Network Working Group                                          G. Huston
Internet-Draft                                             G. Michaelson
Intended status: Informational                                     APNIC
Expires: January 3, 2015                                     C. Martinez
                                                          T. Bruijnzeels
                                                                RIPE NCC
                                                               A. Newton
                                                                 A. Aina
                                                            July 2, 2014

                      RPKI Validation Reconsidered


   This document reviews the certificate validation procedure specified
   in RFC6487 and highlights aspects of potentially acute operational
   fragility in the management of certificates in the RPKI in response
   to the movement of resources across registries, and the associated
   actions of Certification Authorities to maintain continuity of
   validation of certification of resources during this movement.

Status of this Memo

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   This Internet-Draft will expire on January 3, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Certificate Validation in the RPKI  . . . . . . . . . . . . . . 3
   3.  Operational Considerations  . . . . . . . . . . . . . . . . . . 4
   4.  Alternatives Approaches . . . . . . . . . . . . . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   This document reviews the certificate validation procedure specified
   in RFC6487 and highlights aspects of potentially acute operational
   fragility in the management of certificates in the RPKI in response
   to the movement of resources across registries, and the associated
   actions of Certification Authorities to maintain continuity of
   validation of certification of resources during this movement.

2.  Certificate Validation in the RPKI

   As currently defined in section 7.2 of [RFC6487], validation of PKIX
   certificates that conform to the RPKI profile relies on the use of a
   path validation process where each certificate in the validation path
   is required to meet the certificate validation criteria.  This can be
   considered to be a recursive validation process where, in the context
   of an ordered sequence of certificates, as defined by each pair of
   certificates in this sequence having a common Issuer and Subject Name
   respectively, a certificate is defined as valid if it satisfies basic
   validation criteria relating to the syntactic correctness, currency
   of validity dates and similar properties of the certificate itself,
   as described in [RFC5280], and also that it satisfies certain
   additional criteria with respect to the previous certificate in the
   sequence (the Issuer part of the pair), and that this previous
   certificate is itself a valid certificate using the same criteria.
   This definition applies recursively to all certificates in the
   sequence apart from the initial sequence element, which is required
   to be a Trust Anchor.

   For RPKI certificates, the additional criteria relating to the
   previous certificate in this sequence is that the certificate's
   number resource set, as defined in [RFC3779], is "encompassed" by the
   number resource set contained in the previous certificate.

   Because [RFC6487] validation demands that all resources in a
   certificate be valid under the parent (and recursively, to the root),
   a digitally signed attestation, such as a Route Origin Authorization
   (ROA) object [RFC6482], which refers only to a subset of RFC3779-
   specified resources from that certificate validation chain can be
   concluded to be invalid, but not by virtue of the relationship
   between the RFC3779 extensions of the certificates on the putative
   certificate validation path and the resources in the ROA, but by
   other resources described in these certificates where the
   "encompassing" relationship of the resources does not hold.  Any such
   invalidity along the certificate validation path can cause this
   outcome, not just at the immediate parent of the end entity
   certificate that attests to the key used to sign the ROA.

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   For example, in the certificate sequence:

     Certificate 1:
       Issuer A, Subject B, Resources, AS64496-AS64500

     Certificate 2:
       Issuer B, Subject C, Resources, AS64496-AS64511

     Certificate 3:
       Issuer C, Subject D, Resources

   Certificate 3 is considered to be an invalid certificate, because the
   resources in Certificate 2 are not encompassed by the resources in
   Certificate 1, by virtue of certificate 2 describing the resources of
   the range AS64501 - AS64511 in this RFC3779 resource extension.
   Obviously, these Autonomous Systems numbers are not related to the
   IPv4 resources contained in Certificate 3.

   Any non-encompassed resource set can cause invalidation, be it an
   ASN, IPv4 or IPv6 resource, if it is not encompassed by the resource
   set in the parent (Issuer) certificate.

   The underlying observation here is that this definition of
   certificate validation treats a collection of resources as
   inseparable, so that a single certificate containing a bundle of
   number resources is semantically distinct from an equivalent set of
   certificates where each certificate contains a single number
   resource.  This semantic distinction between the whole and the sum of
   its parts is an artifice introduced by the particular choice of a
   certificate validation procedure, as distinct from meeting any
   particular operational requirement, and the result is the
   introduction of operational fragility into the handling of RPKI
   certificates, particularly in the case where number resources are
   moved between the corresponding registries, as described here.

