RIPE NCC's Implementation of Resource Public Key Infrastructure (RPKI) Certificate Tree Validation
RFC 8488

Internet Engineering Task Force (IETF)                      O. Muravskiy
Request for Comments: 8488                                      RIPE NCC
Category: Informational                                   T. Bruijnzeels
ISSN: 2070-1721                                               NLnet Labs
                                                           December 2018

 RIPE NCC's Implementation of Resource Public Key Infrastructure (RPKI)
                      Certificate Tree Validation

Abstract

   This document describes an approach to validating the content of the
   Resource Public Key Infrastructure (RPKI) certificate tree, as it is
   implemented in the RIPE NCC RPKI Validator.  This approach is
   independent of a particular object retrieval mechanism, which allows
   it to be used with repositories available over the rsync protocol,
   the RPKI Repository Delta Protocol (RRDP), and repositories that use
   a mix of both.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8488.

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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  General Considerations  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Hash Comparisons  . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Discovery of RPKI Objects Issued by a CA  . . . . . . . .   5
     2.3.  Manifest Entries versus Repository Content  . . . . . . .   5
   3.  Top-Down Validation of a Single Trust Anchor Certificate Tree   6
     3.1.  Fetching the Trust Anchor Certificate Using the Trust
           Anchor Locator  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  CA Certificate Validation . . . . . . . . . . . . . . . .   7
       3.2.1.  Finding the Most Recent Valid Manifest and CRL  . . .   8
       3.2.2.  Validating Manifest Entries . . . . . . . . . . . . .   9
     3.3.  Object Store Cleanup  . . . . . . . . . . . . . . . . . .  10
   4.  Remote Objects Fetcher  . . . . . . . . . . . . . . . . . . .  11
     4.1.  Fetcher Operations  . . . . . . . . . . . . . . . . . . .  11
       4.1.1.  Fetch Repository Objects  . . . . . . . . . . . . . .  12
       4.1.2.  Fetch Single Repository Object  . . . . . . . . . . .  12
   5.  Local Object Store  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Store Operations  . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Store Repository Object . . . . . . . . . . . . . . .  12
       5.1.2.  Get Objects by Hash . . . . . . . . . . . . . . . . .  12
       5.1.3.  Get Certificate Objects by URI  . . . . . . . . . . .  13
       5.1.4.  Get Manifest Objects by AKI . . . . . . . . . . . . .  13
       5.1.5.  Delete Objects for a URI  . . . . . . . . . . . . . .  13
       5.1.6.  Delete Outdated Objects . . . . . . . . . . . . . . .  13
       5.1.7.  Update Object's Validation Time . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  Hash Collisions . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  Algorithm Agility . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Mismatch between the Expected and Actual Location of an
           Object in the Repository  . . . . . . . . . . . . . . . .  14
     7.4.  Manifest Content versus Publication Point Content . . . .  14
     7.5.  Possible Denial of Service  . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   This document describes how the RIPE NCC RPKI Validator version 2.25
   has been implemented.  Source code for this software can be found at
   [rpki-validator].  The purpose of this document is to provide
   transparency to users of (and contributors to) this software tool.

   In order to use information published in RPKI repositories, Relying
   Parties (RPs) need to retrieve and validate the content of
   certificates, Certificate Revocation Lists (CRLs), and other RPKI
   signed objects.  To validate a particular object, one must ensure
   that all certificates in the certificate chain up to the Trust Anchor
   (TA) are valid.  Therefore, the validation of a certificate tree is
   performed top-down, starting from the TA certificate and descending
   the certificate chain, validating every encountered certificate and
   its products.  The result of this process is a list of all
   encountered RPKI objects with a validity status attached to each of
   them.  These results may later be used by an RP in making routing
   decisions, etc.

   Traditionally, RPKI data is made available to RPs through the
   repositories [RFC6481] accessible over the rsync protocol [rsync].
   RPs are advised to keep a local copy of repository data and perform
   regular updates of this copy from the repository (see Section 5 of
   [RFC6481]).  The RRDP [RFC8182] introduces another method to fetch
   repository data and keep the local copy up to date with the
   repository.

