANA Considerations

This document has no IANA actions.¶

Acknowledgements

The authors wish to thank Tom Harrison whose observations prompted this internet-draft proposal.¶

Authors' Addresses