Resource Public Key Infrastructure (RPKI) Manifest Number Handling

The Resource Public Key Infrastructure (RPKI) makes use of signed objects called manifests . A manifest lists each file that an issuer intends to include within an RPKI repository , and can be used to detect certain forms of attack against a repository. Manifests include a "manifest number" (manifestNumber), which an issuer must increment by one whenever it issues a new manifest, and Relying Parties (RPs) are required to verify that a newly-retrieved manifest for a given Certification Authority (CA) has a higher manifestNumber than the previously-validated manifest (). However, the manifestNumber field is 20 octets in length (i.e., bounded), and no behaviour is specified for when a manifestNumber reaches the largest possible value (2¹⁵⁹-1). When that value is reached, some RP implementations will accept a new manifest for the CA only once the current manifest has expired, while others will not accept a new manifest at all. While it is practically impossible for an issuer to reach the largest possible value under normal operating conditions (it would require that the issuer issue one manifest per second for 23,171,956,451,847,141,650,870 quintillion years), there is still a chance that it could be reached due to bugs in the issuance or publication systems or incorrect/inadvertent use of those systems. For example:

Incrementing by large values when issuing manifests, such that the time to reach that largest value is reduced.
Reissuing new manifests in an infinite delay-free loop, such that the manifestNumber increases by a large value in a comparatively short period of time.
Inadvertently setting the manifestNumber to the largest possible value, such that the issuer will no longer be able to publish usable manifests for that repository.

These scenarios might also arise in combination and be more severe as a result. For example, a CA might increase the manifestNumber by a large value on reissuance, and also reissue the manifest more frequently than is necessary. For a subordinate CA, the risk of repository invalidation due to such a problem can be addressed by the issuer using the key rollover process to get a new CA certificate. RPs will treat this new certificate as though it represents a distinct CA, and the manifestNumber can be reset at that point. However, this option is not available for RPKI Trust Anchors (TAs). If a TA publishes a manifest with the largest-possible manifestNumber value, then it is difficult to rely on the TA after that point, since (as described previously) some RPs will not accept a new manifest until the current one has expired, while others will reject all new manifests indefinitely. Particularly in the case of TAs, the manifest validity period may be quite long, too. Issuing a new TA and distributing the associated Trust Anchor Locator (TAL) to clients would involve a large amount of work for TA operators and RPs. Additionally, depending on the RP implementation being used, there would be a limited degree of RPKI protection by way of that TA for the time between the issuance of the problematic manifest and the installation of the new TAL. In order to avoid these problems, this document updates by defining how issuers and RPs can handle this scenario in order to facilitate ongoing use of an affected repository.

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 when, and only when, they appear in all capitals, as shown here.

For a given CA, an RP MUST NOT reject a new manifest issued by that CA on the basis of it not having a higher manifestNumber than a previously-validated manifest if the new manifest has a different filename from that of the previously-validated manifest. In other words, an RP has to reset its stored manifestNumber for a given CA if the CA changes the filename of its manifest. With this behaviour, it is possible for a CA to be configured such that any time it issues a new manifest, it uses a new filename for that manifest. If a CA were configured in this way, the manifestNumber validation set out in would have no purpose. To avoid this outcome, CAs SHOULD NOT use new filenames for manifests except in situations where such a change is necessary to address the invalidity problem described in this document. Similarly, an RP SHOULD alert its operators when a manifest filename changes for a given CA. To avoid certain forms of replay attack, RPs MUST verify that the URI in the accessLocation in one of the id-ad-signedObject accessMethod instances in the manifest's Subject Information Access (SIA) extension exactly matches the URI presented in the RPKI Repository Delta Protocol (RRDP) "publish" element or the path presented by remote rsync servers. A CA specifies its manifest URI by way of an SIA entry with an accessMethod of id-ad-rpkiManifest (). For the purposes of this document, the manifest filename is the final segment of the path of the accessLocation URI from that SIA entry. states that a CA may include in its certificate multiple id-ad-rpkiManifest SIA entries. For comparisons, an RP may use the filename from any one of the id-ad-rpkiManifest SIA entries in the previously-validated CA certificate. If that filename does not appear in any of the id-ad-rpkiManifest SIA entries in the CA certificate that is currently being validated, then the manifest filename has changed for the purposes of this document. The corollary of this behaviour is that a CA that includes multiple id-ad-rpkiManifest SIA entries in its certificate and wants to rely on the behaviour defined in this document MUST ensure that none of the manifest filenames in the previous CA certificate appear in the newly-issued CA certificate. describes a manifest selection approach for RPs that involves collecting all unexpired, valid manifests for a CA, and then selecting from that collection the manifest that has the highest manifestNumber. The approach set out in the current document is different from that approach.

contains a specific update to for the handling of manifest numbers, in order to address one potential permanent invalidity scenario. RPs that encounter other permanent invalidity scenarios SHOULD also consider how those can be addressed such that the scenario does not require the relevant CA or TA to perform a key rollover operation. For example, in the event that an RP recognises that a permanent invalidity scenario has occurred, the RP could alert the operator and provide an option to the operator to stop relying on cached data for the affected repository, so that the CA can rectify the problem.

CA software may opt to support the manifest number reset functionality in various ways. For example, it could change the manifest filename when the manifestNumber reaches a certain threshold, or it could alert the operator in this scenario and request confirmation that the filename should be changed.

The RPKI primarily exists to support and improve security of the global Internet routing system. Reliability improvements to the RPKI itself, such as outlined in this document, strengthen its dependability (see ). requires that RPs perform two replay-related checks on newly-retrieved manifests: firstly, that the purported new manifest has a greater manifestNumber than the cached manifest, and secondly, that the purported new manifest has a more recent thisUpdate than the cached manifest. An RP that implements the behaviour in Section 2 will momentarily omit the manifestNumber check following a manifest filename change. So long as the RP still performs the second check described above, it will be protected against replay attacks. Although this document updates , the security considerations from remain relevant.

This document has no actions for IANA.

Implementation status 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 . 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 , "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".

OpenBSD
Routinator

The authors would like to thank , , , , , , and for their review and feedback on this document.