   This document describes how the RIPE NCC RPKI Validator discovers
   RPKI objects to download, builds certificate paths, and validates
   RPKI objects, independently of what repository access protocol is
   used.  To achieve this, it puts downloaded RPKI objects in an object
   store, where each RPKI object can be found by its URI, the hash of
   its content, the value of its Authority Key Identifier (AKI)
   extension, or a combination of these.  It also keeps track of the
   download and validation time for every object, to decide which
   locally stored objects are not used in the RPKI tree validation and
   could be removed.

2.  General Considerations

2.1.  Hash Comparisons

   This algorithm relies on the collision resistance properties of the
   hash algorithm (defined in [RFC7935]) to compute the hash of
   repository objects.  It assumes that any two objects for which the
   hash value is the same are identical.

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   The hash comparison is used when matching objects in the repository
   with entries on the manifest (Section 3.2.2) and when looking up
   objects in the object store (Section 5).

2.2.  Discovery of RPKI Objects Issued by a CA

   There are several possible ways of discovering potential products of
   a Certification Authority (CA) certificate: one could 1) use all
   objects located in a repository directory designated as a publication
   point for a CA, 2) only use objects mentioned on the manifest located
   at that publication point (see Section 6 of [RFC6486]), or 3) use all
   known repository objects whose AKI extension matches the Subject Key
   Identifier (SKI) extension (Section 4.2.1 of [RFC5280]) of a CA
   certificate.

   For publication points whose content is consistent with the manifest
   and issuing certificate, all of these approaches should produce the
   same result.  For inconsistent publication points, the results might
   be different.  Section 6 of [RFC6486] leaves the decision on how to
   deal with inconsistencies to a local policy.

   The implementation described here does not rely on content of
   repository directories but uses the Authority Key Identifier (AKI)
   extension of a manifest and a CRL to find in an object store
   (Section 5) a manifest and a CRL issued by a particular CA (see
   Section 3.2.1).  It further uses the hashes of the manifest's
   fileList entries (Section 4.2.1 of [RFC6486]) to find other objects
   issued by the CA, as described in Section 3.2.2.

2.3.  Manifest Entries versus Repository Content

   Since the current set of RPKI standards (see [RFC6481], [RFC6486],
   and [RFC6487]) requires use of the manifest [RFC6486] to describe the
   content of a publication point, this implementation requires strict
   consistency between the publication point content and manifest
   content.  (This is a more stringent requirement than established in
   [RFC6486].)  Therefore, it will not process objects that are found in
   the publication point but do not match any of the entries of that
   publication point's manifest (see Section 3.2.2).  It will also issue
   warnings for all found mismatches, so that the responsible operators
   could be made aware of inconsistencies and fix them.

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3.  Top-Down Validation of a Single Trust Anchor Certificate Tree

   When several Trust Anchors are configured, validation of their
   corresponding certificate trees is performed concurrently and
   independently from each other.  For every configured Trust Anchor,
   the following steps are performed:

   1.  The validation of a TA certificate tree starts from its TA
       certificate.  To retrieve the TA certificate, a Trust Anchor
       Locator (TAL) object is used, as described in Section 3.1.

   2.  If the TA certificate is retrieved, it is validated according to
       Section 7 of [RFC6487] and Section 2.2 of [RFC7730].  Otherwise,
       the validation of the certificate tree is aborted and an error is
       issued.

   3.  If the TA certificate is valid, then all its subordinate objects
       are validated as described in Section 3.2.  Otherwise, the
       validation of the certificate tree is aborted and an error is
       issued.

   4.  For each repository object that was validated during this
       validation run, the validation timestamp is updated in the object
       store (see Section 5.1.7).

   5.  Outdated objects are removed from the store as described in
       Section 3.3.  This completes the validation of the TA certificate
       tree.

3.1.  Fetching the Trust Anchor Certificate Using the Trust Anchor
      Locator

   The following steps are performed in order to fetch a Trust Anchor
   certificate:

   1.  (Optional) If the TAL contains a prefetch.uris field, pass the
       URIs contained in that field to the fetcher (see Section 4.1.1).
       (This field is a non-standard addition to the TAL format.  It
       helps with fetching non-hierarchical rsync repositories more
       efficiently.)

   2.  Extract the first TA certificate URI from the TAL's URI section
       (see Section 2.1 of [RFC7730]) and pass it to the object fetcher
       (Section 4.1.2).  If the fetcher returns an error, repeat this
       step for every URI in the URI section until no error is
       encountered or no more URIs are left.