3.  Operational Considerations

   There are two areas of operational concern with the current RPKI
   validation definition.

   The first is that of the robustness of the operational management
   procedures in the issuance of certificates.  If a subordinate
   Certification Authority (CA) issues a certificate that contains an
   Internet Number Resource (INR) collection that is not either exactly
   equal to, or a strict subset of, its parent CA, then this issued
   certificate, and all subordinate certificates of this issued
   certificate are invalid.  These certificates are not only defined as

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   invalid when being considered to validate an INR that is not in the
   parent CA certificate, but are defined as invalid for all INRs in the

   This constraint creates a degree of operational fragility in the
   issuance of certificates, as all CA's are now required to exercise
   extreme care in the issuance and reissuance of certificates to ensure
   that at no time do they overclaim on the resources described in the
   parent CA, as the consequences of an operational lapse or oversight
   implies that all the subordinate certificates from the point of INR
   mismatch are invalid.  It would be preferred if the consequences of
   such an operational lapse were limited in scope to the specific INRs
   that formed the mismatch, rather than including the entire set of
   INRs within the scope of damage from this point of mismatch downward
   across the entire sub-tree of descendant certificates in the RPKI
   certificate hierarchy.

   The second operational consideration described here relates to the
   situation where a registry withdraws a resource from the current
   holder, and the resource to transferred to another registry, to be
   registered to a new holder in that registry.  The reason why this is
   a consideration in operational deployments of the RPKI lies in the
   movement of the "home" registry of number resources during cases of
   mergers, acquisitions, business re-alignments, and resource transfers
   and the desire to ensure that during this movement all other
   resources can continue to be validated.

   If the original registry's certification actions are simply to issue
   a new certificate for the current holder with a reduced resource set,
   and to revoke the original certificate, then there is a distinct
   possibility of encountering the situation illustrated by the example
   in the previous section.  This is a result of an operational process
   for certificate issuance by the parent CA being de-coupled from the
   certificate operations of child CA.

   This de-coupled operation of CAs introduces a risk of unintended
   third party damage: since a CA certificate can refer to holdings
   which relate to two or more unrelated subordinate certificates, if
   this CA certificate becomes invalid due to the reduction in the
   resources allocated to this CA relating to one subordinate resource
   set, all other subordinate certificates are invalid until the CA
   certificate is reissued with a reduced resource set.

   In the example provided in the previous section, all subordinate
   certificates issued by CA B are invalid, including all certificates
   issued by CA C, until CA A issues a new certificate for CA B with a
   reduced resource set.

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   At the lower levels of the RPKI hierarchy the resource sets affected
   by such movements of resources may not encompass significantly large
   pools of resources.  However, as one ascends through this
   certification hierarchy towards the apex, the larger the resource set
   that is going to be affected by a period of invalidity by virtue of
   such uncoordinated certificate management actions.  In the case of a
   Regional Internet Registry (RIR) or National Internet Registry (NIR),
   the potential risk arising from uncoordinated certification actions
   relating to a transfer of resources is that the entire set of
   subordinate certificates that refer to resources administered by the
   RIR or the NIR cannot be validated during this period.

   Avoiding such situations requires that CA's adhere to a very specific
   ordering of certificate issuance.  In this framework, the common
   registry CA that describes (directly or indirectly) the resources
   being shifted from one registry to the other, and also contains in
   subordinate certificates (direct or indirect) the certificates for
   both registries who are parties to the resource transfer has to
   coordinate a specific sequence of actions.

   This common registry CA has to first issue a new certificate towards
   the "receiving" registry that adds to the RFC3779 extension resource
   set the specific resource being transferred into this receiving
   registry.  The common registry CA then has to wait until all
   registries in the subordinate certificate chain to the receiving
   registry have also performed a similar issuance of new certificates,
   and in each case a registry must await the issuance of the immediate
   superior certificate with the augmented resource set before it, in
   turn, can issue its own augmented certificate to its subordinate CA.
   This is a "top down" issuance sequence."