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   3.  From the object store (see Section 5.1.3), retrieve all
       certificate objects for which the URI matches the URI extracted
       from the TAL in the previous step and the public key matches the
       subjectPublicKeyInfo extension of the TAL (see Section 2.1 of
       [RFC7730]).

   4.  If no such objects are found or if more than one such objects are
       found, issue an error and abort the certificate tree validation
       process with an error.  Otherwise, use the single found object as
       the TA certificate.

3.2.  CA Certificate Validation

   The following steps describe the validation of a single CA resource
   certificate:

   1.  If both the caRepository (Section 4.8.8.1 of [RFC6487]) and the
       id-ad-rpkiNotify (Section 3.2 of [RFC8182]) instances of an
       accessMethod are present in the Subject Information Access
       extension of the CA certificate, use a local policy to determine
       which pointer to use.  Extract the URI from the selected pointer
       and pass it to the object fetcher (that will then fetch all
       objects available from that repository; see Section 4.1.1).

   2.  For the CA certificate, find the current manifest and certificate
       revocation list (CRL) using the procedure described in
       Section 3.2.1.  If no such manifest and CRL could be found, stop
       validation of this certificate, consider it invalid, and issue an
       error.

   3.  Compare the URI found in the id-ad-rpkiManifest field
       (Section 4.8.8.1 of [RFC6487]) of the SIA extension of the
       certificate with the URI of the manifest found in the previous
       step.  If they are different, issue a warning but continue the
       validation process using the manifest found in the previous step.
       (This warning indicates that there is a mismatch between the
       expected and the actual location of an object in a repository.
       See Section 7.3 for the explanation of this mismatch and the
       decision made.)

   4.  Perform discovery and validation of manifest entries as described
       in Section 3.2.2.

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   5.  Validate all resource certificate objects found on the manifest
       using the CRL object:

       *  If the strict validation option is enabled by the operator,
          the validation is performed according to Section 7 of
          [RFC6487].

       *  Otherwise, the validation is performed according to Section 7
          of [RFC6487] but with the exception of the resource
          certification path validation, which is performed according to
          Section 4.2.4.4 of [RFC8360].

       (Note that this implementation uses the operator configuration to
       decide which algorithm to use for path validation.  It applies
       the selected algorithm to all resource certificates, rather than
       applying an appropriate algorithm per resource certificate based
       on the object identifier (OID) for the Certificate Policy found
       in that certificate, as specified in [RFC8360].)

   6.  Validate all Route Origin Authorization (ROA) objects found on
       the manifest using the CRL object found on the manifest,
       according to Section 4 of [RFC6482].

   7.  Validate all Ghostbusters Record objects found on the manifest
       using the CRL object found on the manifest, according to
       Section 7 of [RFC6493].

   8.  For every valid CA certificate object found on the manifest,
       apply the procedure described in this section, recursively,
       provided that this CA certificate (identified by its SKI) has not
       yet been validated during current tree validation run.

3.2.1.  Finding the Most Recent Valid Manifest and CRL

   To find the most recent issued manifest and CRL objects of a
   particular CA certificate, the following steps are performed:

   1.  From the store (see Section 5.1.4), fetch all objects of type
       manifest whose certificate's AKI extension matches the SKI of the
       current CA certificate.  If no such objects are found, stop
       processing the current CA certificate and issue an error.

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   2.  Among found objects, find the manifest object with the highest
       manifestNumber field (Section 4.2.1 of [RFC6486]) for which all
       following conditions are met:

       *  There is only one entry in the manifest for which the store
          contains exactly one object of type CRL, the hash of which
          matches the hash of the entry.

       *  The manifest's certificate AKI equals the above CRL's AKI.

       *  The above CRL is a valid object according to Section 6.3 of
          [RFC5280].

       *  The manifest is a valid object according to Section 4.4 of
          [RFC6486], and its EE certificate is not in the CRL found
          above.

   3.  If there is an object that matches the above criteria, consider
       this object to be the valid manifest, and consider the CRL found
       at the previous step to be the valid CRL for the current CA
       certificate's publication point.

   4.  Report an error for every other manifest with a number higher
       than the number of the valid manifest.

3.2.2.  Validating Manifest Entries

   For every entry in the manifest object:

   1.  Construct an entry's URI by appending the entry name to the
       current CA's publication point URI.

   2.  Get all objects from the store whose hash attribute equals the
       entry's hash (see Section 5.1.2).

   3.  If no such objects are found, issue an error for this manifest
       entry and progress to the next entry.  This case indicates that
       the repository does not have an object at the location listed in
       the manifest or that the object's hash does not match the hash
       listed in the manifest.

   4.  For every found object, compare its URI with the URI of the
       manifest entry.

       *  For every object with a non-matching URI, issue a warning.
          This case indicates that the object from the manifest entry is
          (also) found at a different location in a (possibly different)
          repository.

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       *  If no objects with a matching URI are found, issue a warning.
          This case indicates that there is no object found in the
          repository at the location listed in the manifest entry (but
          there is at least one matching object found at a different
          location).

   5.  Use all found objects for further validation as per Section 3.2.

   Please note that the above steps will not reject objects whose hash
   matches the hash listed in the manifest but whose URI does not.  See
   Section 7.3 for additional information.

3.3.  Object Store Cleanup

   At the end of every TA tree validation, some objects are removed from
   the store using the following rules:

   1.  Given all objects that were encountered during the current
       validation run, remove from the store (Section 5.1.6) all objects
       whose URI attribute matches the URI of one of the encountered
       objects but whose content's hash does not match the hash of any
       of the encountered objects.  This removes from the store objects
       that were replaced in the repository by their newer versions with
       the same URIs.

   2.  Remove from the store all objects that were last encountered
       during validation a long time ago (as specified by the local
       policy).  This removes objects that do not appear on any valid
       manifest anymore (but possibly are still published in a
       repository).

   3.  Remove from the store all objects that were downloaded recently
       (as specified by the local policy) but that have never been used
       in the validation process.  This removes objects that have never
       appeared on any valid manifest.

   Shortening the time interval used in step 2 will free more disk space
   used by the store, at the expense of downloading removed objects
   again if they are still published in the repository.

   Extending the time interval used in step 3 will prevent repeated
   downloads of unused repository objects.  However, it will also extend
   the interval at which unused objects are removed.  This creates a
   risk that such objects will fill up all available disk space if a
   large enough amount of such objects is published in the repository
   (either by mistake or with a malicious intent).

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4.  Remote Objects Fetcher

   The fetcher is responsible for downloading objects from remote
   repositories (described in Section 3 of [RFC6481]) using the rsync
   protocol [rsync] or RRDP [RFC8182].

4.1.  Fetcher Operations

   For every visited URI, the fetcher keeps track of the last time a
   successful fetch occurred.

4.1.1.  Fetch Repository Objects

   This operation receives one parameter -- a URI.  For an rsync
   repository, this URI points to a directory.  For an RRDP repository,
   it points to the repository's notification file.

   The fetcher follows these steps:

   1.  If data associated with the URI has been downloaded recently (as
       specified by the local policy), skip the following steps.

   2.  Download remote objects using the URI provided (for an rsync
       repository, use recursive mode).  If the URI contains the "https"
       schema and download has failed, issue a warning, replace the
       "https" schema in the URI with "http", and try to download
       objects again using the resulting URI.

   3.  If remote objects cannot be downloaded, issue an error and skip
       the following steps.

   4.  Perform syntactic verification of fetched objects.  The type of
       every object (certificate, manifest, CRL, ROA, or Ghostbusters
       Record) is determined based on the object's filename extension
       (.cer, .mft, .crl, .roa, and .gbr, respectively).  The syntax of
       the object is described in Section 4 of [RFC6487] for resource
       certificates, step 1 of Section 3 of [RFC6488] for signed
       objects, Section 4 of [RFC6486] for manifests, [RFC5280] for
       CRLs, Section 3 of [RFC6482] for ROAs, and Section 5 of [RFC6493]
       for Ghostbusters Records.

   5.  Put every downloaded and syntactically correct object in the
       object store (Section 5.1.1).

   The time interval used in step 1 should be chosen based on the
   acceptable delay in receiving repository updates.

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4.1.2.  Fetch Single Repository Object

   This operation receives one parameter -- a URI that points to an
   object in a repository.