   It is possible for the common registry to issue a certificate to the
   "sending" registry with the reduced resource set at any time, but it
   should not revoke the previously issued certificate, nor overwrite
   this previously issued certificate in its repository publication
   point without specific coordination.  Only when the common registry
   is assured that the top down certificate issuance process to the
   receiving registry CA chain has been completed can the common
   registry commence the revocation of the original certificate for the
   sending registry, However, it should not so until it is assured that
   the immediate subordinate registry CA in the path to the sending
   registry has issued a certificate with a reduced resource set, and so
   on.  This implies that on the sending side the certificate issuance
   and revocation is a "bottom up" process.

   If this process is not carefully followed, then the risk is that some
   or all or the subordinate certificates of this common registry CA
   will be unable to be validated until the entire process of

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   certificate issuance and revocation has been completed.  While this
   sequenced process is intended to preserve validity of certificates in
   the RPKI, it is a complex, fragile and operationally cumbersome

   The underlying consideration here is that the operational
   coordination of these certificate issuance and revocation actions to
   effect a smooth resource transfer across registries is mandated by
   the nature of the particular choice of certificate validation process
   described in [RFC6487].

4.  Alternatives Approaches

   If the current definition of the RPKI certificate validation
   procedure is considered to introduce unacceptable levels of fragility
   and risk into the operational environment, what alternatives exist?

   One approach is to remove the semantic requirement to consider the
   collection of resources in the extension field of the RPKI
   certificate as an indivisible bundle.  This would allow for a
   certificate to be considered as valid for some subset of the
   resources listed in this extension, without necessarily being
   considered as valid for all such described resources.  The
   implications of this approach is that any mismatch between parent and
   subordinate over resources where the subordinate certificate lists
   resources that are not contained in the parent certificate would
   affect validity questions relating to only those particular
   resources, rather than invaliding the subordinate certificate for all
   resources, and all of its subordinate products.  This would appear to
   offer a relatively precise match tot he defined problem space, and
   limits the scope of consequent third party damage in the event of a
   INR mismatch in the RPKI certification hierarchy.

   Another approach may involve the alteration of the RPKI provisioning
   protocol [RFC6492] to include a specific signal from child to parent
   ("bottom up") relating to readiness for certificate revocation.  At
   this stage it is entirely unclear how this signalling mechanism would
   operate, nor is it clear that it would alter the elements of
   operational fragility nor mitigate to any meaningful extent the risks
   of failure to ensure strict INR consistency at all times.  This is a
   topic for further study.

5.  Security Considerations

   The Security Considerations of [RFC6487] and [RFC6492] do not address
   the topic described here.  Obviously, within the current RPKI

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   validation procedure, any inconsistency in certificates located
   towards the apex of the RPKI hierarchy would invalidate the entirety
   of the sub-tree located below the point of this inconsistency.  If
   the RPKI was used to control inter-domain routing in the context of a
   secure routing protocol, then the implications of this large scale
   invalidation of certificates would have a corresponding massive
   impact on the stability of routing.  This appears to be a serious

6.  IANA Considerations

   No updates to the registries are suggested by this document.

7.  Acknowledgements


8.  References

8.1.  Normative References

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779, June 2004.

   [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, May 2008.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              February 2012.

8.2.  Informative References

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482, February 2012.

   [RFC6492]  Huston, G., Loomans, R., Ellacott, B., and R. Austein, "A
              Protocol for Provisioning Resource Certificates",
              RFC 6492, February 2012.

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Authors' Addresses

   Geoff Huston
   Asia Pacific Network Information Centre
   6 Cordelia St
   South Brisbane, QLD  4101

   Phone: +61 7 3858 3100

   George Michaelson
   Asia Pacific Network Information Centre
   6 Cordelia St
   South Brisbane, QLD  4101

   Phone: +61 7 3858 3100

   Carlos M. Martinez
   Latin American and Caribbean IP Address Regional Registry
   Rambla Mexico 6125
   Montevideo  11400

   Phone: +598 2604 2222

   Tim Bruijnzeels
   RIPE Network Coordination Centre
   Singel 258
   Amsterdam  1016 AB
   The Netherlands


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   Andrew Lee Newton
   American Registry for Internet Numbers
   3635 Concorde Parkway
   Chantilly, VA  20151


   Alain Aina
   African Network Information Centre (AFRINIC)
   11th Floor, Raffles Tower
   Cybercity, Ebene

   Phone: +230 403 51 00

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