   The fetcher follows these steps:

   1.  Download a remote object using the URI provided.  If the URI
       contains the "https" schema and download failed, issue a warning,
       replace the "https" schema in the URI with "http", and try to
       download the object using the resulting URI.

   2.  If the remote object cannot be downloaded, issue an error and
       skip the following steps.

   3.  Perform syntactic verification of the fetched object.  The type
       of object (certificate, manifest, CRL, ROA, or Ghostbusters
       Record) is determined based on the object's filename extension
       (.cer, .mft, .crl, .roa, and .gbr, respectively).  The syntax of
       the object is described in Section 4 of [RFC6487] for resource
       certificates, step 1 of Section 3 of [RFC6488] for signed
       objects, Section 4 of [RFC6486] for manifests, [RFC5280] for
       CRLs, Section 3 of [RFC6482] for ROAs, and Section 5 of [RFC6493]
       for Ghostbusters Records.

   4.  If the downloaded object is not syntactically correct, issue an
       error and skip further steps.

   5.  Delete all objects from the object store (Section 5.1.5) whose
       URI matches the URI given.

   6.  Put the downloaded object in the object store (Section 5.1.1).

5.  Local Object Store

5.1.  Store Operations

5.1.1.  Store Repository Object

   Put the given object in the store if there is no record with the same
   hash and URI fields.  Note that in the (unlikely) event of hash
   collision, the given object will not replace the object in the store.

5.1.2.  Get Objects by Hash

   Retrieve all objects from the store whose hash attribute matches the
   given hash.

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5.1.3.  Get Certificate Objects by URI

   Retrieve from the store all objects of type certificate whose URI
   attribute matches the given URI.

5.1.4.  Get Manifest Objects by AKI

   Retrieve from the store all objects of type manifest whose AKI
   attribute matches the given AKI.

5.1.5.  Delete Objects for a URI

   For a given URI, delete all objects in the store with a matching URI
   attribute.

5.1.6.  Delete Outdated Objects

   For a given URI and a list of hashes, delete all objects in the store
   with a matching URI whose hash attribute is not in the given list of
   hashes.

5.1.7.  Update Object's Validation Time

   For all objects in the store whose hash attribute matches the given
   hash, set the last validation time attribute to the given timestamp.

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

7.1.  Hash Collisions

   This implementation will not detect possible hash collisions in the
   hashes of repository objects (calculated using the file hash
   algorithm specified in [RFC7935]).  It considers objects with same
   hash values to be identical.

7.2.  Algorithm Agility

   This implementation only supports hash algorithms and key sizes
   specified in [RFC7935].  Algorithm agility described in [RFC6916] is
   not supported.

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7.3.  Mismatch between the Expected and Actual Location of an Object in
      the Repository

   According to Section 2 of [RFC6481], all objects issued by a
   particular CA certificate are expected to be located in one
   repository publication point, specified in the SIA extension of that
   CA certificate.  The manifest object issued by that CA certificate
   enumerates all other issued objects, listing their filenames and
   content hashes.

   However, it is possible that an object whose content hash matches the
   hash listed in the manifest either has a different filename or is
   located at a different publication point in a repository.

   On the other hand, all RPKI objects, either explicitly or within
   their embedded EE certificate, have an AKI extension that contains
   the key identifier of their issuing CA certificate.  Therefore, it is
   always possible to perform an RPKI validation of the object whose
   expected location does not match its actual location, provided that
   the certificate that matches the AKI of the object in question is
   known to the system that performs validation.

   In the case of a mismatch as described above, this implementation
   will not exclude an object from further validation merely because its
   actual location or filename does not match the expected location or
   filename.  This decision was made because the actual location of a
   file in a repository is taken from the repository retrieval
   mechanism, which, in the case of an rsync repository, does not
   provide any cryptographic security, and in the case of an RRDP
   repository, provides only a transport-layer security with the
   fallback to unsecured transport.  On the other hand, the manifest is
   an RPKI signed object, and its content could be verified in the
   context of the RPKI validation.

7.4.  Manifest Content versus Publication Point Content

   This algorithm uses the content of a manifest object to determine
   other objects issued by a CA certificate.  It verifies that the
   manifest is located in the publication point designated in the CA
   certificate's SIA extension.  However, if there are other (not listed
   in the manifest) objects located in the same publication point
   directory, they are ignored even if they might be valid and issued by
   the same CA as the manifest.  (This RP behavior is allowed, but not
   required, by [RFC6486].)

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7.5.  Possible Denial of Service

   The store cleanup procedure described in Section 3.3 tries to
   minimize removal and subsequent re-fetch of objects that are
   published in a repository but not used in the validation.  Once such
   objects are removed from the remote repository, they will be
   discarded from the local object store after a period of time
   specified by a local policy.  By generating an excessive amount of
   syntactically valid RPKI objects, a man-in-the-middle attack between
   a validating tool and a repository could force an implementation to
   fetch and store those objects in the object store (see Section 4.1.1)
   before they are validated and discarded, leading to out-of-memory or
   out-of-disk-space conditions and, subsequently, a denial of service.

8.  References

8.1.  Normative References

   [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, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              DOI 10.17487/RFC6481, February 2012,
              <https://www.rfc-editor.org/info/rfc6481>.

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482,
              DOI 10.17487/RFC6482, February 2012,
              <https://www.rfc-editor.org/info/rfc6482>.

   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, DOI 10.17487/RFC6486, February 2012,
              <https://www.rfc-editor.org/info/rfc6486>.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,
              <https://www.rfc-editor.org/info/rfc6487>.

   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
              <https://www.rfc-editor.org/info/rfc6488>.

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   [RFC6493]  Bush, R., "The Resource Public Key Infrastructure (RPKI)
              Ghostbusters Record", RFC 6493, DOI 10.17487/RFC6493,
              February 2012, <https://www.rfc-editor.org/info/rfc6493>.

   [RFC6916]  Gagliano, R., Kent, S., and S. Turner, "Algorithm Agility
              Procedure for the Resource Public Key Infrastructure
              (RPKI)", BCP 182, RFC 6916, DOI 10.17487/RFC6916, April
              2013, <https://www.rfc-editor.org/info/rfc6916>.

   [RFC7730]  Huston, G., Weiler, S., Michaelson, G., and S. Kent,
              "Resource Public Key Infrastructure (RPKI) Trust Anchor
              Locator", RFC 7730, DOI 10.17487/RFC7730, January 2016,
              <https://www.rfc-editor.org/info/rfc7730>.

   [RFC7935]  Huston, G. and G. Michaelson, Ed., "The Profile for
              Algorithms and Key Sizes for Use in the Resource Public
              Key Infrastructure", RFC 7935, DOI 10.17487/RFC7935,
              August 2016, <https://www.rfc-editor.org/info/rfc7935>.

   [RFC8182]  Bruijnzeels, T., Muravskiy, O., Weber, B., and R. Austein,
              "The RPKI Repository Delta Protocol (RRDP)", RFC 8182,
              DOI 10.17487/RFC8182, July 2017,
              <https://www.rfc-editor.org/info/rfc8182>.

   [RFC8360]  Huston, G., Michaelson, G., Martinez, C., Bruijnzeels, T.,
              Newton, A., and D. Shaw, "Resource Public Key
              Infrastructure (RPKI) Validation Reconsidered", RFC 8360,
              DOI 10.17487/RFC8360, April 2018,
              <https://www.rfc-editor.org/info/rfc8360>.

8.2.  Informative References

   [rpki-validator]
              "RIPE-NCC/rpki-validator source code",
              <https://github.com/RIPE-NCC/rpki-validator>.

   [rsync]    "rsync", October 2018, <https://rsync.samba.org>.

Acknowledgements

   This document describes the algorithm as it is implemented by the
   software development team at the RIPE NCC, which, over time, included
   Mikhail Puzanov, Erik Rozendaal, Miklos Juhasz, Misja Alma, Thiago da
   Cruz Pereira, Yannis Gonianakis, Andrew Snare, Varesh Tapadia, Paolo
   Milani, Thies Edeling, Hans Westerbeek, Rudi Angela, and Constantijn
   Visinescu.  The authors would also like to acknowledge contributions
   by Carlos Martinez, Andy Newton, Rob Austein, and Stephen Kent.

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Authors' Addresses

   Oleg Muravskiy
   RIPE NCC

   Email: oleg@ripe.net
   URI:   https://www.ripe.net/

   Tim Bruijnzeels
   NLnet Labs

   Email: tim@nlnetlabs.nl
   URI:   https://www.nlnetlabs.nl/

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